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linux.pdf

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The document discusses various commands for working with directories in Linux/Unix systems such as pwd, cd, ls, mkdir and rmdir. It explains how to use these commands to view the current working directory, change directories, list directory contents, and create/remove directories. It also covers the differences between absolute and relative paths and the use of tab completion in bash.

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https://www.linkedin.com/in/vijaykumar-biradar-29b710161/ Linux Fundamentals for beginners Why Use Linux
Unix Vs Linux Today Linux is in great demand. You can see the use of Linux everywhere. It's dominating on our servers, desktop, smartphones and even used in some electrical devices like refrigerators. Some people think Unix and Linux as synonyms, but that's not true. Many operating systems were developed to be like Unix but none of them got the popularity as Linux. Linux is the clone of Unix. It has several features similar to Unix, still have some key differences. Before Linux and Windows, computer world was dominated by Unix. Unix is a copyrighted name and IBM AIX, HP-UX and Sun Solaris are only Unix operating system remained till date. Difference between Linux and Unix Comparison Linux Unix Definition It is an open-source operating system which is freely available to everyone. It is an operating system which can be only used by its copyrighters. Examples It has different distros like Ubuntu, Redhat, Fedora, etc IBM AIX, HP-UX and Sun Solaris. Users Nowadays, Linux is in great demand. Anyone can use Linux whether a home user, developer or a student. It was developed mainly for servers, workstations and mainframes.
Usage Linux is used everywhere from servers, PC, smartphones, tablets to mainframes and supercomputers. It is used in servers, workstations and PCs. Cost Linux is freely distributed, downloaded, and distributed through magazines also. And priced distros of Linux are also cheaper than Windows. Unix copyright vendors decide different costs for their respective Unix Operating systems. Development As it is open source, it is developed by sharing and collaboration of codes by world-wide developers. Unix was developed by AT&T Labs, various commercial vendors and non- profit organizations. Manufacturer Linux kernel is developed by the community of developers from different parts of the world. Although the father of Linux, Linus Torvalds oversees things. Unix has three distributions IBM AIX, HP-UX and Sun Solaris. Apple also uses Unix to make OSX operating system. GUI Linux is command based but some distros provide GUI based Linux. Gnome and KDE are mostly used GUI. Initially it was command based OS, but later Common Desktop Environment was created. Most Unix distributions use Gnome. Interface The default interface is BASH (Bourne Again SHell). But some distros have developed their own interfaces. It originally used Bourne shell. But is also compatible with other GUIs. File system support Linux supports more file system than Unix. It also supports file system but lesser than Linux. Coding Linux is a Unix clone,behaves like Unix but doesn't contain its code. Unix contain a completely different coding developed by AT&T Labs. Operating system Linux is just the kernel. Unix is a complete package of Operating system. Security It provides higher security. Linux has about 60-100 viruses listed till date. Unix is also highly secured. It has about 85-120 viruses listed till date Error detection and solution As Linux is open-source,whenever a user post any kind of threat, developers from all over the world start working on it. And hence, it provides faster solution. In Unix, users have to wait for some time for the problem to be resolved.
Linux Distributions (Distros) Choosing a Linux Distro Distribution Why To Use UBuntu It works like Mac OS and easy to use. Linux mint It works like windows and should be use by new comers. Debian It provides stability but not recommended to a new user. Fedora If you want to use red hat and latest software. Red hat enterprise To be used commercially. CentOS If you want to use red hat but without its trademark. OpenSUSE It works same as Fedora but slightly older and more stable. Arch Linux It is not for the beginners because every package has to be installed by yourself.
Top Linux Distribution
Chapter 1. man pages This chapter will explain the use of man pages (also called manual pages) on your Unix or Linux computer. You will learn the man command together with related commands like whereis, whatis and mandb. Most Unix files and commands have pretty good man pages to explain their use. Man pages also come in handy when you are using multiple flavours of Unix or several Linux distributions since options and parameters sometimes vary.
1.1. man $command Type man followed by a command (for which you want help) and start reading. Press q to quit the manpage. Some man pages contain examples (near the end). 1.2. man $configfile Most configuration files have their own manual. 1.3. man $daemon This is also true for most daemons (background programs) on your system.. 1.4. man -k (apropos) man -k (or apropos) shows a list of man pages containing a string. 1.5. whatis To see just the description of a manual page, use whatis followed by a string. 1.6. whereis The location of a manpage can be revealed with whereis. This file is directly readable by man. paul@laika:~$ man /usr/share/man/man1/whois.1.gz paul@laika:~$ man whois Reformatting whois(1), please wait... paul@laika:~$ man syslog.conf Reformatting syslog.conf(5), please wait... paul@laika:~$ man syslogd Reformatting syslogd(8), please wait... paul@laika:~$ man -k syslog lm-syslog-setup (8) - configure laptop mode to switch syslog.conf ... logger (1) - a shell command interface to the syslog(3) ... syslog-facility (8) - Setup and remove LOCALx facility for sysklogd syslog.conf (5) - syslogd(8) configuration file syslogd (8) - Linux system logging utilities. syslogd-listfiles (8) - list system logfiles paul@u810:~$ whatis route route (8) - show / manipulate the IP routing table paul@laika:~$ whereis -m whois whois: /usr/share/man/man1/whois.1.gz
1.7. man sections By now you will have noticed the numbers between the round brackets. man man will explain to you that these are section numbers. Executable programs and shell commands reside in section one. 1.8. man $section $file Therefor, when referring to the man page of the passwd command, you will see it written as passwd(1); when referring to the passwd file, you will see it written as passwd(5). The screenshot explains how to open the man page in the correct section. 1.9. man man If you want to know more about man, then Read The Fantastic Manual (RTFM). Unfortunately, manual pages do not have the answer to everything... 1.10. mandb Should you be convinced that a man page exists, but you can't access it, then try running mandb on Debian/Mint. Or run makewhatis on CentOS/Redhat. [root@centos65 ~]# apropos scsi scsi: nothing appropriate [root@centos65 ~]# makewhatis [root@centos65 ~]# apropos scsi hpsa (4) - HP Smart Array SCSI driver lsscsi (8) - list SCSI devices (or hosts) and their attributes sd (4) - Driver for SCSI Disk Drives st (4) - SCSI tape device 1 Executable programs or shell commands 2 System calls (functions provided by the kernel) 3 Library calls (functions within program libraries) 4 Special files (usually found in /dev) 5 File formats and conventions eg /etc/passwd 6 Games 7 Miscellaneous (including macro packages and conventions), e.g. man(7) 8 System administration commands (usually only for root) 9 Kernel routines [Non standard] [paul@RHEL52 ~]$ man passwd [paul@RHEL52 ~]$ man 5 passwd # opens the first manual found # opens a page from section 5 paul@laika:~$ man woman No manual entry for woman root@laika:~# mandb 0 man subdirectories contained newer manual pages. 0 manual pages were added. 0 stray cats were added. 0 old database entries were purged.
Chapter 2. working with directories This module is a brief overview of the most common commands to work with directories: pwd, cd, ls, mkdir and rmdir. These commands are available on any Linux (or Unix) system. This module also discusses absolute and relative paths and path completion in the bash shell.
2.1. pwd The you are here sign can be displayed with the pwd command (Print Working Directory). Go ahead, try it: Open a command line interface (also called a terminal, console or xterm) and type pwd. The tool displays your current directory. 2.2. cd You can change your current directory with the cd command (Change Directory). 2.2.1. cd ~ The cd is also a shortcut to get back into your home directory. Just typing cd without a target directory, will put you in your home directory. Typing cd ~ has the same effect. 2.2.2. cd .. To go to the parent directory (the one just above your current directory in the directory tree), type cd .. . To stay in the current directory, type cd . ;-) We will see useful use of the . character representing the current directory later. paul@debian8:~$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd /bin paul@debian8$ pwd /bin paul@debian8$ cd /home/paul/ paul@debian8$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd paul@debian8$ pwd /home/paul paul@debian8$ cd ~ paul@debian8$ pwd /home/paul paul@debian8$ pwd /usr/share/games paul@debian8$ cd .. paul@debian8$ pwd /usr/share
2.2.3. cd - Another useful shortcut with cd is to just type cd - to go to the previous directory. 2.3. absolute and relative paths You should be aware of absolute and relative paths in the file tree. When you type a path starting with a slash (/), then the root of the file tree is assumed. If you don't start your path with a slash, then the current directory is the assumed starting point. The screenshot below first shows the current directory /home/paul. From within this directory, you have to type cd /home instead of cd home to go to the /home directory. When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory paul of the current directory /home. In case your current directory is the root directory /, then both cd /home and cd home will get you in the /home directory. This was the last screenshot with pwd statements. From now on, the current directory will often be displayed in the prompt. Later in this book we will explain how the shell variable $PS1 can be configured to show this. paul@debian8$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd - /home/paul paul@debian8$ cd - /etc paul@debian8$ pwd /home/paul paul@debian8$ cd home bash: cd: home: No such file or directory paul@debian8$ cd /home paul@debian8$ pwd /home paul@debian8$ pwd /home paul@debian8$ cd /paul bash: cd: /paul: No such file or directory paul@debian8$ cd paul paul@debian8$ pwd /home/paul paul@debian8$ pwd / paul@debian8$ cd home paul@debian8$ pwd /home paul@debian8$ cd / paul@debian8$ cd /home paul@debian8$ pwd /home
2.4. path completion The tab key can help you in typing a path without errors. Typing cd /et followed by the tab key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key, nothing will happen because you typed the wrong path (upper case E). You will need fewer key strokes when using the tab key, and you will be sure your typed path is correct! 2.5. ls You can list the contents of a directory with ls. 2.5.1. ls -a A frequently used option with ls is -a to show all files. Showing all files means including the hidden files. When a file name on a Linux file system starts with a dot, it is considered a hidden file and it doesn't show up in regular file listings. 2.5.2. ls -l Many times you will be using options with ls to display the contents of the directory in different formats or to display different parts of the directory. Typing just ls gives you alist of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a long listing. paul@debian8:~$ ls -l total 17296 -rw-r--r-- 1 paul paul 17584442 Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 96650 Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 19558 Sep 17 00:04 services drwxr-xr-x 2 paul paul 4096 Sep 17 00:04 stuff -rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt paul@debian8:~$ ls allfiles.txt dmesg.txt services stuff summer.txt paul@debian8:~$ paul@debian8:~$ ls allfiles.txt dmesg.txt services paul@debian8:~$ ls -a . allfiles.txt .bash_profile .. .bash_history .bashrc paul@debian8:~$ stuff summer.txt dmesg.txt services .lesshst stuff .ssh summer.txt
2.5.3. ls -lh Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human readable format. Also shown below is some variation in the way you can give the options to ls. We will explain the details of the output later in this book. Note that we use the letter L as an option in this screenshot, not the number 1. paul@debian8:~$ ls -l -h total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul paul@debian8:~$ ls -lh 0 Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul paul@debian8:~$ ls -hl 0 Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul 0 paul@debian8:~$ ls -h -l Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul@debian8:~$ paul 0 Sep 17 00:04 summer.txt
2.6. mkdir Walking around the Unix file tree is fun, but it is even more fun to create your own directories with mkdir. You have to give at least one parameter to mkdir, the name of the new directory to be created. Think before you type a leading / . 2.6.1. mkdir -p The following command will fail, because the parent directory of threedirsdeep does not exist. When given the option -p, then mkdir will create parent directories as needed. 2.7. rmdir When a directory is empty, you can use rmdir to remove the directory. paul@debian8:~$ mkdir mydir paul@debian8:~$ cd mydir paul@debian8:~/mydir$ ls -al total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:07 . drwxr-xr-x 48 paul paul 4096 Sep 17 00:07 .. paul@debian8:~/mydir$ mkdir stuff paul@debian8:~/mydir$ mkdir otherstuff paul@debian8:~/mydir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff paul@debian8:~/mydir$ paul@debian8:~$ mkdir mydir2/mysubdir2/threedirsdeep mkdir: cannot create directory ‘mydir2/mysubdir2/threedirsdeep’: No such fi le or directory paul@debian8:~$ mkdir -p mydir2/mysubdir2/threedirsdeep paul@debian8:~$ cd mydir2 paul@debian8:~/mydir2$ ls -l total 4 drwxr-xr-x 3 paul paul 4096 Sep 17 00:11 mysubdir2 paul@debian8:~/mydir2$ cd mysubdir2 paul@debian8:~/mydir2/mysubdir2$ ls -l total 4 drwxr-xr-x 2 paul paul 4096 Sep 17 00:11 threedirsdeep paul@debian8:~/mydir2/mysubdir2$ cd threedirsdeep/ paul@debian8:~/mydir2/mysubdir2/threedirsdeep$ pwd /home/paul/mydir2/mysubdir2/threedirsdeep paul@debian8:~/mydir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff paul@debian8:~/mydir$ rmdir otherstuff paul@debian8:~/mydir$ cd .. paul@debian8:~$ rmdir mydir rmdir: failed to remove ‘mydir’: Directory not empty paul@debian8:~$ rmdir mydir/stuff paul@debian8:~$ rmdir mydir paul@debian8:~$
2.7.1. rmdir -p And similar to the mkdir -p option, you can also use rmdir to recursively remove directories. 2.8. solution: working with directories 1. Display your current directory. 2. Change to the /etc directory. 3. Now change to your home directory using only three key presses. 4. Change to the /boot/grub directory using only eleven key presses. 5. Go to the parent directory of the current directory. 6. Go to the root directory. 7. List the contents of the root directory. 8. List a long listing of the root directory. 9. Stay where you are, and list the contents of /etc. 10. Stay where you are, and list the contents of /bin and /sbin. 11. Stay where you are, and list the contents of ~. 12. List all the files (including hidden files) in your home directory. 13. List the files in /boot in a human readable format. paul@debian8:~$ mkdir -p test42/subdir paul@debian8:~$ rmdir -p test42/subdir paul@debian8:~$ pwd cd /etc cd (and the enter key) cd /boot/grub (use the tab key) cd .. (with space between cd and ..) cd / ls ls -l ls /etc ls /bin /sbin ls ~ ls -al ~
14. Create a directory testdir in your home directory. 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. ls -lh /boot mkdir ~/testdir
16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). 17. Remove the directory testdir. 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. The Bash shell has two built-in commands called pushd and popd. Both commands work with a common stack of previous directories. Pushd adds a directory to the stack and changes to a new current directory, popd removes a directory from the stack and sets the current directory. cd /etc ; mkdir ~/newdir mkdir -p ~/dir1/dir2/dir3 rmdir testdir man bash /pushd n # opens the manual # searches for pushd # next (do this two/three times) paul@debian7:/etc$ cd /bin paul@debian7:/bin$ pushd /lib /lib /bin paul@debian7:/lib$ pushd /proc /proc /lib /bin paul@debian7:/proc$ popd /lib /bin paul@debian7:/lib$ popd /bin
linux.pdf
Chapter 3. working with files In this chapter we learn how to recognise, create, remove, copy and move files using commands like file, touch, rm, cp, mv and rename.
3.1. all files are case sensitive Files on Linux (or any Unix) are case sensitive. This means that FILE1 is different from file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical Linux computer). This screenshot shows the difference between two files, one with upper case W, the other with lower case w. 3.2. everything is a file A directory is a special kind of file, but it is still a (case sensitive!) file. Each terminal window (for example /dev/pts/4), any hard disk or partition (for example /dev/sdb1) and any process are all represented somewhere in the file system as a file. It will become clear throughout this course that everything on Linux is a file. 3.3. file The file utility determines the file type. Linux does not use extensions to determine the file type. The command line does not care whether a file ends in .txt or .pdf. As a system administrator, you should use the file command to determine the file type. Here are some examples on a typical Linux system. The file command uses a magic file that contains patterns to recognise file types. The magic file is located in /usr/share/file/magic. Type man 5 magic for more information. It is interesting to point out file -s for special files like those in /dev and /proc. paul@laika:~/Linux$ ls winter.txt Winter.txt paul@laika:~/Linux$ cat winter.txt It is cold. paul@laika:~/Linux$ cat Winter.txt It is very cold! paul@laika:~$ file pic33.png pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced paul@laika:~$ file /etc/passwd /etc/passwd: ASCII text paul@laika:~$ file HelloWorld.c HelloWorld.c: ASCII C program text root@debian6~# file /dev/sda /dev/sda: block special root@debian6~# file -s /dev/sda /dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead... root@debian6~# file /proc/cpuinfo /proc/cpuinfo: empty root@debian6~# file -s /proc/cpuinfo /proc/cpuinfo: ASCII C++ program text
3.4. touch 3.4.1. create an empty file One easy way to create an empty file is with touch. (We will see many other ways for creating files later in this book.) This screenshot starts with an empty directory, creates two files with touch and the lists those files. 3.4.2. touch -t The touch command can set some properties while creating empty files. Can you determine what is set by looking at the next screenshot? If not, check the manual for touch. paul@debian7:~$ touch -t 200505050000 SinkoDeMayo paul@debian7:~$ touch -t 130207111630 BigBattle.txt paul@debian7:~$ ls -l total 0 -rw-r--r-- 1 paul paul 0 Jul 11 1302 BigBattle.txt -rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33 -rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42 -rw-r--r-- 1 paul paul@debian7:~$ paul 0 May 5 2005 SinkoDeMayo paul@debian7:~$ ls -l total 0 paul@debian7:~$ touch file42 paul@debian7:~$ touch file33 paul@debian7:~$ ls -l total 0 -rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33 -rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42 paul@debian7:~$
3.5. rm 3.5.1. remove forever When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces, the command line in general does not have a waste bin or trash can to recover files. When you use rm to remove a file, the file is gone. Therefore, be careful when removing files! 3.5.2. rm -i To prevent yourself from accidentally removing a file, you can type rm -i. 3.5.3. rm -rf By default, rm -r will not remove non-empty directories. However rm accepts several options that will allow you to remove any directory. The rm -rf statement is famous because it will erase anything (providing that you have the permissions to do so). When you are logged on as root, be very careful with rm -rf (the f means force and the r means recursive) since being root implies that permissions don't apply to you. You can literally erase your entire file system by accident. paul@debian7:~$ ls BigBattle.txt file33 file42 SinkoDeMayo paul@debian7:~$ rm BigBattle.txt paul@debian7:~$ ls file33 file42 SinkoDeMayo paul@debian7:~$ paul@debian7:~$ ls file33 file42 SinkoDeMayo paul@debian7:~$ rm -i file33 rm: remove regular empty file `file33'? yes paul@debian7:~$ rm -i SinkoDeMayo rm: remove regular empty file `SinkoDeMayo'? n paul@debian7:~$ ls file42 SinkoDeMayo paul@debian7:~$ paul@debian7:~$ mkdir test paul@debian7:~$ rm test rm: cannot remove `test': Is a directory paul@debian7:~$ rm -rf test paul@debian7:~$ ls test ls: cannot access test: No such file or directory paul@debian7:~$
3.6. cp 3.6.1. copy one file To copy a file, use cp with a source and a target argument. 3.6.2. copy to another directory If the target is a directory, then the source files are copied to that target directory. 3.6.3. cp -r To copy complete directories, use cp -r (the -r option forces recursive copying of all files in all subdirectories). 3.6.4. copy multiple files to directory You can also use cp to copy multiple files into a directory. In this case, the last argument (a.k.a. the target) must be a directory. 3.6.5. cp -i To prevent cp from overwriting existing files, use the -i (for interactive) option. paul@debian7:~$ ls file42 SinkoDeMayo paul@debian7:~$ cp file42 file42.copy paul@debian7:~$ ls file42 file42.copy SinkoDeMayo paul@debian7:~$ mkdir dir42 paul@debian7:~$ cp SinkoDeMayo dir42 paul@debian7:~$ ls dir42/ SinkoDeMayo paul@debian7:~$ ls dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ cp -r dir42/ dir33 paul@debian7:~$ ls dir33 dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ ls dir33/ SinkoDeMayo paul@debian7:~$ cp file42 file42.copy SinkoDeMayo dir42/ paul@debian7:~$ ls dir42/ file42 file42.copy SinkoDeMayo paul@debian7:~$ cp SinkoDeMayo file42 paul@debian7:~$ cp SinkoDeMayo file42 paul@debian7:~$ cp -i SinkoDeMayo file42 cp: overwrite `file42'? n paul@debian7:~$
3.7. mv 3.7.1. rename files with mv Use mv to rename a file or to move the file to another directory. When you need to rename only one file then mv is the preferred command to use. 3.7.2. rename directories with mv The same mv command can be used to rename directories. paul@debian7:~$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir33 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42 -rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33 -rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy -rw-r--r-- 1 paul paul 0 May 5 2005 SinkoDeMayo paul@debian7:~$ mv dir33 backup paul@debian7:~$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 backup drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42 -rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33 -rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy -rw-r--r-- 1 paul paul@debian7:~$ paul 0 May 5 2005 SinkoDeMayo 3.7.3. mv -i The mv also has a -i switch similar to cp and rm. this screenshot shows that mv -i will ask permission to overwrite an existing file. paul@debian7:~$ ls dir33 dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ mv file42 file33 paul@debian7:~$ ls dir33 dir42 file33 file42.copy SinkoDeMayo paul@debian7:~$ paul@debian7:~$ mv -i file33 SinkoDeMayo mv: overwrite `SinkoDeMayo'? no paul@debian7:~$
3.8. rename 3.8.1. about rename The rename command is one of the rare occasions where the Linux Fundamentals book has to make a distinction between Linux distributions. Almost every command in the Fundamentals part of this book works on almost every Linux computer. But rename is different. Try to use mv whenever you need to rename only a couple of files. 3.8.2. rename on Debian/Ubuntu The rename command on Debian uses regular expressions (regular expression or shor regex are explained in a later chapter) to rename many files at once. Below a rename example that switches all occurrences of txt to png for all file names ending in .txt. This second example switches all (first) occurrences of file into document for all file names ending in .png. 3.8.3. rename on CentOS/RHEL/Fedora On Red Hat Enterprise Linux, the syntax of rename is a bit different. The first example below renames all *.conf files replacing any occurrence of .conf with .backup. The second example renames all (*) files replacing one with ONE. paul@debian7:~/test42$ ls abc.txt file33.txt file42.txt paul@debian7:~/test42$ rename 's/.txt/.png/' *.txt paul@debian7:~/test42$ ls abc.png file33.png file42.png paul@debian7:~/test42$ ls abc.png file33.png file42.png paul@debian7:~/test42$ rename 's/file/document/' *.png paul@debian7:~/test42$ ls abc.png document33.png document42.png paul@debian7:~/test42$ [paul@centos7 ~]$ touch one.conf two.conf three.conf [paul@centos7 ~]$ rename .conf .backup *.conf [paul@centos7 ~]$ ls one.backup three.backup two.backup [paul@centos7 ~]$ [paul@centos7 ~]$ ls one.backup three.backup two.backup [paul@centos7 ~]$ rename one ONE * [paul@centos7 ~]$ ls ONE.backup three.backup two.backup [paul@centos7 ~]$
3.9. practice: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux- training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/ LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. 10. Use one command to remove the directory ~/testbackup and all files into it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf
3.10. solution: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux- training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/ LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. 10. Use one command to remove the directory ~/testbackup and all files into it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? ls /bin file /bin/cat /etc/passwd /usr/bin/passwd wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf file wolf.jpg LinuxFun.pdf mv wolf.jpg wolf.pdf file wolf.pdf LinuxFun.pdf mkdir ~/touched ; cd ~/touched touch today.txt yesterday.txt touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday) cp yesterday.txt copy.yesterday.txt mv copy.yesterday.txt kim mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/ rm -rf ~/testbackup
cp -r /etc/*.conf ~/etcbackup Only *.conf files that are directly in /etc/ are copied. 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf
Chapter 4. working with file contents In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less and strings. We will also get a glimpse of the possibilities of tools like cat on the command line.
4.1. head You can use head to display the first ten lines of a file. The head command can also display the first n lines of a file. And head can also display the first n bytes. 4.2. tail Similar to head, the tail command will display the last ten lines of a file. You can give tail the number of lines you want to see. The tail command has other useful options, some of which we will use during this course. paul@debian7~$ head /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh root@debian7~# paul@debian7~$ head -4 /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh paul@debian7~$ paul@debian7~$ head -c14 /etc/passwd root:x:0:0:roopaul@debian7~$ paul@debian7~$ tail /etc/services vboxd 20012/udp binkp 24554/tcp asp 27374/tcp asp 27374/udp csync2 30865/tcp dircproxy 57000/tcp tfido 60177/tcp fido 60179/tcp # binkp fidonet protocol # Address Search Protocol # cluster synchronization tool # Detachable IRC Proxy # fidonet EMSI over telnet # fidonet EMSI over TCP # Local services paul@debian7~$ paul@debian7~$ tail -3 /etc/services fido 60179/tcp # fidonet EMSI over TCP # Local services paul@debian7~$
4.3. cat The cat command is one of the most universal tools, yet all it does is copy standard input to standard output. In combination with the shell this can be very powerful and diverse. Some examples will give a glimpse into the possibilities. The first example is simple, you can use cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end. 4.3.1. concatenate cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger (or complete) file. 4.3.2. create files You can use cat to create flat text files. Type the cat > winter.txt command as shown in the screenshot below. Then type one or more lines, finishing each line with the enter key. After the last line, type and hold the Control (Ctrl) key and press d. The Ctrl d key combination will send an EOF (End of File) to the running process ending the cat command. paul@debian8:~$ cat /etc/resolv.conf domain linux-training.be search linux-training.be nameserver 192.168.1.42 paul@debian8:~$ echo one >part1 paul@debian8:~$ echo two >part2 paul@debian8:~$ echo three >part3 paul@debian8:~$ cat part1 one paul@debian8:~$ cat part2 two paul@debian8:~$ cat part3 three paul@debian8:~$ cat part1 part2 part3 one two three paul@debian8:~$ cat part1 part2 part3 >all paul@debian8:~$ cat all one two three paul@debian8:~$ paul@debian8:~$ cat > winter.txt It is very cold today! paul@debian8:~$ cat winter.txt It is very cold today! paul@debian8:~$
4.3.3. custom end marker You can choose an end marker for cat with << as is shown in this screenshot. This construction is called a here directive and will end the cat command. 4.3.4. copy files In the third example you will see that cat can be used to copy files. We will explain in detail what happens here in the bash shell chapter. 4.4. tac Just one example will show you the purpose of tac (cat backwards). paul@debian8:~$ cat > hot.txt <<stop > It is hot today! > Yes it is summer. > stop paul@debian8:~$ cat hot.txt It is hot today! Yes it is summer. paul@debian8:~$ paul@debian8:~$ cat winter.txt It is very cold today! paul@debian8:~$ cat winter.txt > cold.txt paul@debian8:~$ cat cold.txt It is very cold today! paul@debian8:~$ paul@debian8:~$ cat count one two three four paul@debian8:~$ tac count four three two one
4.5. more and less The more command is useful for displaying files that take up more than one screen. More will allow you to see the contents of the file page by page. Use the space bar to see the next page, or q to quit. Some people prefer the less command to more. 4.6. strings With the strings command you can display readable ascii strings found in (binary) files. This example locates the ls binary then displays readable strings in the binary file (output is truncated). 4.7. solution: file contents 1. Display the first 12 lines of /etc/services. 2. Display the last line of /etc/passwd. 3. Use cat to create a file named count.txt that looks like this: 4. Use cp to make a backup of this file to cnt.txt. 5. Use cat to make a backup of this file to catcnt.txt. 6. Display catcnt.txt, but with all lines in reverse order (the last line first). 7. Use more to display /etc/services. paul@laika:~$ which ls /bin/ls paul@laika:~$ strings /bin/ls /lib/ld-linux.so.2 librt.so.1 gmon_start _Jv_RegisterClasses clock_gettime libacl.so.1 ... head -12 /etc/services tail -1 /etc/passwd cat > count.txt One Two Three Four Five (followed by Ctrl-d) cp count.txt cnt.txt cat count.txt > catcnt.txt tac catcnt.txt
8. Display the readable character strings from the /usr/bin/passwd command. 9. Use ls to find the biggest file in /etc. 10. Open two terminal windows (or tabs) and make sure you are in the same directory in both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f tailing.txt in the second terminal. Now go back to the first terminal and type echo This is another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal shows both lines. Stop the tail -f with Ctrl-C. 11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed by the contents of /etc/passwd. 12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded by the contents of /etc/passwd. more /etc/services strings /usr/bin/passwd ls -lrS /etc cat /etc/passwd >> tailing.txt mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt
linux.pdf
101 Chapter 5. the Linux file tree This chapter takes a look at the most common directories in the Linux file tree. It also shows that on Unix everything is a file.
5.1. filesystem hierarchy standard Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS may help make more Unix/Linux file system trees conform better in the future. The FHS is available online at http://www.pathname.com/fhs/ where we read: "The filesystem hierarchy standard has been designed to be used by Unix distribution developers, package developers, and system implementers. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy." 5.2. man hier There are some differences in the filesystems between Linux distributions. For help about your machine, enter man hier to find information about the file system hierarchy. This manual will explain the directory structure on your computer. 5.3. the root directory / All Linux systems have a directory structure that starts at the root directory. The root directory is represented by a forward slash, like this: /. Everything that exists on your Linux system can be found below this root directory. Let's take a brief look at the contents of the root directory. [paul@RHELv4u3 ~]$ ls / bin dev home media mnt proc sbin srv tftpboot usr boot etc lib misc opt root selinux sys tmp var
the Linux file tree 103 5.4. binary directories Binaries are files that contain compiled source code (or machine code). Binaries can be executed on the computer. Sometimes binaries are called executables. 5.4.1. /bin The /bin directory contains binaries for use by all users. According to the FHS the /bin directory should contain /bin/cat and /bin/date (among others). In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd, echo, grep, and so on. Many of these will be covered in this book. paul@laika:~$ ls /bin archdetect egrep mt setupcon autopartition false mt-gnu sh bash fgconsole mv sh.distrib bunzip2 fgrep nano sleep bzcat fuser nc stralign bzcmp fusermount nc.traditional stty bzdiff get_mountoptions netcat su bzegrep grep netstat sync bzexe gunzip ntfs-3g sysfs bzfgrep gzexe ntfs-3g.probe tailf bzgrep gzip parted_devices tar bzip2 hostname parted_server tempfile bzip2recover hw-detect partman touch bzless ip partman-commit true bzmore kbd_mode perform_recipe ulockmgr cat kill pidof umount ... 5.4.2. other /bin directories You can find a /bin subdirectory in many other directories. A user named serena could put her own programs in /home/serena/bin. Some applications, often when installed directly from source will put themselves in /opt. A samba server installation can use /opt/samba/bin to store its binaries. 5.4.3. /sbin /sbin contains binaries to configure the operating system. Many of the system binaries require root privilege to perform certain tasks. Below a screenshot containing system binaries to change the ip address, partition a disk and create an ext4 file system. paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4 -rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk -rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig -rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4
5.4.4. /lib Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a screenshot of the partial contents of /lib. /lib/modules Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel-version/. This directory is discussed in detail in the Linux kernel chapter. /lib32 and /lib64 We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may encounter directories named /lib32 and /lib64 which clarify the register size used during compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and libraries for compatibility with legacy applications. This screenshot uses the file utility to demonstrate the difference. The ELF (Executable and Linkable Format) is used in almost every Unix-like operating system since System V. 5.4.5. /opt The purpose of /opt is to store optional software. In many cases this is software from outside the distribution repository. You may find an empty /opt directory on many systems. A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/ $packagename/. If for example the package is called wp, then it installs in /opt/wp, putting binaries in /opt/wp/bin and manpages in /opt/wp/man. paul@laika:~$ ls /lib/libc* /lib/libc-2.5.so /lib/libcfont.so.0.0.0 /lib/libcom_err.so.2.1 /lib/libcap.so.1 /lib/libcidn-2.5.so /lib/libconsole.so.0 /lib/libcap.so.1.10 /lib/libcidn.so.1 /lib/libconsole.so.0.0.0 /lib/libcfont.so.0 /lib/libcom_err.so.2 /lib/libcrypt-2.5.so paul@laika:~$ file /lib32/libc-2.5.so /lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.0, stripped paul@laika:~$ file /lib64/libcap.so.1.10 /lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, version 1 (SYSV), stripped
the Linux file tree 105 5.5. configuration directories 5.5.1. /boot The /boot directory contains all files needed to boot the computer. These files don't change very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which defines the boot menu that is displayed before the kernel starts. 5.5.2. /etc All of the machine-specific configuration files should be located in /etc. Historically /etc stood for etcetera, today people often use the Editable Text Configuration backronym. Many times the name of a configuration files is the same as the application, daemon, or protocol with .conf added as the extension. paul@laika:~$ ls /etc/*.conf /etc/adduser.conf /etc/ld.so.conf /etc/scrollkeeper.conf /etc/brltty.conf /etc/lftp.conf /etc/sysctl.conf /etc/ccertificates.conf /etc/libao.conf /etc/syslog.conf /etc/cvs-cron.conf /etc/logrotate.conf /etc/ucf.conf /etc/ddclient.conf /etc/ltrace.conf /etc/uniconf.conf /etc/debconf.conf /etc/mke2fs.conf /etc/updatedb.conf /etc/deluser.conf /etc/netscsid.conf /etc/usplash.conf /etc/fdmount.conf /etc/nsswitch.conf /etc/uswsusp.conf /etc/hdparm.conf /etc/pam.conf /etc/vnc.conf /etc/host.conf /etc/pnm2ppa.conf /etc/wodim.conf /etc/inetd.conf /etc/povray.conf /etc/wvdial.conf /etc/kernel-img.conf /etc/resolv.conf paul@laika:~$ There is much more to be found in /etc. /etc/init.d/ A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start and stop daemons. This directory could disappear as Linux migrates to systems that replace the old init way of starting all daemons. /etc/X11/ The graphical display (aka X Window System or just X) is driven by software from the X.org foundation. The configuration file for your graphical display is /etc/X11/xorg.conf. /etc/skel/ The skeleton directory /etc/skel is copied to the home directory of a newly created user. It usually contains hidden files like a .bashrc script. /etc/sysconfig/ This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise Linux configuration files. We will discuss some of them in greater detail. The screenshot below is the /etc/sysconfig directory from RHELv4u4 with everything installed.
paul@RHELv4u4:~$ ls /etc/sysconfig/ apmd firstboot irda network saslauthd apm-scripts grub irqbalance networking selinux authconfig hidd keyboard ntpd spamassassin autofs httpd kudzu openib.conf squid bluetooth hwconf lm_sensors pand syslog clock i18n mouse pcmcia sys-config-sec console init mouse.B pgsql sys-config-users crond installinfo named prelink sys-logviewer desktop ipmi netdump rawdevices tux diskdump iptables netdump_id_dsa rhn vncservers dund iptables-cfg paul@RHELv4u4:~$ netdump_id_dsa.p samba xinetd The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file, and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install the latest updates, create a user account, join the Red Hat Network and more. It will then create the /etc/sysconfig/firstboot file again. The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file explains itself. You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from Red Hat for automatic discovery and configuration of hardware. The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more console keyboard information, check the manual pages of keymaps(5), dumpkeys(1), loadkeys(1) and the directory /lib/kbd/keymaps/. We will discuss networking files in this directory in the networking chapter. paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot RUN_FIRSTBOOT=NO root@RHELv4u4:/etc/sysconfig# cat keyboard KEYBOARDTYPE="pc" KEYTABLE="us"
the Linux file tree 107 5.6. data directories 5.6.1. /home Users can store personal or project data under /home. It is common (but not mandatory by the fhs) practice to name the users home directory after the user name in the format /home/ $USERNAME. For example: Besides giving every user (or every project or group) a location to store personal files, the home directory of a user also serves as a location to store the user profile. A typical Unix user profile contains many hidden files (files whose file name starts with a dot). The hidden files of the Unix user profiles contain settings specific for that user. 5.6.2. /root On many systems /root is the default location for personal data and profile of the root user. If it does not exist by default, then some administrators create it. 5.6.3. /srv You may use /srv for data that is served by your system. The FHS allows locating cvs, rsync, ftp and www data in this location. The FHS also approves administrative naming in / srv, like /srv/project55/ftp and /srv/sales/www. On Sun Solaris (or Oracle Solaris) /export is used for this purpose. 5.6.4. /media The /media directory serves as a mount point for removable media devices such as CD- ROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the Unix world, you could very well encounter systems running without this directory. Solaris 9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable media in /media. 5.6.5. /mnt The /mnt directory should be empty and should only be used for temporary mount points (according to the FHS). paul@ubu606:~$ ls /home geert annik sandra paul tom paul@ubu606:~$ ls -d /home/paul/.* /home/paul/. /home/paul/.. /home/paul/.bash_history /home/paul/.bash_profile /home/paul/.ssh /home/paul/.bashrc /home/paul/.lesshst /home/paul/.viminfo paul@debian5:~$ ls /media/ cdrom cdrom0 usbdisk
Unix and Linux administrators used to create many directories here, like /mnt/something/. You likely will encounter many systems with more than one directory created and/or mounted inside /mnt to be used for various local and remote filesystems. 5.6.6. /tmp Applications and users should use /tmp to store temporary data when needed. Data stored in /tmp may use either disk space or RAM. Both of which are managed by the operating system. Never use /tmp to store data that is important or which you wish to archive.
the Linux file tree 109 5.7. in memory directories 5.7.1. /dev Device files in /dev appear to be ordinary files, but are not actually located on the hard disk. The /dev directory is populated with files as the kernel is recognising hardware. common physical devices Common hardware such as hard disk devices are represented by device files in /dev. Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop. (The detailed meaning of these devices will be discussed later.) Besides representing physical hardware, some device files are special. These special devices can be very useful. /dev/tty and /dev/pts For example, /dev/tty1 represents a terminal or console attached to the system. (Don't break your head on the exact terminology of 'terminal' or 'console', what we mean here is a command line interface.) When typing commands in a terminal that is part of a graphical interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can be another number). /dev/null On Linux you will find other special devices such as /dev/null which can be considered a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard unwanted output from commands. /dev/null is not a good location to store your backups ;-). 5.7.2. /proc conversation with the kernel /proc is another special directory, appearing to be ordinary files, but not taking up disk space. It is actually a view of the kernel, or better, what the kernel manages, and is a means to interact with it directly. /proc is a proc filesystem. # # SATA or SCSI or USB # paul@laika:~$ ls /dev/sd* /dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sdb /dev/sdb1 /dev/sdb2 # # IDE or ATAPI # paul@barry:~$ ls /dev/hd* /dev/hda /dev/hda1 /dev/hda2 /dev/hdb /dev/hdb1 /dev/hdb2 /dev/hdc paul@RHELv4u4:~$ mount -t proc
When listing the /proc directory you will see many numbers (on any Unix) and some interesting files (on Linux) mul@laika:~$ ls /proc 1 2339 4724 5418 6587 7201 cmdline mounts 10175 2523 4729 5421 6596 7204 cpuinfo mtrr 10211 2783 4741 5658 6599 7206 crypto net 10239 2975 4873 5661 6638 7214 devices pagetypeinfo 141 29775 4874 5665 6652 7216 diskstats partitions 15045 29792 4878 5927 6719 7218 dma sched_debug 1519 2997 4879 6 6736 7223 driver scsi 1548 3 4881 6032 6737 7224 execdomains self 1551 30228 4882 6033 6755 7227 fb slabinfo 1554 3069 5 6145 6762 7260 filesystems stat 1557 31422 5073 6298 6774 7267 fs swaps 1606 3149 5147 6414 6816 7275 ide sys 180 31507 5203 6418 6991 7282 interrupts sysrq-trigger 181 3189 5206 6419 6993 7298 iomem sysvipc 182 3193 5228 6420 6996 7319 ioports timer_list 18898 3246 5272 6421 7157 7330 irq timer_stats 19799 3248 5291 6422 7163 7345 kallsyms tty 19803 3253 5294 6423 7164 7513 kcore uptime 19804 3372 5356 6424 7171 7525 key-users version 1987 4 5370 6425 7175 7529 kmsg version_signature 1989 42 5379 6426 7188 9964 loadavg vmcore 2 45 5380 6430 7189 acpi locks vmnet 20845 4542 5412 6450 7191 asound meminfo vmstat 221 46 5414 6551 7192 buddyinfo misc zoneinfo 2338 4704 5416 6568 7199 bus modules Let's investigate the file properties inside /proc. Looking at the date and time will display the current date and time showing the files are constantly updated (a view on the kernel). Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see this by executing cat on files like /proc/cpuinfo, which contains information about the CPU. none on /proc type proc (rw) paul@RHELv4u4:~$ date Mon Jan 29 18:06:32 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:06 /proc/cpuinfo paul@RHELv4u4:~$ paul@RHELv4u4:~$ ...time passes... paul@RHELv4u4:~$ paul@RHELv4u4:~$ date Mon Jan 29 18:10:00 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:10 /proc/cpuinfo paul@RHELv4u4:~$ file /proc/cpuinfo /proc/cpuinfo: empty paul@RHELv4u4:~$ cat /proc/cpuinfo processor : 0 vendor_id : AuthenticAMD cpu family : 15 model : 43
the Linux file tree 111 Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000... Most of the files in /proc are read only, some require root privileges, some files are writable, and many files in /proc/sys are writable. Let's discuss some of the files in /proc. model name : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+ stepping : 1 cpu MHz : 2398.628 cache size : 512 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge... bogomips : 4803.54 paul@pasha:~$ cat /proc/cpuinfo cpu : TI UltraSparc III (Cheetah) fpu : UltraSparc III integrated FPU promlib : Version 3 Revision 2 prom : 4.2.2 type : sun4u ncpus probed : 2 ncpus active : 2 Cpu0Bogo : 498.68 Cpu0ClkTck : 000000002cb41780 Cpu1Bogo : 498.68 Cpu1ClkTck : 000000002cb41780 MMU Type : Cheetah State: CPU0: online CPU1: online
/proc/interrupts On the x86 architecture, /proc/interrupts displays the interrupts. On a machine with two CPU's, the file looks like this. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 860013 0 IO-APIC-edge timer 1: 4533 0 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 6588227 0 IO-APIC-edge rtc 10: 2314 0 IO-APIC-fasteoi acpi 12: 133 0 IO-APIC-edge i8042 14: 0 0 IO-APIC-edge libata 15: 72269 0 IO-APIC-edge libata 18: 1 0 IO-APIC-fasteoi yenta 19: 115036 0 IO-APIC-fasteoi eth0 20: 126871 0 IO-APIC-fasteoi libata, ohci1394 21: 30204 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2 22: 1334 0 IO-APIC-fasteoi saa7133[0], saa7133[0] 24: 234739 0 IO-APIC-fasteoi nvidia NMI: 72 42 LOC: 860000 859994 ERR: 0 /proc/kcore The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes. paul@RHELv4u4:~$ cat /proc/interrupts CPU0 177: NMI: LOC: ERR: MIS: IO-APIC-edge timer IO-APIC-edge i8042 IO-APIC-edge rtc IO-APIC-level acpi IO-APIC-edge i8042 IO-APIC-edge ide0 IO-APIC-edge IO-APIC-level ioc0 IO-APIC-level eth0 paul@laika:~$ ls -lh /proc/kcore -r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore paul@laika:~$
the Linux file tree 113 5.7.3. /sys Linux 2.6 hot plugging The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux- hotplug.sourceforge.net/ ). Basically the /sys directory contains kernel information about hardware.
5.8. /usr Unix System Resources Although /usr is pronounced like user, remember that it stands for Unix System Resources. The /usr hierarchy should contain shareable, read only data. Some people choose to mount /usr as read only. This can be done from its own partition or from a read only NFS share (NFS is discussed later). 5.8.1. /usr/bin The /usr/bin directory contains a lot of commands. (On Solaris the /bin directory is a symbolic link to /usr/bin.) 5.8.2. /usr/include The /usr/include directory contains general use include files for C. paul@ubu1010:~$ ls /usr/include/ aalib.h expat_config.h math.h search.h af_vfs.h expat_external.h mcheck.h semaphore.h aio.h expat.h memory.h setjmp.h AL fcntl.h menu.h sgtty.h aliases.h features.h mntent.h shadow.h ... 5.8.3. /usr/lib The /usr/lib directory contains libraries that are not directly executed by users or scripts. 5.8.4. /usr/local The /usr/local directory can be used by an administrator to install software locally. 5.8.5. /usr/share The /usr/share directory contains architecture independent data. As you can see, this is a fairly large directory. paul@deb508:~$ ls /usr/share/ | wc -l paul@deb508:~$ ls /usr/bin | wc -l 1395 paul@deb508:~$ ls /usr/lib | head -7 4Suite ao apt arj aspell avahi bonobo paul@deb508:~$ ls /usr/local/ bin etc games include lib man sbin share src paul@deb508:~$ du -sh /usr/local/ 128K /usr/local/
the Linux file tree 115 This directory typically contains /usr/share/man for manual pages. And it contains /usr/share/games for all static game data (so no high-scores or play logs). 5.8.6. /usr/src The /usr/src directory is the recommended location for kernel source files. 263 paul@deb508:~$ du -sh /usr/share/ 1.3G /usr/share/ paul@deb508:~$ ls /usr/share/man cs de es fi fr hu it.UTF-8 man2 man6 pl.ISO8859-2 sv fr.ISO8859-1 id ja man3 man7 pl.UTF-8 tr fr.UTF-8 it ko man4 man8 pt_BR gl it.ISO8859-1 man1 man5 pl ru zh_CN zh_TW paul@ubu1010:~$ ls /usr/share/games/ openttd wesnoth paul@deb508:~$ ls -l /usr/src/ total 12 drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686 drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26
5.9. /var variable data Files that are unpredictable in size, such as log, cache and spool files, should be located in /var. 5.9.1. /var/log The /var/log directory serves as a central point to contain all log files. 5.9.2. /var/log/messages A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/ log/messages file. By default this file will contain information on what just happened to the system. The file is called /var/log/syslog on Debian and Ubuntu. 5.9.3. /var/cache The /var/cache directory can contain cache data for several applications. paul@ubu1010:~$ ls /var/cache/ apt dictionaries-common gdm man software-center binfmts flashplugin-installer hald pm-utils cups fontconfig jockey pppconfig debconf fonts ldconfig samba 5.9.4. /var/spool The /var/spool directory typically contains spool directories for mail and cron, but also serves as a parent directory for other spool files (for example print spool files). [paul@RHEL4b ~]$ ls /var/log acpid cron.2 amanda cron.3 anaconda.log cron.4 anaconda.syslog cups anaconda.xlog audit boot.log boot.log.1 boot.log.2 boot.log.3 boot.log.4 canna cron cron.1 dmesg exim gdm httpd iiim iptraf lastlog mail maillog maillog.2 maillog.3 maillog.4 mailman messages messages.1 messages.2 messages.3 messages.4 mysqld.log news pgsql ppp quagga radius rpmpkgs rpmpkgs.1 rpmpkgs.2 rpmpkgs.3 rpmpkgs.4 sa samba secure.4 spooler spooler.1 spooler.2 spooler.3 spooler.4 squid uucp vbox scrollkeeper.log vmware-tools-guestd secure secure.1 secure.2 maillog.1 prelink.log secure.3 wtmp wtmp.1 Xorg.0.log Xorg.0.log.old [root@RHEL4b ~]# tail /var/log/messages Jul 30 05:13:56 anacron: anacron startup succeeded Jul 30 05:13:56 atd: atd startup succeeded Jul 30 05:13:57 messagebus: messagebus startup succeeded Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded Jul 30 05:13:58 haldaemon: haldaemon startup succeeded Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for... Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for... Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by... Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul
the Linux file tree 117 5.9.5. /var/lib The /var/lib directory contains application state information. Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/. 11.9.6. /var/... /var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary files that survive a reboot in /var/tmp and information about file locks in /var/lock. There
5.10. solution: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? 3. Create a directory ~/test. Then issue the following commands: dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? /dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send something to /dev/zero, but you can read zeroes from it. 4. Now issue the following command: dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/ random to ~/test/random.txt. Can you describe the functionality of /dev/random ? /dev/random acts as a random number generator on your Linux machine. 5. Issue the following two commands, and look at the first character of each output line. The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512 bytes are common. Character devices act as a stream of characters (or bytes). Mouse and keyboard are typical character devices. 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? ls /bin/cat ; file /bin/cat ls /bin/dd ; file /bin/dd ls /bin/echo ; file /bin/echo ls -lh /boot/vm* cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* /etc/hosts contains hostnames with their ip address /etc/resolv.conf should contain the ip address of a DNS name server.
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7. Are there any files in /etc/skel/ ? Check also for hidden files. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? The size is zero, yet the file contains data. It is not stored anywhere because /proc is a virtual file system that allows you to talk with the kernel. (If you answered "stored in RAM- memory, that is also correct...). 10. Can you enter the /root directory ? Are there (hidden) files ? Try "cd /root". The /root directory is not accessible for normal users on most modern Linux sy 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? 12. Is /var/log a file or a directory ? What about /var/spool ? 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. 14. Read the man page of random and explain the difference between /dev/random and / dev/urandom. Issue "ls -al /etc/skel/". Yes, there should be hidden files there. The file should contain at least one line with Intel or other cpu. Because those files are only meant for system administrators. Both are directories. tty-terminal: echo Hello > /dev/tty1 pts-terminal: echo Hello > /dev/pts/1 man 4 random
125 Chapter 6. commands and arguments This chapter introduces you to shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed. The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell). This chapter frequently uses the echo command to demonstrate shell features. The echo command is very simple: it echoes the input that it receives. paul@laika:~$ echo Burtonville Burtonville paul@laika:~$ echo Smurfs are blue Smurfs are blue
6.1. arguments One of the primary features of a shell is to perform a command line scan. When you enter a command at the shell's command prompt and press the enter key, then the shell will start scanning that line, cutting it up in arguments. While scanning the line, the shell maymake many changes to the arguments you typed. This process is called shell expansion. When the shell has finished scanning and modifying that line, then it will be executed. 6.2. white space removal Parts that are separated by one or more consecutive white spaces (or tabs) are considered separate arguments, any white space is removed. The first argument is the command to be executed, the other arguments are given to the command. The shell effectively cuts your command into one or more arguments. This explains why the following four different command lines are the same after shell expansion. The echo command will display each argument it receives from the shell. The echo command will also add a new white space between the arguments it received. [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ Hello World echo Hello World
commands and arguments 127 6.3. single quotes You can prevent the removal of white spaces by quoting the spaces. The contents of the quoted string are considered as one argument. In the screenshot below the echo receives only one argument. 6.4. double quotes You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument. Later in this book, when discussing variables we will see important differences between single and double quotes. 6.5. echo and quotes Quoted lines can include special escaped characters recognised by the echo command (when using echo -e). The screenshot below shows how to use n for a newline and t for a tab (usually eight white spaces). The echo command can generate more than white spaces, tabs and newlines. Look in the man page for a list of options. [paul@RHEL4b ~]$ echo 'A line with single quotes' A line with [paul@RHEL4b ~]$ single quotes [paul@RHEL4b ~]$ echo "A line with double quotes" A line with [paul@RHEL4b ~]$ double quotes [paul@RHEL4b ~]$ echo -e "A line with na newline" A line with a newline [paul@RHEL4b ~]$ echo -e 'A line with na newline' A line with a newline [paul@RHEL4b ~]$ echo -e "A line with ta tab" A line with a tab [paul@RHEL4b ~]$ echo -e 'A line with ta tab' A line with a tab [paul@RHEL4b ~]$
6.6. commands 6.6.1. external or built-in commands? Not all commands are external to the shell, some are builtin. External commands are programs that have their own binary and reside somewhere in the file system. Many external commands are located in /bin or /sbin. Builtin commands are an integral part of the shell program itself. 6.6.2. type To find out whether a command given to the shell will be executed as an external command or as a builtin command, use the type command. As you can see, the cd command is builtin and the cat command is external. You can also use this command to show you whether the command is aliased or not. 6.6.3. running external commands Some commands have both builtin and external versions. When one of these commands is executed, the builtin version takes priority. To run the external version, you must enter the full path to the command. 6.6.4. which The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands. paul@laika:~$ type cd cd is a shell builtin paul@laika:~$ type cat cat is /bin/cat paul@laika:~$ type ls ls is aliased to `ls --color=auto' paul@laika:~$ type -a echo echo is a shell builtin echo is /bin/echo paul@laika:~$ /bin/echo Running the external echo command... Running the external echo command... [root@RHEL4b ~]# which cp ls cd mkdir pwd /bin/cp /bin/ls /usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:... /bin/mkdir /bin/pwd
commands and arguments 129 6.7. aliases 6.7.1. create an alias The shell allows you to create aliases. Aliases are often used to create an easier to remember name for an existing command or to easily supply parameters. 6.7.2. abbreviate commands An alias can also be useful to abbreviate an existing command. 6.7.3. default options Aliases can be used to supply commands with default options. The example below shows how to set the -i option default when typing rm. Some distributions enable default aliases to protect users from accidentally erasing files ('rm -i', 'mv -i', 'cp -i') 6.7.4. viewing aliases You can provide one or more aliases as arguments to the alias command to get their definitions. Providing no arguments gives a complete list of current aliases. [paul@RHELv4u3 ~]$ cat count.txt one two three [paul@RHELv4u3 ~]$ alias dog=tac [paul@RHELv4u3 ~]$ dog count.txt three two one paul@laika:~$ alias ll='ls -lh --color=auto' paul@laika:~$ alias c='clear' paul@laika:~$ [paul@RHELv4u3 ~]$ rm -i winter.txt rm: remove regular file `winter.txt'? no [paul@RHELv4u3 ~]$ rm winter.txt [paul@RHELv4u3 ~]$ ls winter.txt ls: winter.txt: No such file or directory [paul@RHELv4u3 ~]$ touch winter.txt [paul@RHELv4u3 ~]$ alias rm='rm -i' [paul@RHELv4u3 ~]$ rm winter.txt rm: remove regular empty file `winter.txt'? no [paul@RHELv4u3 ~]$ paul@laika:~$ alias c ll alias c='clear' alias ll='ls -lh --color=auto'
6.7.5. unalias You can undo an alias with the unalias command. 6.8. displaying shell expansion You can display shell expansion with set -x, and stop displaying it with set +x. You might want to use this further on in this course, or when in doubt about exactly what the shell is doing with your command. [paul@RHEL4b ~]$ which rm /bin/rm [paul@RHEL4b ~]$ alias rm='rm -i' [paul@RHEL4b ~]$ which rm alias rm='rm -i' /bin/rm [paul@RHEL4b ~]$ unalias rm [paul@RHEL4b ~]$ which rm /bin/rm [paul@RHEL4b ~]$ [paul@RHELv4u3 ~]$ set -x ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo paul paul ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo '$USER' $USER ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ set +x + set +x [paul@RHELv4u3 ~]$ echo $USER paul
commands and arguments 131 6.9. solution: commands and arguments 1. How many arguments are in this line (not counting the command itself). 2. Is tac a shell builtin command ? 3. Is there an existing alias for rm ? 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? 6. List all current aliases. 7a. Create an alias called 'city' that echoes your hometown. 7b. Use your alias to test that it works. 8. Execute set -x to display shell expansion for every command. 9. Test the functionality of set -x by executing your city and rm aliases. 10 Execute set +x to stop displaying shell expansion. 11. Remove your city alias. touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" answer: three type tac alias rm man rm touch testfile rm -i testfile touch testfile rm testfile (should ask for confirmation) alias alias city='echo Antwerp' city (it should display Antwerp) set -x shell should display the resolved aliases and then execute the command: paul@deb503:~$ set -x paul@deb503:~$ city + echo antwerp antwerp set +x
12. What is the location of the cat and the passwd commands ? 13. Explain the difference between the following commands: The echo command will be interpreted by the shell as the built-in echo command. The /bin/ echo command will make the shell execute the echo binary located in the /bin directory. 14. Explain the difference between the following commands: The -n option of the echo command will prevent echo from echoing a trailing newline. echo Hello will echo six characters in total, echo -n hello only echoes five characters. (The -n option might not work in the Korn shell.) 15. Display A B C with two spaces between B and C. 16. Complete the following command (do not use spaces) to display exactly the following output: The solution is to use tabs with t. 17. Use echo to display the following exactly: Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra). 18. Use one echo command to display three words on three lines. unalias city which cat (probably /bin/cat) which passwd (probably /usr/bin/passwd) echo /bin/echo echo Hello echo -n Hello echo "A B C" 4+4 =8 10+14 =24 echo -e "4+4t=8" ; echo -e "10+14t=24" ?? echo '??' echo -e '??' echo "??" echo -e "??" echo ?? echo -e "one ntwo nthree"
135 Chapter 7. control operators In this chapter we put more than one command on the command line using control operators. We also briefly discuss related parameters ($?) and similar special characters(&).
7.1. ; semicolon You can put two or more commands on the same line separated by a semicolon ; . The shell will scan the line until it reaches the semicolon. All the arguments before this semicolon will be considered a separate command from all the arguments after the semicolon. Both series will be executed sequentially with the shell waiting for each command to finish before starting the next one. 7.2. & ampersand When a line ends with an ampersand &, the shell will not wait for the command to finish. You will get your shell prompt back, and the command is executed in background. You will get a message when this command has finished executing in background. The technical explanation of what happens in this case is explained in the chapter about processes. 7.3. $? dollar question mark The exit code of the previous command is stored in the shell variable $?. Actually $? is a shell parameter and not a variable, since you cannot assign a value to $?. [paul@RHELv4u3 ~]$ echo Hello Hello [paul@RHELv4u3 ~]$ echo World World [paul@RHELv4u3 ~]$ echo Hello ; echo World Hello World [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ sleep 20 & [1] 7925 [paul@RHELv4u3 ~]$ ...wait 20 seconds... [paul@RHELv4u3 ~]$ [1]+ Done sleep 20 paul@debian5:~/test$ touch file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 rm: cannot remove `file1': No such file or directory paul@debian5:~/test$ echo $? 1 paul@debian5:~/test$
control operators 137 7.4. && double ampersand The shell will interpret && as a logical AND. When using && the second command is executed only if the first one succeeds (returns a zero exit status). Another example of the same logical AND principle. This example starts with a working cd followed by ls, then a non-working cd which is not followed by ls. 7.5. || double vertical bar The || represents a logical OR. The second command is executed only when the first command fails (returns a non-zero exit status). Another example of the same logical OR principle. 7.6. combining && and || You can use this logical AND and logical OR to write an if-then-else structure on the command line. This example uses echo to display whether the rm command was successful. paul@barry:~$ echo first && echo second first second paul@barry:~$ zecho first && echo second -bash: zecho: command not found [paul@RHELv4u3 ~]$ cd gen && ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ cd gen && ls -bash: cd: gen: No such file or directory paul@barry:~$ echo first || echo second ; echo third first third paul@barry:~$ zecho first || echo second ; echo third -bash: zecho: command not found second third paul@barry:~$ [paul@RHELv4u3 ~]$ cd gen || ls [paul@RHELv4u3 gen]$ cd gen || ls -bash: cd: gen: No such file or directory file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! It worked! paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! rm: cannot remove `file1': No such file or directory It failed! paul@laika:~/test$
7.7. # pound sign Everything written after a pound sign (#) is ignored by the shell. This is useful to write a shell comment, but has no influence on the command execution or shell expansion. 7.8. escaping special characters The backslash character enables the use of control characters, but without the shell interpreting it, this is called escaping characters. 7.8.1. end of line backslash Lines ending in a backslash are continued on the next line. The shell does not interpret the newline character and will wait on shell expansion and execution of the command line until a newline without backslash is encountered. paul@debian4:~$ mkdir test paul@debian4:~$ cd test paul@debian4:~/test$ ls paul@debian4:~/test$ # we create a directory #### we enter the directory # is it empty ? [paul@RHELv4u3 ~]$ echo hello ; world hello ; world [paul@RHELv4u3 ~]$ echo hello world hello world [paul@RHELv4u3 ~]$ echo escaping # & " ' escaping # & " ' [paul@RHELv4u3 ~]$ echo escaping ?*"' escaping ?*"' [paul@RHEL4b ~]$ echo This command line > is split in three > parts This command line is split in three parts [paul@RHEL4b ~]$
control operators 139 7.9. practice: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes)
7.10. solution: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) which passwd file /usr/bin/passwd pwd ; pwd cd /etc && ls cd etc || cd /etc paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed it worked paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed touch: cannot touch `test42': Permission denied it failed pausing for six seconds sleep 200 & rm file55 && echo success || echo failed echo "Hello World with strange' characters * [ } ~ . " or echo ""Hello World with strange' characters * [ } ~ . ""
141 Chapter 8. shell variables In this chapter we learn to manage environment variables in the shell. These variables are often needed by applications.
8.1. $ dollar sign Another important character interpreted by the shell is the dollar sign $. The shell willlook for an environment variable named like the string following the dollar sign and replaceit with the value of the variable (or with nothing if the variable does not exist). These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME. 8.2. case sensitive This example shows that shell variables are case sensitive! 8.3. creating variables This example creates the variable $MyVar and sets its value. It then uses echo to verify the value. [paul@RHELv4u3 ~]$ echo This is the $SHELL shell This is the /bin/bash shell [paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME This is /bin/bash on computer RHELv4u3.localdomain [paul@RHELv4u3 ~]$ echo The userid of $USER is $UID The userid of paul is 500 [paul@RHELv4u3 ~]$ echo My homedir is $HOME My homedir is /home/paul [paul@RHELv4u3 ~]$ echo Hello $USER Hello paul [paul@RHELv4u3 ~]$ echo Hello $user Hello [paul@RHELv4u3 gen]$ MyVar=555 [paul@RHELv4u3 gen]$ echo $MyVar 555 [paul@RHELv4u3 gen]$
shell variables 143 8.4. quotes Notice that double quotes still allow the parsing of variables, whereas single quotes prevent this. The bash shell will replace variables with their value in double quoted lines, but not in single quoted lines. 8.5. set You can use the set command to display a list of environment variables. On Ubuntu and Debian systems, the set command will also list shell functions after the shell variables.Use set | more to see the variables then. 8.6. unset Use the unset command to remove a variable from your shell environment. [paul@RHELv4u3 ~]$ MyVar=555 [paul@RHELv4u3 ~]$ echo $MyVar 555 [paul@RHELv4u3 ~]$ echo "$MyVar" 555 [paul@RHELv4u3 ~]$ echo '$MyVar' $MyVar paul@laika:~$ city=Burtonville paul@laika:~$ echo "We are in $city today." We are in Burtonville today. paul@laika:~$ echo 'We are in $city today.' We are in $city today. [paul@RHEL4b ~]$ MyVar=8472 [paul@RHEL4b ~]$ echo $MyVar 8472 [paul@RHEL4b ~]$ unset MyVar [paul@RHEL4b ~]$ echo $MyVar [paul@RHEL4b ~]$
8.7. $PS1 The $PS1 variable determines your shell prompt. You can use backslash escaped special characters like u for the username or w for the working directory. The bash manual has a complete reference. In this example we change the value of $PS1 a couple of times. To avoid unrecoverable mistakes, you can set normal user prompts to green and the root prompt to red. Add the following to your .bashrc for a green user prompt: paul@deb503:~$ PS1=prompt prompt promptPS1='prompt ' prompt prompt PS1='> ' > > PS1='u@h$ ' paul@deb503$ paul@deb503$ PS1='u@h:W$' paul@deb503:~$ # color prompt by paul RED='[033[01;31m]' WHITE='[033[01;00m]' GREEN='[033[01;32m]' BLUE='[033[01;34m]' export PS1="${debian_chroot:+($debian_chroot)}$GREENu$WHITE@$BLUEh$WHITEw$ "
shell variables 145 8.8. $PATH The $PATH variable is determines where the shell is looking for commands to execute (unless the command is builtin or aliased). This variable contains a list of directories, separated by colons. The shell will not look in the current directory for commands to execute! (Looking for executables in the current directory provided an easy way to hack PC-DOS computers). If you want the shell to look in the current directory, then add a . at the end of your $PATH. Your path might be different when using su instead of su - because the latter will take on the environment of the target user. The root user typically has /sbin directories added to the $PATH variable. [[paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin: [paul@RHEL4b ~]$ PATH=$PATH:. [paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:. [paul@RHEL4b ~]$ [paul@RHEL3 ~]$ su Password: [root@RHEL3 paul]# echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin [root@RHEL3 paul]# exit [paul@RHEL3 ~]$ su - Password: [root@RHEL3 ~]# echo $PATH /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: [root@RHEL3 ~]#
8.9. env The env command without options will display a list of exported variables. The difference with set with options is that set lists all variables, including those not exported to child shells. But env can also be used to start a clean shell (a shell without any inherited environment). The env -i command clears the environment for the subshell. Notice in this screenshot that bash will set the $SHELL variable on startup. You can use the env command to set the $LANG, or any other, variable for just one instance of bash with one command. The example below uses this to show the influence of the $LANG variable on file globbing (see the chapter on file globbing). 8.10. export You can export shell variables to other shells with the export command. This will export the variable to child shells. But it will not export to the parent shell (previous screenshot continued). [paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER' /bin/bash /home/paul paul [paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER' /bin/bash [paul@RHEL4b ~]$ [paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]' Filea Fileb [paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]' Filea FileA Fileb FileB [paul@RHEL4b test]$ [paul@RHEL4b ~]$ var3=three [paul@RHEL4b ~]$ var4=four [paul@RHEL4b ~]$ export var4 [paul@RHEL4b ~]$ echo $var3 $var4 three four [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $var3 $var4 four [paul@RHEL4b ~]$ export var5=five [paul@RHEL4b ~]$ echo $var3 $var4 $var5 four five [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ echo $var3 $var4 $var5 three four [paul@RHEL4b ~]$
shell variables 147 8.11. delineate variables Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing until the first occurrence of a non-alphanumeric character that is not an underscore. In some situations, this can be a problem. This issue can be resolved with curly braces like in this example. 8.12. unbound variables The example below tries to display the value of the $MyVar variable, but it fails because the variable does not exist. By default the shell will display nothing when a variable is unbound (does not exist). There is, however, the nounset shell option that you can use to generate an error when a variable does not exist. In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set +o nounset. [paul@RHEL4b ~]$ prefix=Super [paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl Hello and [paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl Hello Superman and Supergirl [paul@RHEL4b ~]$ [paul@RHELv4u3 gen]$ echo $MyVar [paul@RHELv4u3 gen]$ paul@laika:~$ set -u paul@laika:~$ echo $Myvar bash: Myvar: unbound variable paul@laika:~$ set +u paul@laika:~$ echo $Myvar paul@laika:~$
8.13. practice: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Create two variables, and export one of them. 8. Display the exported variable in an interactive child shell. 9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt.
shell variables 149 8.14. solution: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Create two variables, and export one of them. 8. Display the exported variable in an interactive child shell. 9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. echo Hello $USER answer=42 MyLANG=$LANG set set|more on Ubuntu/Debian env export declare -x env | more set | more unset answer var1=1; export var2=2 bash echo $var2 varx=Dumb; vary=do echo ${varx}le${vary}re solution by Yves from Dexia : echo $varx'le'$vary're' solution by Erwin from Telenet : echo "$varx"le"$vary"re PS1='t u@h W$ '
Chapter 9. shell embedding and options This chapter takes a brief look at child shells, embedded shells and shell options.
shell embedding and options 151 9.1. shell embedding Shells can be embedded on the command line, or in other words, the command line scan can spawn new processes containing a fork of the current shell. You can use variables to prove that new shells are created. In the screenshot below, the variable $var1 only exists in the (temporary) sub shell. You can embed a shell in an embedded shell, this is called nested embedding of shells. This screenshot shows an embedded shell inside an embedded shell. 9.1.1. backticks Single embedding can be useful to avoid changing your current directory. The screenshot below uses backticks instead of dollar-bracket to embed. You can only use the $() notation to nest embedded shells, backticks cannot do this. 9.1.2. backticks or single quotes Placing the embedding between backticks uses one character less than the dollar and parenthesis combo. Be careful however, backticks are often confused with single quotes. The technical difference between ' and ` is significant! paul@deb503:~$ A=shell paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(C=sub;echo $C$B$A)) shell subshell subsubshell [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1) 5 [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ [paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass` passwd passwd- passwd.OLD [paul@RHELv4u3 ~]$ [paul@RHELv4u3 gen]$ echo `var1=5;echo $var1` 5 [paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1' var1=5;echo $var1 [paul@RHELv4u3 gen]$
9.2. shell options Both set and unset are builtin shell commands. They can be used to set options of the bash shell itself. The next example will clarify this. By default, the shell will treat unset variables as a variable having no value. By setting the -u option, the shell will treat any reference to unset variables as an error. See the man page of bash for more information. To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be explained later in this book (in the I/O redirection chapter). When typing set without options, you get a list of all variables without function when the shell is on posix mode. You can set bash in posix mode typing set -o posix. 9.3. solution: shell embedding 1. Find the list of shell options in the man page of bash. What is the difference betweenset -u and set -o nounset? read the manual of bash (man bash), search for nounset -- both mean the same thing. 2. Activate nounset in your shell. Test that it shows an error message when using non- existing variables. Both these lines have the same effect. 3. Deactivate nounset. 4. Execute cd /var and ls in an embedded shell. echo $(cd /var ; ls) [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ set -u [paul@RHEL4b ~]$ echo $var123 -bash: var123: unbound variable [paul@RHEL4b ~]$ set +u [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ set -C ; set -u [paul@RHEL4b ~]$ echo $- himuBCH [paul@RHEL4b ~]$ set +C ; set +u [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ set -u OR set -o nounset set +u OR set +o nounset
shell embedding and options 153 The echo command is only needed to show the result of the ls command. Omitting will result in the shell trying to execute the first file as a command. 5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? echo $(embvar=emb;echo $embvar) ; echo $embvar #the last echo fails $embvar does not exist in your current shell 6. Explain what "set -x" does. Can this be useful ? It displays shell expansion for troubleshooting your command. (optional)7. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last echo -n First; echo -n Middle; echo Last 8. Display a long listing (ls -l) of the passwd command using the which command inside an embedded shell. ls -l $(which passwd)
linux.pdf
155 Chapter 10. shell history The shell makes it easy for us to repeat commands, this chapter explains how.
10.1. repeating the last command To repeat the last command in bash, type !!. This is pronounced as bang bang. 10.2. repeating other commands You can repeat other commands using one bang followed by one or more characters. The shell will repeat the last command that started with those characters. 10.3. history To see older commands, use history to display the shell command history (or use history n to see the last n commands). 10.4. !n When typing ! followed by the number preceding the command you want repeated, then the shell will echo the command and execute it. paul@debian5:~/test42$ echo this will be repeated > file42.txt paul@debian5:~/test42$ !! echo this will be repeated > file42.txt paul@debian5:~/test42$ paul@debian5:~/test42$ touch file42 paul@debian5:~/test42$ cat file42 paul@debian5:~/test42$ !to touch file42 paul@debian5:~/test42$ paul@debian5:~/test$ history 10 38 mkdir test 39 cd test 40 touch file1 41 echo hello > file2 42 echo It is very cold today > winter.txt 43 ls 44 ls -l 45 cp winter.txt summer.txt 46 ls -l 47 history 10 paul@debian5:~/test$ !43 ls file1 file2 summer.txt winter.txt
shell history 157 10.5. Ctrl-r Another option is to use ctrl-r to search in the history. In the screenshot below i only typed ctrl-r followed by four characters apti and it finds the last command containing these four consecutive characters. 10.6. $HISTSIZE The $HISTSIZE variable determines the number of commands that will be remembered in your current environment. Most distributions default this variable to 500 or 1000. You can change it to any value you like. 10.7. $HISTFILE The $HISTFILE variable points to the file that contains your history. The bash shell defaults this value to ~/.bash_history. A session history is saved to this file when you exit the session! Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your terminal's history. 10.8. $HISTFILESIZE The number of commands kept in your history file can be set using $HISTFILESIZE. paul@debian5:~$ (reverse-i-search)`apti': sudo aptitude install screen paul@debian5:~$ echo $HISTSIZE 500 paul@debian5:~$ HISTSIZE=15000 paul@debian5:~$ echo $HISTSIZE 15000 paul@debian5:~$ echo $HISTFILE /home/paul/.bash_history paul@debian5:~$ echo $HISTFILESIZE 15000
10.9. prevent recording a command You can prevent a command from being recorded in history using a space prefix. paul@debian8:~/github$ echo abc abc paul@debian8:~/github$ echo def def paul@debian8:~/github$ echo ghi ghi paul@debian8:~/github$ history 3 9501 echo abc 9502 echo ghi 9503 history 3 10.10. (optional)regular expressions It is possible to use regular expressions when using the bang to repeat commands. The screenshot below switches 1 into 2. 10.11. (optional) Korn shell history Repeating a command in the Korn shell is very similar. The Korn shell also has the history command, but uses the letter r to recall lines from history. This screenshot shows the history command. Note the different meaning of the parameter. Repeating with r can be combined with the line numbers given by the history command, or with the first few letters of the command. paul@debian5:~/test$ cat file1 paul@debian5:~/test$ !c:s/1/2 cat file2 hello paul@debian5:~/test$ $ history 17 17 clear 18 echo hoi 19 history 12 20 echo world 21 history 17 $ r e echo world world $ cd /etc $ r cd /etc $
shell history 159 10.12. practice: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. 2. Repeat the previous command using only two characters (there are two solutions!) 3. Display the last 5 commands you typed. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? 6. Where are these commands stored when exiting the shell ? 7. How many commands can be written to the history file when exiting your current shell session ? 8. Make sure your current bash shell remembers the next 5000 commands you type. 9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ?
10.13. solution: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. echo The answer to the meaning of life, the universe and everything is 42 2. Repeat the previous command using only two characters (there are two solutions!) 3. Display the last 5 commands you typed. You will receive different line numbers. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? echo $HISTSIZE 6. Where are these commands stored when exiting the shell ? echo $HISTFILE 7. How many commands can be written to the history file when exiting your current shell session ? echo $HISTFILESIZE 8. Make sure your current bash shell remembers the next 5000 commands you type. HISTSIZE=5000 9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ? when you type exit !! OR !e paul@ubu1010:~$ history 5 52 ls -l 53 ls 54 df -h | grep sda 55 echo The answer to the meaning of life, the universe and everything is 42 56 history 5 paul@ubu1010:~$ !55 echo The answer to the meaning of life, the universe and everything is 42 The answer to the meaning of life, the universe and everything is 42
161 Chapter 11. file globbing The shell is also responsible for file globbing (or dynamic filename generation). This chapter will explain file globbing.
11.1. * asterisk The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk to any combination of characters (even none). When no path is given, the shell will use filenames in the current directory. See the man page of glob(7) for more information.(This is part of LPI topic 1.103.3.) 11.2. ? question mark Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate filenames, matching the question mark with exactly one character. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File* File4 File55 FileA Fileab FileAB [paul@RHELv4u3 gen]$ ls file* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls *ile55 File55 [paul@RHELv4u3 gen]$ ls F*ile55 File55 [paul@RHELv4u3 gen]$ ls F*55 File55 [paul@RHELv4u3 gen]$ [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File? File4 FileA [paul@RHELv4u3 gen]$ ls Fil?4 File4 [paul@RHELv4u3 gen]$ ls Fil?? File4 FileA [paul@RHELv4u3 gen]$ ls File?? File55 Fileab FileAB [paul@RHELv4u3 gen]$
file globbing 163 11.3. [] square brackets The square bracket [ is interpreted by the shell as a sign to generate filenames, matching any of the characters between [ and the first subsequent ]. The order in this list between the brackets is not important. Each pair of brackets is replaced by exactly one character. You can also exclude characters from a list between square brackets with the exclamation mark !. And you are allowed to make combinations of these wild cards. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File[5A] FileA [paul@RHELv4u3 gen]$ ls File[A5] FileA [paul@RHELv4u3 gen]$ ls File[A5][5b] File55 [paul@RHELv4u3 gen]$ ls File[a5][5b] File55 Fileab [paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm] ls: File[a5][5b][abcdefghijklm]: No such file or directory [paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm] fileabc [paul@RHELv4u3 gen]$ [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls file[a5][!Z] fileab [paul@RHELv4u3 gen]$ ls file[!5]* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls file[!5]? fileab [paul@RHELv4u3 gen]$
11.4. a-z and 0-9 ranges The bash shell will also understand ranges of characters between brackets. 11.5. $LANG and square brackets But, don't forget the influence of the LANG variable. Some languages include lower case letters in an upper case range (and vice versa). If $LC_ALL is set, then this will also need to be reset to prevent file globbing. [paul@RHELv4u3 gen]$ ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ ls file[a-z]* fileab fileab2 fileabc [paul@RHELv4u3 gen]$ ls file[0-9] file1 file2 file3 [paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]* fileab2 [paul@RHELv4u3 gen]$ paul@RHELv4u4:~/test$ ls [A-Z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ echo $LANG en_US.UTF-8 paul@RHELv4u4:~/test$ LANG=C paul@RHELv4u4:~/test$ echo $LANG C paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 paul@RHELv4u4:~/test$ ls [A-Z]ile? File4 paul@RHELv4u4:~/test$
file globbing 165 11.6. preventing file globbing The screenshot below should be no surprise. The echo * will echo a * when in an empty directory. And it will echo the names of all files when the directory is not empty. Globbing can be prevented using quotes or by escaping the special characters, as shown in this screenshot. paul@ubu1010:~$ mkdir test42 paul@ubu1010:~$ cd test42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ touch file42 file33 paul@ubu1010:~/test42$ echo * file33 file42 paul@ubu1010:~/test42$ echo * file33 file42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ echo '*' * paul@ubu1010:~/test42$ echo "*" *
11.7. solution: shell globbing 1. Create a test directory and enter it. mkdir testdir; cd testdir 2. Create the following files : (the last one has 6 characters including a space) 3. List (with ls) all files starting with file ls file* 4. List (with ls) all files starting with File ls File* 5. List (with ls) all files starting with file and ending in a number. ls file*[0-9] 6. List (with ls) all files starting with file and ending with a letter ls file*[a-z] 7. List (with ls) all files starting with File and having a digit as fifth character. ls File[0-9]* 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. ls File[0-9] 9. List (with ls) all files starting with a letter and ending in a number. ls [a-z]*[0-9] 10. List (with ls) all files that have exactly five characters. file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 touch file1 file10 file11 file2 File2 File3 touch file33 fileAB filea fileA fileAAA touch "file(" touch "file 2"
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11. List (with ls) all files that start with f or F and end with 3 or A. 12. List (with ls) all files that start with f have i or R as second character and end in a number. 13. List all files that do not start with the letter F. 14. Copy the value of $LANG to $MyLANG. 15. Show the influence of $LANG in listing A-Z or a-z ranges. 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? 17. Is there another command besides cd to change directories ? ls ????? ls [fF]*[3A] ls f[iR]*[0-9] ls [!F]* MyLANG=$LANG see example in book echo * pushd popd
171 Chapter 12. I/O redirection One of the powers of the Unix command line is the use of input/output redirection and pipes. This chapter explains redirection of input, output and error streams.
12.1. stdin, stdout, and stderr The bash shell has three basic streams; it takes input from stdin (stream 0), it sends output to stdout (stream 1) and it sends error messages to stderr (stream 2) . The drawing below has a graphical interpretation of these three streams. The keyboard often serves as stdin, whereas stdout and stderr both go to the display. This can be confusing to new Linux users because there is no obvious way to recognize stdout from stderr. Experienced users know that separating output from errors can be very useful. The next sections will explain how to redirect these streams. stdout (1) stdin (0) bash stderr (2)
I/O redirection 173 12.2. output redirection 12.2.1. > stdout stdout can be redirected with a greater than sign. While scanning the line, the shell will see the > sign and will clear the file. The > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1). Note that the bash shell effectively removes the redirection from the command line before argument 0 is executed. This means that in the case of this command: echo hello > greetings.txt the shell only counts two arguments (echo = argument 0, hello = argument 1). The redirection is removed before the argument counting takes place. 12.2.2. output file is erased While scanning the line, the shell will see the > sign and will clear the file! Since this happens before resolving argument 0, this means that even when the command fails, the file will have been cleared! [paul@RHELv4u3 ~]$ echo It is cold today! It is cold today! [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt -bash: zcho: command not found [paul@RHELv4u3 ~]$ cat winter.txt [paul@RHELv4u3 ~]$
12.2.3. noclobber Erasing a file while using > can be prevented by setting the noclobber option. 12.2.4. overruling noclobber The noclobber can be overruled with >|. 12.2.5. >> append Use >> to append output to a file. [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ set +o noclobber [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is very cold today! [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! Where is the summer ? [paul@RHELv4u3 ~]$
I/O redirection 175 12.3. error redirection 12.3.1. 2> stderr Redirecting stderr is done with 2>. This can be very useful to prevent error messages from cluttering your screen. The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing 1> is the same as >. 18.3.2. 2>&1 To redirect both stdout and stderr to the same file, use 2>&1. Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error made a copy of the standard output before the standard output was redirected to dirlist. [paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1 [paul@RHELv4u3 ~]$
12.4. output redirection and pipes By default you cannot grep inside stderr when using pipes on the command line, because only stdout is passed. With 2>&1 you can force stderr to go to stdout. This enables the next command in the pipe to act on both streams. You cannot use both 1>&2 and 2>&1 to switch stdout and stderr. You need a third stream to switch stdout and stderr after a pipe symbol. 12.5. joining stdout and stderr The &> construction will put both stdout and stderr in one stream (to a file). paul@debian7:~$ rm file42 file33 file1201 | grep file42 rm: cannot remove ‘file42’: No such file or directory rm: cannot remove ‘file33’: No such file or directory rm: cannot remove ‘file1201’: No such file or directory paul@debian7:~$ rm file42 file33 file1201 2>&1 | grep file42 rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ rm file42 file33 file1201 2>&1 1>&2 | grep file42 rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ echo file42 2>&1 1>&2 | sed 's/file42/FILE42/' FILE42 paul@debian7:~$ echo file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' file42 paul@debian7:~$ rm file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' rm: cannot remove ‘FILE42’: No such file or directory paul@debian7:~$ rm file42 &> out_and_err paul@debian7:~$ cat out_and_err rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ echo file42 &> out_and_err paul@debian7:~$ cat out_and_err file42 paul@debian7:~$
I/O redirection 177 12.6. input redirection 12.6.1. < stdin Redirecting stdin is done with < (short for 0<). 12.6.2. << here document The here document (sometimes called here-is-document) is a way to append input until a certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or can be called with Ctrl-D. 12.6.3. <<< here string The here string can be used to directly pass strings to a command. The result is the same as using echo string | command (but you have one less process running). See rfc 3548 for more information about base64. [paul@RHEL4b ~]$ cat < text.txt one two [paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt ONE ZZO [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ cat <<EOF > text.txt > one > two > EOF [paul@RHEL4b ~]$ cat text.txt one two [paul@RHEL4b ~]$ cat <<brol > text.txt > brel > brol [paul@RHEL4b ~]$ cat text.txt brel [paul@RHEL4b ~]$ paul@ubu1110~$ base64 <<< linux-training.be bGludXgtdHJhaW5pbmcuYmUK paul@ubu1110~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK linux-training.be
12.7. confusing redirection The shell will scan the whole line before applying redirection. The following command line is very readable and is correct. cat winter.txt > snow.txt 2> errors.txt But this one is also correct, but less readable. 2> errors.txt cat winter.txt > snow.txt Even this will be understood perfectly by the shell. < winter.txt > snow.txt 2> errors.txt cat 12.8. quick file clear So what is the quickest way to clear a file ? >foo And what is the quickest way to clear a file when the noclobber option is set ?
I/O redirection 179 12.9. solution: input/output redirection 1. Activate the noclobber shell option. 2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file. 4. When listing all shell options, which character represents the noclobber option ? echo $- (noclobber is visible as C) 5. Deactivate the noclobber option. set +o noclobber 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. set -o noclobber set -C ls /etc > etc.txt ls /etc > etc.txt (should not work) paul@deb503:~$ > tailing.txt paul@deb503:~$ tail -f tailing.txt hello world in the other shell: paul@deb503:~$ echo hello >> tailing.txt paul@deb503:~$ echo world >> tailing.txt paul@deb503:~$ cat > tennis.txt << ace > Justine Henin > Venus Williams > Serena Williams > Martina Hingis > Kim Clijsters > ace paul@deb503:~$ cat tennis.txt Justine Henin Venus Williams Serena Williams Martina Hingis Kim Clijsters paul@deb503:~$
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181 Chapter 13. filters Commands that are created to be used with a pipe are often called filters. These filters are very small programs that do one specific thing very efficiently. They can be used as building blocks. This chapter will introduce you to the most common filters. The combination of simple commands and filters in a long pipe allows you to design elegant solutions.
13.1. cat When between two pipes, the cat command does nothing (except putting stdin on stdout). 13.2. tee Writing long pipes in Unix is fun, but sometimes you may want intermediate results. This is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is almost the same as cat, except that it has two identical outputs. 13.3. grep The grep filter is famous among Unix users. The most common use of grep is to filter lines of text containing (or not containing) a certain string. You can write this without the cat. One of the most useful options of grep is grep -i which filters in a case insensitive way. [paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat five four three two one [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac one two three four five [paul@RHEL4b pipes]$ cat temp.txt five four three two one [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | grep Williams Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ grep Williams tennis.txt Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ grep Bel tennis.txt Justine Henin, Bel [paul@RHEL4b pipes]$ grep -i Bel tennis.txt
filters 183 Another very useful option is grep -v which outputs lines not matching the string. And of course, both options can be combined to filter all lines not containing a case insensitive string. With grep -A1 one line after the result is also displayed. With grep -B1 one line before the result is also displayed. With grep -C1 (context) one line before and one after are also displayed. All three options (A,B, and C) can display any number of lines (using e.g. A2, B4 or C20). Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ grep -v Fra tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ grep -vi usa tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ paul@debian5:~/pipes$ grep -A1 Henin tennis.txt Justine Henin, Bel Serena Williams, usa paul@debian5:~/pipes$ grep -B1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel paul@debian5:~/pipes$ grep -C1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa
13.4. cut The cut filter can select columns from files, depending on a delimiter or a count of bytes. The screenshot below uses cut to filter for the username and userid in the /etc/passwd file. It uses the colon as a delimiter, and selects fields 1 and 3. When using a space as the delimiter for cut, you have to quote the space. This example uses cut to display the second to the seventh character of /etc/passwd. 13.5. tr You can translate characters with tr. The screenshot shows the translation of all occurrences of e to E. Here we set all letters to uppercase by defining two ranges. Here we translate all newlines to spaces. [[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4 Figo:510 Pfaff:511 Harry:516 Hermione:517 [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt Amelie Kim Justine Serena Venus [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4 igo:x: faff:x arry:x ermion [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E' AmEliE MaurEsmo, Fra Kim ClijstErs, BEL JustinE HEnin, BEl SErEna Williams, usa VEnus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z' AMELIE MAURESMO, FRA KIM CLIJSTERS, BEL JUSTINE HENIN, BEL SERENA WILLIAMS, USA VENUS WILLIAMS, USA [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat count.txt one two
filters 185 The tr -s filter can also be used to squeeze multiple occurrences of a character to one. You can also use tr to 'encrypt' texts with rot13. This last example uses tr -d to delete characters. 13.6. wc Counting words, lines and characters is easy with wc. three four five [paul@RHEL4b pipes]$ cat count.txt | tr 'n' ' ' one two three four five [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat spaces.txt one two three four five six [paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' ' one two three four five six [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ paul@debian5:~/pipes$ cat tennis.txt | tr -d e Amli Maursmo, Fra Kim Clijstrs, BEL Justin Hnin, Bl Srna Williams, usa Vnus Williams, USA [paul@RHEL4b pipes]$ wc tennis.txt 5 15 100 tennis.txt [paul@RHEL4b pipes]$ wc -l tennis.txt 5 tennis.txt [paul@RHEL4b pipes]$ wc -w tennis.txt 15 tennis.txt [paul@RHEL4b pipes]$ wc -c tennis.txt 100 tennis.txt [paul@RHEL4b pipes]$
13.7. sort The sort filter will default to an alphabetical sort. But the sort filter has many options to tweak its usage. This example shows sorting different columns (column 1 or column 2). The screenshot below shows the difference between an alphabetical sort and a numerical sort (both on the third column). paul@debian5:~/pipes$ cat music.txt Queen Brel Led Zeppelin Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Led Zeppelin Queen [paul@RHEL4b pipes]$ sort -k1 country.txt Belgium, Brussels, 10 France, Paris, 60 Germany, Berlin, 100 Iran, Teheran, 70 Italy, Rome, 50 [paul@RHEL4b pipes]$ sort -k2 country.txt Germany, Berlin, 100 Belgium, Brussels, 10 France, Paris, 60 Italy, Rome, 50 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -k3 country.txt Belgium, Brussels, 10 Germany, Berlin, 100 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -n -k3 country.txt Belgium, Brussels, 10 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 Germany, Berlin, 100
filters 187 13.8. uniq With uniq you can remove duplicates from a sorted list. uniq can also count occurrences with the -c option. paul@debian5:~/pipes$ cat music.txt Queen Brel Queen Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Queen Queen paul@debian5:~/pipes$ sort music.txt |uniq Abba Brel Queen paul@debian5:~/pipes$ sort music.txt |uniq -c 1 Abba 1 Brel 2 Queen
13.9. comm Comparing streams (or files) can be done with the comm. By default comm will output three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet are only in the first file, Turner is only in the second. The output of comm can be easier to read when outputting only a single column. The digits point out which output columns should not be displayed. paul@debian5:~/pipes$ cat > list1.txt Abba Bowie Cure Queen Sweet paul@debian5:~/pipes$ cat > list2.txt Abba Cure Queen Turner paul@debian5:~/pipes$ comm list1.txt list2.txt Abba Bowie Cure Queen Sweet Turner paul@debian5:~/pipes$ comm -12 list1.txt list2.txt Abba Cure Queen paul@debian5:~/pipes$ comm -13 list1.txt list2.txt Turner paul@debian5:~/pipes$ comm -23 list1.txt list2.txt Bowie Sweet
filters 189 13.10. od European humans like to work with ascii characters, but computers store files in bytes.The example below creates a simple file, and then uses od to show the contents of the file in hexadecimal bytes The same file can also be displayed in octal bytes. And here is the file in ascii (or backslashed) characters. paul@laika:~/test$ cat > text.txt abcdefg 1234567 paul@laika:~/test$ od -t x1 text.txt 0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a 0000020 paul@laika:~/test$ od -b text.txt 0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012 0000020 paul@laika:~/test$ od -c text.txt 0000000 0000020 b c d e f g n 1 2 3 4 5 6 7 n
13.11. sed The stream editor sed can perform editing functions in the stream, using regular expressions. Add g for global replacements (all occurrences of the string per line). With d you can remove lines from a stream containing a character. paul@debian5:~/pipes$ echo level5 | sed 's/5/42/' level42 paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/' jump5 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/' jump5 level7 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g' jump5 jump7 paul@debian5:~/test42$ cat tennis.txt Venus Williams, USA Martina Hingis, SUI Justine Henin, BE Serena williams, USA Kim Clijsters, BE Yanina Wickmayer, BE paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d' Venus Williams, USA Martina Hingis, SUI Serena williams, USA
filters 191 13.12. pipe examples 13.12.1. who | wc How many users are logged on to this system ? 13.12.2. who | cut | sort Display a sorted list of logged on users. Display a sorted list of logged on users, but every user only once . 13.12.3. grep | cut Display a list of all bash user accounts on this computer. Users accounts are explained in detail later. [paul@RHEL4b pipes]$ who root tty1 Jul 25 10:50 paul pts/0 Jul 25 09:29 (laika) Harry pts/1 Jul 25 12:26 (barry) paul pts/2 Jul 25 12:26 (pasha) [paul@RHEL4b pipes]$ who | wc -l 4 [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort Harry paul paul root [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq Harry paul root paul@debian5:~$ grep bash /etc/passwd root:x:0:0:root:/root:/bin/bash paul:x:1000:1000:paul,,,:/home/paul:/bin/bash serena:x:1001:1001::/home/serena:/bin/bash paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1 root paul serena
13.13. solution: filters 1. Put a sorted list of all bash users in bashusers.txt. grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt 2. Put a sorted list of all logged on users in onlineusers.txt. who | cut -d' ' -f1 | sort > onlineusers.txt 3. Make a list of all filenames in /etc that contain the string conf in their filename. ls /etc | grep conf 4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their filename. ls /etc | grep -i conf | sort 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. /sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5 6. Write a line that removes all non-letters from a stream. 7. Write a line that receives a text file, and outputs all words on a separate line. 8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/ dict/ .) paul@deb503:~$ cat text This is, yes really! , a text with ?&* too many str$ange# characters ;-) paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-' This is yes really a text with too many strange characters paul@deb503:~$ cat text2 it is very cold today without the sun paul@deb503:~$ cat text2 | tr ' ' 'n' it is very cold today without the sun paul@rhel ~$ echo "The zun is shining today" > text paul@rhel ~$ cat > DICT is shining sun the
filters 193 You could also add the solution from question number 6 to remove non-letters, and tr -s ' ' to remove redundant spaces. today paul@rhel ~$ cat text | tr 'A-Z ' 'a-zn' | sort | uniq | comm -23 - DICT zun
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195 Chapter 14. basic Unix tools This chapter introduces commands to find or locate files and to compress files, together with other common tools that were not discussed before. While the tools discussed hereare technically not considered filters, they can be used in pipes.
14.1. find The find command can be very useful at the start of a pipe to search for files. Here are some examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your screen with error messages. Find all files in /etc and put the list in etcfiles.txt Find all files of the entire system and put the list in allfiles.txt Find files that end in .conf in the current directory (and all subdirs). Find files of type file (not directory, pipe or etc.) that end in .conf. Find files of type directory that end in .bak . Find files that are newer than file42.txt Find can also execute another command on every file found. This example will look for *.odf files and copy them to /backup/. Find can also execute, after your confirmation, another command on every file found. This example will remove *.odf files if you approve of it for every file found. find /etc > etcfiles.txt find / > allfiles.txt find . -name "*.conf" find . -type f -name "*.conf" find /data -type d -name "*.bak" find . -newer file42.txt find /data -name "*.odf" -exec cp {} /backup/ ; find /data -name "*.odf" -ok rm {} ;
basic Unix tools 197 14.2. locate The locate tool is very different from find in that it uses an index to locate files. This is a lot faster than traversing all the directories, but it also means that it is always outdated. If the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux) with the updatedb command. Most Linux distributions will schedule the updatedb to run once every day. 14.3. date The date command can display the date, time, time zone and more. A date string can be customised to display the format of your choice. Check the man page for more options. Time on any Unix is calculated in number of seconds since 1969 (the first second being the first second of the first of January 1970). Use date +%s to display Unix time in seconds. When will this seconds counter reach two thousand million ? [paul@RHEL4b ~]$ locate Samba warning: locate: could not open database: /var/lib/slocate/slocate.db:... warning: You need to run the 'updatedb' command (as root) to create th... Please have a look at /etc/updatedb.conf to enable the daily cron job. [paul@RHEL4b ~]$ updatedb fatal error: updatedb: You are not authorized to create a default sloc... [paul@RHEL4b ~]$ su - Password: [root@RHEL4b ~]# updatedb [root@RHEL4b ~]# paul@rhel55 ~$ date Sat Apr 17 12:44:30 CEST 2010 paul@rhel55 ~$ date +'%A %d-%m-%Y' Saturday 17-04-2010 paul@rhel55 ~$ date +%s 1271501080 paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds' Wed May 18 04:33:20 CEST 2033
14.4. cal The cal command displays the current month, with the current day highlighted. paul@rhel55 ~$ cal April 2010 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 You can select any month in the past or the future. paul@rhel55 ~$ cal 2 1970 February 1970 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 14.5. sleep The sleep command is sometimes used in scripts to wait a number of seconds. This example shows a five second sleep. paul@rhel55 ~$ sleep 5 paul@rhel55 ~$
basic Unix tools 199 14.6. time The time command can display how long it takes to execute a command. The date command takes only a little time. The sleep 5 command takes five real seconds to execute, but consumes little cpu time. This bzip2 command compresses a file and uses a lot of cpu time. paul@rhel55 ~$ time date Sat Apr 17 13:08:27 CEST 2010 real user sys 0m0.014s 0m0.008s 0m0.006s paul@rhel55 ~$ time sleep 5 real user sys 0m5.018s 0m0.005s 0m0.011s paul@rhel55 ~$ time bzip2 text.txt real user sys 0m2.368s 0m0.847s 0m0.539s
14.7. gzip - gunzip Users never have enough disk space, so compression comes in handy. The gzip command can make files take up less space. You can get the original back with gunzip. 14.8. zcat - zmore Text files that are compressed with gzip can be viewed with zcat and zmore. paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ ls -lh text.txt.gz -rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz paul@rhel55 ~$ gunzip text.txt.gz paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ head -4 text.txt / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ zcat text.txt.gz | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh
basic Unix tools 201 14.9. bzip2 - bunzip2 Files can also be compressed with bzip2 which takes a little more time than gzip, but compresses better. Files can be uncompressed again with bunzip2. 14.10. bzcat - bzmore And in the same way bzcat and bzmore can display files compressed with bzip2. paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ ls -lh text.txt.bz2 -rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2 paul@rhel55 ~$ bunzip2 text.txt.bz2 paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ bzcat text.txt.bz2 | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh
14.11. solution: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt When *.txt is quoted then the shell will not touch it. The find tool will look in the /data for all files ending in .txt. When *.txt is not quoted then the shell might expand this (when one or more files that ends in .txt exist in the current directory). The find might show a different result, or can result in a syntax error. 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt The first find will output all .odf filenames in /data and all subdirectories. The shell will redirect this to a file. The second find will output all files named .odf in /data and will also output all files that exist in directories named *.odf (in /data). With two million files the command line would be expanded beyond the maximum that the shell can accept. The last part of the command line would be lost. 3. Write a find command that finds all files created after January 30th 2010. 4. Write a find command that finds all *.odf files created in September 2009. The exclamation mark ! -newer can be read as not newer. 5. Count the number of *.conf files in /etc and all its subdirs. find /etc -type f -name '*.conf' | wc -l 6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ touch -t 201001302359 marker_date find . -type f -newer marker_date There is another solution : find . -type f -newerat "20100130 23:59:59" touch -t 200908312359 marker_start touch -t 200910010000 marker_end find . -type f -name "*.odf" -newer marker_start ! -newer marker_end
basic Unix tools 203 find /data -name "*.odf" -exec cp {} /backup/ ; The first might fail when there are too many files to fit on one command line. 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? You cannot locate this with locate because it is not yet in the index. updatedb 8. Use find and -exec to rename all .htm files to .html. 9. Issue the date command. Now display the date in YYYY/MM/DD format. date +%Y/%m/%d 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? cal 1582 The calendars are different depending on the country. Check http://linux-training.be/files/ studentfiles/dates.txt paul@rhel55 ~$ find . -name '*.htm' ./one.htm ./two.htm paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l ; paul@rhel55 ~$ find . -name '*.htm*' ./one.html ./two.html
linux.pdf
205 Chapter 15. regular expressions Regular expressions are a very powerful tool in Linux. They can be used with a variety of programs like bash, vi, rename, grep, sed, and more. This chapter introduces you to the basics of regular expressions.
15.1. regex versions There are three different versions of regular expression syntax: Depending on the tool being used, one or more of these syntaxes can be used. For example the grep tool has the -E option to force a string to be read as ERE while -G forces BRE and -P forces PRCE. Note that grep also has -F to force the string to be read literally. The sed tool also has options to choose a regex syntax. Read the manual of the tools you use! BRE: Basic Regular Expressions ERE: Extended Regular Expressions PRCE: Perl Regular Expressions
regular expressions 207 15.2. grep 15.2.1. print lines matching a pattern grep is a popular Linux tool to search for lines that match a certain pattern. Below are some examples of the simplest regular expressions. This is the contents of the test file. This file contains three lines (or three newline characters). When grepping for a single character, only the lines containing that character are returned. The pattern matching in this example should be very straightforward; if the given character occurs on a line, then grep will return that line. 15.2.2. concatenating characters Two concatenated characters will have to be concatenated in the same way to have a match. This example demonstrates that ia will match Tania but not Valentina and in will match Valentina but not Tania. paul@rhel65:~$ cat names Tania Laura Valentina paul@rhel65:~$ grep u names Laura paul@rhel65:~$ grep e names Valentina paul@rhel65:~$ grep i names Tania Valentina paul@rhel65:~$ grep a names Tania Laura Valentina paul@rhel65:~$ grep ia names Tania paul@rhel65:~$ grep in names Valentina paul@rhel65:~$
15.2.3. one or the other PRCE and ERE both use the pipe symbol to signify OR. In this example we grep for lines containing the letter i or the letter a. Note that we use the -E switch of grep to force interpretion of our string as an ERE. We need to escape the pipe symbol in a BRE to get the same logical OR. 15.2.4. one or more The * signifies zero, one or more occurences of the previous and the + signifies one or more of the previous. paul@debian7:~$ grep -G 'i|a' list paul@debian7:~$ grep -G 'i|a' list Tania Laura paul@debian7:~$ cat list Tania Laura paul@debian7:~$ grep -E 'i|a' list Tania Laura paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o*' list2 ll lol lool loool paul@debian7:~$ grep -E 'o+' list2 lol lool loool paul@debian7:~$
regular expressions 209 15.2.5. match the end of a string For the following examples, we will use this file. The two examples below show how to use the dollar character to match the end of a string. 15.2.6. match the start of a string The caret character (^) will match a string at the start (or the beginning) of a line. Given the same file as above, here are two examples. Both the dollar sign and the little hat are called anchors in a regex. paul@debian7:~$ grep ^Val names Valentina paul@debian7:~$ grep ^F names Fleur Floor paul@debian7:~$ cat names Tania Laura Valentina Fleur Floor paul@debian7:~$ grep a$ names Tania Laura Valentina paul@debian7:~$ grep r$ names Fleur Floor
15.2.7. separating words Regular expressions use a b sequence to reference a word separator. Take for example this file: Simply grepping for over will give too many results. Surrounding the searched word with spaces is not a good solution (because other characters can be word separators). This screenshot below show how to use b to find only the searched word: Note that grep also has a -w option to grep for words. paul@debian7:~$ cat text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep over text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep 'boverb' text The winter is over. Can you get over there? paul@debian7:~$ paul@debian7:~$ cat text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep -w over text The winter is over. Can you get over there? paul@debian7:~$
regular expressions 211 15.2.8. grep features Sometimes it is easier to combine a simple regex with grep options, than it is to write a more complex regex. These options where discussed before: 15.2.9. preventing shell expansion of a regex The dollar sign is a special character, both for the regex and also for the shell (remember variables and embedded shells). Therefore it is advised to always quote the regex, this prevents shell expansion. grep -i grep -v grep -w grep -A5 grep -B5 grep -C5 paul@debian7:~$ grep 'r$' names Fleur Floor
15.3. rename 15.3.1. the rename command On Debian Linux the /usr/bin/rename command is a link to /usr/bin/prename installed by the perl package. Red Hat derived systems do not install the same rename command, so this section does not describe rename on Red Hat (unless you copy the perl script manually). There is often confusion on the internet about the rename command because solutions that work fine in Debian (and Ubuntu, xubuntu, Mint, ...) cannot be used in Red Hat (and CentOS, Fedora, ...). 15.3.2. perl The rename command is actually a perl script that uses perl regular expressions. The complete manual for these can be found by typing perldoc perlrequick (after installing perldoc). paul@pi ~ $ dpkg -S $(readlink -f $(which rename)) perl: /usr/bin/prename root@pi:~# aptitude install perl-doc The following NEW packages will be installed: perl-doc 0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded. Need to get 8,170 kB of archives. After unpacking 13.2 MB will be used. Get: 1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main perl-do... Fetched 8,170 kB in 19s (412 kB/s) Selecting previously unselected package perl-doc. (Reading database ... 67121 files and directories currently installed.) Unpacking perl-doc (from .../perl-doc_5.14.2-21+rpi2_all.deb) ... Adding 'diversion of /usr/bin/perldoc to /usr/bin/perldoc.stub by perl-doc' Processing triggers for man-db ... Setting up perl-doc (5.14.2-21+rpi2) ... root@pi:~# perldoc perlrequick
regular expressions 213 15.3.3. well known syntax The most common use of the rename is to search for filenames matching a certain string and replacing this string with an other string. This is often presented as s/string/other string/ as seen in this example: And here is another example that uses rename with the well know syntax to change the extensions of the same files once more: These two examples appear to work because the strings we used only exist at the end of the filename. Remember that file extensions have no meaning in the bash shell. The next example shows what can go wrong with this syntax. Only the first occurrence of the searched string is replaced. paul@pi ~ $ ls abc allfiles.TXT bllfiles.TXT Scratch abc.conf backup cllfiles.TXT temp.TXT tennis2.TXT tennis.TXT paul@pi ~ $ rename 's/TXT/text/' * paul@pi ~ $ ls abc allfiles.text bllfiles.text Scratch tennis2.text abc.conf backup cllfiles.text temp.text tennis.text paul@pi ~ $ ls abc allfiles.text bllfiles.text Scratch tennis2.text abc.conf backup cllfiles.text temp.text tennis.text paul@pi ~ $ rename 's/text/txt/' *.text paul@pi ~ $ ls abc allfiles.txt bllfiles.txt Scratch tennis2.txt abc.conf backup cllfiles.txt temp.txt tennis.txt paul@pi ~ $ paul@pi ~ $ touch atxt.txt paul@pi ~ $ rename 's/txt/problem/' atxt.txt paul@pi ~ $ ls abc allfiles.txt backup cllfiles.txt temp.txt tennis.txt abc.conf aproblem.txt bllfiles.txt Scratch tennis2.txt paul@pi ~ $
15.3.4. a global replace The syntax used in the previous example can be described as s/regex/replacement/. This is simple and straightforward, you enter a regex between the first two slashes and a replacement string between the last two. This example expands this syntax only a little, by adding a modifier. The syntax we use now can be described as s/regex/replacement/g where s signifies switch and g stands for global. Note that this example used the -n switch to show what is being done (instead of actually renaming the file). 15.3.5. case insensitive replace Another modifier that can be useful is i. this example shows how to replace a case insensitive string with another string. 15.3.6. renaming extensions Command line Linux has no knowledge of MS-DOS like extensions, but many end users and graphical application do use them. Here is an example on how to use rename to only rename the file extension. It uses the dollar sign to mark the ending of the filename. Note that the dollar sign in the regex means at the end. Without the dollar sign this command would fail on the really.txt.txt file. paul@pi ~ $ rename -n 's/TXT/txt/g' aTXT.TXT aTXT.TXT renamed as atxt.txt paul@pi ~ $ paul@debian7:~/files$ ls file1.text file2.TEXT file3.txt paul@debian7:~/files$ rename 's/.text/.txt/i' * paul@debian7:~/files$ ls file1.txt file2.txt file3.txt paul@debian7:~/files$ paul@pi ~ $ ls *.txt allfiles.txt bllfiles.txt cllfiles.txt really.txt.txt temp.txt tennis.txt paul@pi ~ $ rename 's/.txt$/.TXT/' *.txt paul@pi ~ $ ls *.TXT allfiles.TXT bllfiles.TXT cllfiles.TXT really.txt.TXT temp.TXT tennis.TXT paul@pi ~ $
regular expressions 215 15.4. sed 15.4.1. stream editor The stream editor or short sed uses regex for stream editing. In this example sed is used to replace a string. The slashes can be replaced by a couple of other characters, which can be handy in some cases to improve readability. 15.4.2. interactive editor While sed is meant to be used in a stream, it can also be used interactively on a file. echo Sunday | sed 's/Sun/Mon/' Monday echo Sunday | sed 's:Sun:Mon:' Monday echo Sunday | sed 's_Sun_Mon_' Monday echo Sunday | sed 's|Sun|Mon|' Monday paul@debian7:~/files$ echo Sunday > today paul@debian7:~/files$ cat today Sunday paul@debian7:~/files$ sed -i 's/Sun/Mon/' today paul@debian7:~/files$ cat today Monday
15.4.3. simple back referencing The ampersand character can be used to reference the searched (and found) string. In this example the ampersand is used to double the occurence of the found string. 15.4.4. back referencing Parentheses (often called round brackets) are used to group sections of the regex so they can leter be referenced. Consider this simple example: 15.4.5. a dot for any character In a regex a simple dot can signify any character. 15.4.6. multiple back referencing When more than one pair of parentheses is used, each of them can be referenced separately by consecutive numbers. This feature is called grouping. echo Sunday | sed 's/Sun/&&/' SunSunday echo Sunday | sed 's/day/&&/' Sundayday paul@debian7:~$ echo Sunday | sed 's_(Sun)_1ny_' Sunnyday paul@debian7:~$ echo Sunday | sed 's_(Sun)_1ny 1_' Sunny Sunday paul@debian7:~$ echo 2014-04-01 | sed 's/....-.... /YYYY-MM-DD/' YYYY-MM-DD paul@debian7:~$ echo abcd-ef-gh | sed 's/....-.... /YYYY-MM-DD/' YYYY-MM-DD paul@debian7:~$ echo 2014-04-01 | sed 's/(... )-(..)-(..)/1+2+3/' 2014+04+01 paul@debian7:~$ echo 2014-04-01 | sed 's/(....)-(..)-(..)/3:2:1/' 01:04:2014
regular expressions 217 15.4.7. white space The s can refer to white space such as a space or a tab. This example looks for white spaces (s) globally and replaces them with 1 space. 15.4.8. optional occurrence A question mark signifies that the previous is optional. The example below searches for three consecutive letter o, but the third o is optional. paul@debian7:~$ echo -e 'todaytistwarm' today is warm paul@debian7:~$ echo -e 'todaytistwarm' | sed 's_s_ _g' today is warm paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'ooo?' list2 lool loool paul@debian7:~$ cat list2 | sed 's/ooo?/A/' ll lol lAl lAl
15.4.9. exactly n times You can demand an exact number of times the oprevious has to occur. This example wants exactly three o's. 15.4.10. between n and m times And here we demand exactly from minimum 2 to maximum 3 times. paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o{3}' list2 loool paul@debian7:~$ cat list2 | sed 's/o{3}/A/' ll lol lool lAl paul@debian7:~$ paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o{2,3}' list2 lool loool paul@debian7:~$ grep 'o{2,3}' list2 lool loool paul@debian7:~$ cat list2 | sed 's/o{2,3}/A/' ll lol lAl lAl paul@debian7:~$
233 paul@debian7:~/hist$ history 6 paul@debian7:~/hist$ !2092 ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !2092:s/1/2 ls -l file2 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file2 paul@debian7:~/hist$ 15.5. bash history The bash shell can also interprete some regular expressions. This example shows how to manipulate the exclamation mask history feature of the bash shell. This also works with the history numbers in bash. 2089 mkdir hist 2090 cd hist/ 2091 touch file1 file2 file3 2092 ls -l file1 2093 ls -l file3 2094 history 6 paul@debian7:~$ mkdir hist paul@debian7:~$ cd hist/ paul@debian7:~/hist$ touch file1 file2 file3 paul@debian7:~/hist$ ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !l ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !l:s/1/3 ls -l file3 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file3 paul@debian7:~/hist$
Chapter 16. scripting introduction Shells like bash and Korn have support for programming constructs that can be saved as scripts. These scripts in turn then become more shell commands. Many Linux commands are scripts. User profile scripts are run when a user logs on and init scripts are run when a daemon is stopped or started. This means that system administrators also need basic knowledge of scripting to understand how their servers and their applications are started, updated, upgraded, patched, maintained, configured and removed, and also to understand how a user environment is built. The goal of this chapter is to give you enough information to be able to read and understand scripts. Not to become a writer of complex scripts.
scripting introduction 235 16.1. prerequisites You should have read and understood part III shell expansion and part IV pipes and commands before starting this chapter. 16.2. hello world Just like in every programming course, we start with a simple hello_world script. The following script will output Hello World. echo Hello World After creating this simple script in vi or with echo, you'll have to chmod +x hello_world to make it executable. And unless you add the scripts directory to your path, you'll have to type the path to the script for the shell to be able to find it. 16.3. she-bang Let's expand our example a little further by putting #!/bin/bash on the first line of the script. The #! is called a she-bang (sometimes called sha-bang), where the she-bang is the first two characters of the script. You can never be sure which shell a user is running. A script that works flawlessly in bash might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell, you can start your script with a she-bang followed by the shell it is supposed to run in. This script will run in a bash shell. This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/ shells file contains a list of shells on your system. [paul@RHEL4a ~]$ echo echo Hello World > hello_world [paul@RHEL4a ~]$ chmod +x hello_world [paul@RHEL4a ~]$ ./hello_world Hello World [paul@RHEL4a ~]$ #!/bin/bash echo Hello World #!/bin/bash echo -n hello echo A bash subshell `echo -n hello` #!/bin/ksh echo -n hello echo a Korn subshell `echo -n hello`
16.4. comment Let's expand our example a little further by adding comment lines. 16.5. variables Here is a simple example of a variable inside a script. Scripts can contain variables, but since scripts are run in their own shell, the variables do not survive the end of the script. 16.6. sourcing a script Luckily, you can force a script to run in the same shell; this is called sourcing a script. The above is identical to the below. #!/bin/bash # # Hello World Script # echo Hello World #!/bin/bash # # simple variable in script # var1=4 echo var1 = $var1 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ [paul@RHEL4a ~]$ source ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$ [paul@RHEL4a ~]$ . ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$
scripting introduction 237 16.7. troubleshooting a script Another way to run a script in a separate shell is by typing bash with the name of the script as a parameter. Expanding this to bash -x allows you to see the commands that the shell is executing (after shell expansion). Notice the absence of the commented (#) line, and the replacement of the variable before execution of echo. 16.8. prevent setuid root spoofing Some user may try to perform setuid based script root spoofing. This is a rare but possible attack. To improve script security and to avoid interpreter spoofing, you need to add -- after the #!/bin/bash, which disables further option processing so the shell will not accept any options. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --. paul@debian6~/test$ bash runme 42 paul@debian6~/test$ bash -x runme + var4=42 + echo 42 42 paul@debian6~/test$ cat runme # the runme script var4=42 echo $var4 paul@debian6~/test$ #!/bin/bash - or #!/bin/bash --
16.9. solution: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. 2. Make sure the script runs in the bash shell. 3. Make sure the script runs in the Korn shell. Note that while first.bash will technically work as a Korn shell script, the name ending in .bash is confusing. 4. Create a script that defines two variables, and outputs their value. 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. source second.bash 6. Is there a shorter way to source the script ? . ./second.bash 7. Comment your scripts so that you know what they are doing. $ echo 'echo Antwerp' > first.bash $ chmod +x first.bash $ ./first.bash Antwerp $ cat first.bash #!/bin/bash echo Antwerp $ cat first.bash #!/bin/ksh echo Antwerp $ cat second.bash #!/bin/bash var33=300 var42=400 echo $var33 $var42 $ cat second.bash #!/bin/bash # script to test variables and sourcing # define two variables var33=300 var42=400 # output the value of these variables echo $var33 $var42
linux.pdf
Chapter 17. scripting loops
scripting loops 241 17.1. test [ ] The test command can test whether something is true or false. Let's start by testing whether 10 is greater than 55. The test command returns 1 if the test fails. And as you see in the next screenshot, test returns 0 when a test succeeds. If you prefer true and false, then write the test like this. The test command can also be written as square brackets, the screenshot below is identical to the one above. Below are some example tests. Take a look at man test to see more options for tests. Tests can be combined with logical AND and OR. [paul@RHEL4b ~]$ test 10 -gt 55 ; echo $? 1 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ test 56 -gt 55 ; echo $? 0 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false true [paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false false [paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false true [paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false false [ -d foo ] [ -e bar ] [ '/etc' = $PWD ] [ $1 != 'secret' ] [ 55 -lt $bar ] [ $foo -ge 1000 ] [ "abc" < $bar ] [ -f foo ] [ -r bar ] [ foo -nt bar ] [ -o nounset ] Does the directory foo exist ? Does the file bar exist ? Is the string /etc equal to the variable $PWD ? Is the first parameter different from secret ? Is 55 less than the value of $bar ? Is the value of $foo greater or equal to 1000 ? Does abc sort before the value of $bar ? Is foo a regular file ? Is bar a readable file ? Is file foo newer than file bar ? Is the shell option nounset set ? paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false true paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false false paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false true
17.2. if then else The if then else construction is about choice. If a certain condition is met, then execute something, else execute something else. The example below tests whether a file exists, and if the file exists then a proper message is echoed. If we name the above script 'choice', then it executes like this. 17.3. if then elif You can nest a new if inside an else with elif. This is a simple example. 17.4. for loop The example below shows the syntax of a classical for loop in bash. An example of a for loop combined with an embedded shell. The same example as above can be written without the embedded shell using the bash {from..to} shorthand. #!/bin/bash if [ -f isit.txt ] then echo isit.txt exists! else echo isit.txt not found! fi [paul@RHEL4a scripts]$ ./choice isit.txt not found! [paul@RHEL4a scripts]$ touch isit.txt [paul@RHEL4a scripts]$ ./choice isit.txt exists! [paul@RHEL4a scripts]$ #!/bin/bash count=42 if [ $count -eq 42 ] then echo "42 is correct." elif [ $count -gt 42 ] then echo "Too much." else echo "Not enough." fi for i in 1 2 4 do echo $i done #!/bin/ksh for counter in `seq 1 20` do echo counting from 1 to 20, now at $counter sleep 1 done
scripting loops 243 This for loop uses file globbing (from the shell expansion). Putting the instruction on the command line has identical functionality. 17.5. while loop Below a simple example of a while loop. Endless loops can be made with while true or while : , where the colon is the equivalent of no operation in the Korn and bash shells. 17.6. until loop Below a simple example of an until loop. #!/bin/bash for counter in {1..20} do echo counting from 1 to 20, now at $counter sleep 1 done kahlan@solexp11$ ls count.ksh go.ksh kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done kahlan@solexp11$ ls count.ksh count.ksh.backup go.ksh go.ksh.backup i=100; while [ $i -ge 0 ] ; do echo Counting down, from 100 to 0, now at $i; let i--; done #!/bin/ksh # endless loop while : do echo hello sleep 1 done let i=100; until [ $i -le 0 ] ; do echo Counting down, from 100 to 1, now at $i; let i--; done
17.7. solution: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. 2. Write a script that uses a for loop to count from 1 to 17000. 3. Write a script that uses a while loop to count from 3 to 7. 4. Write a script that uses an until loop to count down from 8 to 4. 5. Write a script that counts the number of files ending in .txt in the current directory. 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. #!/bin/bash for i in 3 4 5 6 7 do echo Counting from 3 to 7, now at $i done #!/bin/bash for i in `seq 1 17000` do echo Counting from 1 to 17000, now at $i done #!/bin/bash i=3 while [ $i -le 7 ] do echo Counting from 3 to 7, now at $i let i=i+1 done #!/bin/bash i=8 until [ $i -lt 4 ] do echo Counting down from 8 to 4, now at $i let i=i-1 done #!/bin/bash let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" #!/bin/bash ls *.txt > /dev/null 2>&1 if [ $? -ne 0 ]
scripting loops 245 then echo "There are 0 files ending in .txt" else let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" fi
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247 Chapter 18. scripting parameters
18.1. script parameters A bash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list. Below is the output of the script above in action. Once more the same script, but with only two parameters. Here is another example, where we use $0. The $0 parameter contains the name of the script. #!/bin/bash echo The first argument is $1 echo The second argument is $2 echo The third argument is $3 echo $ $$ PID of the script echo # $# count arguments echo ? $? last return code echo * $* all the arguments [paul@RHEL4a scripts]$ ./pars one two three The first argument is one The second argument is two The third argument is three $ 5610 PID of the script # 3 count arguments ? 0 last return code * one two three all the arguments [paul@RHEL4a scripts]$ ./pars 1 2 The first argument is 1 The second argument is 2 The third argument is $ 5612 PID of the script # 2 count arguments ? 0 last return code * 1 2 all the arguments [paul@RHEL4a scripts]$ paul@debian6~$ cat myname echo this script is called $0 paul@debian6~$ ./myname this script is called ./myname paul@debian6~$ mv myname test42 paul@debian6~$ ./test42 this script is called ./test42
scripting parameters 249 18.2. shift through parameters The shift statement can parse all parameters one by one. This is a sample script. Below is some sample output of the script above. 18.3. runtime input You can ask the user for input with the read command in a script. kahlan@solexp11$ cat shift.ksh #!/bin/ksh if [ "$#" == "0" ] then echo You have to give at least one parameter. exit 1 fi while (( $# )) do echo You gave me $1 shift done kahlan@solexp11$ ./shift.ksh one You gave me one kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42" You gave me one You gave me two You gave me three You gave me 1201 You gave me 33 42 kahlan@solexp11$ ./shift.ksh You have to give at least one parameter. #!/bin/bash echo -n Enter a number: read number
18.4. sourcing a config file The source (as seen in the shell chapters) can be used to source a configuration file. Below a sample configuration file for an application. And here an application that uses this file. The running application can use the values inside the sourced configuration file. [paul@RHEL4a scripts]$ cat myApp.conf # The config file of myApp # Enter the path here myAppPath=/var/myApp # Enter the number of quines here quines=5 [paul@RHEL4a scripts]$ cat myApp.bash #!/bin/bash # # Welcome to the myApp application # . ./myApp.conf echo There are $quines quines [paul@RHEL4a scripts]$ ./myApp.bash There are 5 quines [paul@RHEL4a scripts]$
scripting parameters 251 18.5. get script options with getopts The getopts function allows you to parse options given to a command. The following script allows for any combination of the options a, f and z. This is sample output from the script above. First we use correct options, then we enter twice an invalid option. kahlan@solexp11$ cat options.ksh #!/bin/ksh while getopts ":afz" option; do case $option in a) echo received -a ;; f) echo received -f ;; z) echo received -z ;; *) echo "invalid option -$OPTARG" ;; esac done kahlan@solexp11$ ./options.ksh kahlan@solexp11$ ./options.ksh -af received -a received -f kahlan@solexp11$ ./options.ksh -zfg received -z received -f invalid option -g kahlan@solexp11$ ./options.ksh -a -b -z received -a invalid option -b received -z
You can also check for options that need an argument, as this example shows. This is sample output from the script above. 18.6. get shell options with shopt You can toggle the values of variables controlling optional shell behaviour with the shopt built-in shell command. The example below first verifies whether the cdspell option is set; it is not. The next shopt command sets the value, and the third shopt command verifies that the option really is set. You can now use minor spelling mistakes in the cd command. The man page of bash has a complete list of options. kahlan@solexp11$ cat argoptions.ksh #!/bin/ksh while getopts ":af:z" option; do case $option in a) echo received -a ;; f) echo received -f with $OPTARG ;; z) echo received -z ;; :) echo "option -$OPTARG needs an argument" ;; *) echo "invalid option -$OPTARG" ;; esac done kahlan@solexp11$ ./argoptions.ksh -a -f hello -z received -a received -f with hello received -z kahlan@solexp11$ ./argoptions.ksh -zaf 42 received -z received -a received -f with 42 kahlan@solexp11$ ./argoptions.ksh -zf received -z option -f needs an argument paul@laika:~$ shopt -q cdspell ; echo $? 1 paul@laika:~$ shopt -s cdspell paul@laika:~$ shopt -q cdspell ; echo $? 0 paul@laika:~$ cd /Etc /etc
scripting parameters 253 18.7. solution: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. echo $4 $3 $2 $1 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. 3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. #!/bin/bash if [ -f $1 ] then echo $1 exists! else echo $1 not found! fi if [ -f $2 ] then echo $2 exists! else echo $2 not found! fi
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255 Chapter 19. more scripting
19.1. eval eval reads arguments as input to the shell (the resulting commands are executed). This allows using the value of a variable as a variable. Both in bash and Korn the arguments can be quoted. Sometimes the eval is needed to have correct parsing of arguments. Consider this example where the date command receives one parameter 1 week ago. When we set this command in a variable, then executing that variable fails unless we use eval. 19.2. (( )) The (( )) allows for evaluation of numerical expressions. paul@deb503:~/test42$ answer=42 paul@deb503:~/test42$ word=answer paul@deb503:~/test42$ eval x=$$word ; echo $x 42 kahlan@solexp11$ answer=42 kahlan@solexp11$ word=answer kahlan@solexp11$ eval "y=$$word" ; echo $y 42 paul@debian6~$ date --date="1 week ago" Thu Mar 8 21:36:25 CET 2012 paul@debian6~$ lastweek='date --date="1 week ago"' paul@debian6~$ $lastweek date: extra operand `ago"' Try `date --help' for more information. paul@debian6~$ eval $lastweek Thu Mar 8 21:36:39 CET 2012 paul@deb503:~/test42$ (( 42 > 33 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false false paul@deb503:~/test42$ var42=42 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false true paul@deb503:~/test42$ var42=33 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false false
more scripting 257 19.3. let The let built-in shell function instructs the shell to perform an evaluation of arithmetic expressions. It will return 0 unless the last arithmetic expression evaluates to 0. The shell can also convert between different bases. There is a difference between assigning a variable directly, or using let to evaluate the arithmetic expressions (even if it is just assigning a value). [paul@RHEL4b ~]$ let x="3 + 4" ; echo $x 7 [paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x 20 [paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x 18 [paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x 30 [paul@RHEL4b ~]$ let x="0xFF" ; echo $x 255 [paul@RHEL4b ~]$ let x="0xC0" ; echo $x 192 [paul@RHEL4b ~]$ let x="0xA8" ; echo $x 168 [paul@RHEL4b ~]$ let x="8#70" ; echo $x 56 [paul@RHEL4b ~]$ let x="8#77" ; echo $x 63 [paul@RHEL4b ~]$ let x="16#c0" ; echo $x 192 kahlan@solexp11$ dec=15 ; oct=017 ; hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 017 0x0f kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 15 15
19.4. case You can sometimes simplify nested if statements with a case construct. [paul@RHEL4b ~]$ ./help What animal did you see ? lion You better start running fast! [paul@RHEL4b ~]$ ./help What animal did you see ? dog Don't worry, give it a cookie. [paul@RHEL4b ~]$ cat help #!/bin/bash # # Wild Animals Helpdesk Advice # echo -n "What animal did you see ? " read animal case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty...'." ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac [paul@RHEL4b ~]$
more scripting 259 19.5. shell functions Shell functions can be used to group commands in a logical way. This is sample output from this script with a function. A shell function can also receive parameters. kahlan@solexp11$ cat addfunc.ksh #!/bin/ksh function plus { let result="$1 + $2" echo } $1 + $2 = $result plus 3 10 plus 20 13 plus 20 22 This script produces the following output. kahlan@solexp11$ cat funcs.ksh #!/bin/ksh function greetings { echo Hello World! echo and hello to $USER to! } echo We will now call a function greetings echo The end kahlan@solexp11$ ./funcs.ksh We will now call a function Hello World! and hello to kahlan to! The end kahlan@solexp11$ ./addfunc.ksh 3 + 10 = 13 20 + 13 = 33 20 + 22 = 42
19.6. solution : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. 3. Improve the previous script to congratulate the user if the sum equals the product. 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. Enter a number: 5 Enter another number: 2 Sum: 5 + 2 = 7 Product: 5 x 2 = 10 #!/bin/bash echo -n "Enter a number : " read n1 echo -n "Enter another number : " read n2 let sum="$n1+$n2" let pro="$n1*$n2" echo -e "Sumt: $n1 + $n2 = $sum" echo -e "Productt: $n1 * $n2 = $pro" echo -n "Enter a number between 1 and 100 : " read n1 if [ $n1 -lt 1 -o $n1 -gt 100 ] then echo Wrong number... exit 1 fi if [ $sum -eq $pro ] then echo Congratulations $sum == $pro fi #!/bin/bash # # Wild Animals Case Insensitive Helpdesk Advice # if shopt -q nocasematch; then nocase=yes; else nocase=no; shopt -s nocasematch; fi echo -n "What animal did you see ? " read animal case $animal in
265 5. If time permits (or if you are waiting for other students to finish this practice), take a look at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty.'" ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac if [ nocase = yes ] ; then shopt -s nocasematch; else shopt -u nocasematch; fi
Chapter 20. introduction to users This little chapter will teach you how to identify your user account on a Unix computer using commands like who am i, id, and more. In a second part you will learn how to become another user with the su command. And you will learn how to run a program as another user with sudo.
introduction to users 267 01, 0.02 20.1. whoami The whoami command tells you your username. 20.2. who The who command will give you information about who is logged on the system. [paul@centos7 ~]$ who root pts/0 2014-10-10 23:07 (10.104.33.101) paul pts/1 2014-10-10 23:30 (10.104.33.101) laura pts/2 2014-10-10 23:34 (10.104.33.96) tania pts/3 [paul@centos7 ~]$ 2014-10-10 23:39 (10.104.33.91) 20.3. who am i With who am i the who command will display only the line pointing to your current session. 20.4. w The w command shows you who is logged on and what they are doing. [paul@centos7 ~]$ w 23:34:07 up 31 min, 2 users, load average: 0.00, 0. USER TTY LOGIN@ IDLE JCPU PCPU WHAT root pts/0 23:07 15.00s 0.01s 0.01s top paul pts/1 23:30 [paul@centos7 ~]$ 7.00s 0.00s 0.00s w 20.5. id The id command will give you your user id, primary group id, and a list of the groups that you belong to. On RHEL/CentOS you will also get SELinux context information with this command. [paul@centos7 ~]$ whoami paul [paul@centos7 ~]$ [paul@centos7 ~]$ who am i paul pts/1 2014-10-10 23:30 (10.104.33.101) [paul@centos7 ~]$ paul@debian7:~$ id uid=1000(paul) gid=1000(paul) groups=1000(paul) [root@centos7 ~]# id uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r :unconfined_t:s0-s0:c0.c1023
20.6. su to another user The su command allows a user to run a shell as another user. 20.7. su to root Yes you can also su to become root, when you know the root password. 20.8. su as root You need to know the password of the user you want to substitute to, unless your are logged in as root. The root user can become any existing user without knowing that user's password. 20.9. su - $username By default, the su command maintains the same shell environment. To become another user and also get the target user's environment, issue the su - command followed by the target username. 20.10. su - When no username is provided to su or su -, the command will assume root is the target. laura@debian7:~$ su tania Password: tania@debian7:/home/laura$ laura@debian7:~$ su root Password: root@debian7:/home/laura# root@debian7:~# id uid=0(root) gid=0(root) groups=0(root) root@debian7:~# su - valentina valentina@debian7:~$ root@debian7:~# su laura laura@debian7:/root$ exit exit root@debian7:~# su - laura laura@debian7:~$ pwd /home/laura tania@debian7:~$ su - Password: root@debian7:~#
introduction to users 269 20.11. run a program as another user The sudo program allows a user to start a program with the credentials of another user. Before this works, the system administrator has to set up the /etc/sudoers file. This can be useful to delegate administrative tasks to another user (without giving the root password). The screenshot below shows the usage of sudo. User paul received the right to run useradd with the credentials of root. This allows paul to create new users on the system without becoming root and without knowing the root password. First the command fails for paul. But with sudo it works. 20.12. visudo Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers is out of scope for this fundamentals book. paul@debian7:~$ /usr/sbin/useradd -m valentina useradd: Permission denied. useradd: cannot lock /etc/passwd; try again later. paul@debian7:~$ sudo /usr/sbin/useradd -m valentina [sudo] password for paul: paul@debian7:~$ paul@rhel65:~$ apropos visudo visudo (8) - edit the sudoers file paul@rhel65:~$
20.13. sudo su - On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password set. This means that it is not possible to login as root (extra security). To perform tasks as root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are members of the admin group can use sudo to run all commands as root. The end result of this is that the user can type sudo su - and become root without having to enter the root password. The sudo command does require you to enter your own password. Thus the password prompt in the screenshot below is for sudo, not for su. 20.14. sudo logging Using sudo without authorization will result in a severe warning: The root user can see this in the /var/log/secure on Red Hat and in /var/log/auth.log on Debian). root@laika:~# grep admin /etc/sudoers # Members of the admin group may gain root privileges %admin ALL=(ALL) ALL paul@laika:~$ sudo su - Password: root@laika:~# paul@rhel65:~$ sudo su - We trust you have received the usual lecture from the local System Administrator. It usually boils down to these three things: #1) Respect the privacy of others. #2) Think before you type. #3) With great power comes great responsibility. [sudo] password for paul: paul is not in the sudoers file. This incident will be reported. paul@rhel65:~$ root@rhel65:~# tail /var/log/secure | grep sudo | tr -s ' ' Apr 13 16:03:42 rhel65 sudo: paul : user NOT in sudoers ; TTY=pts/0 ; PWD= /home/paul ; USER=root ; COMMAND=/bin/su - root@rhel65:~#
introduction to users 271 20.15. solution: introduction to users 1. Run a command that displays only your currently logged on user name. 2. Display a list of all logged on users. 3. Display a list of all logged on users including the command they are running at this very moment. laura@debian7:~$ w 07:47:02 up 16 min, 2 users, USER TTY FROM load average: 0.00, 0.00, LOGIN@ IDLE JCPU 0.00 PCPU WHAT root pts/0 10.104.33.101 07:30 6.00s 0.04s 0.00s w root pts/1 10.104.33.101 laura@debian7:~$ 07:46 6.00s 0.01s 0.00s sleep 42 4. Display your user name and your unique user identification (userid). 5. Use su to switch to another user account (unless you are root, you will need the password of the other account). And get back to the previous account. 6. Now use su - to switch to another user and notice the difference. Note that su - gets you into the home directory of Tania. laura@debian7:~$ whoami laura laura@debian7:~$ echo $USER laura laura@debian7:~$ who laura pts/0 2014-10-13 07:22 (10.104.33.101) laura@debian7:~$ laura@debian7:~$ id uid=1005(laura) gid=1007(laura) groups=1007(laura) laura@debian7:~$ laura@debian7:~$ su tania Password: tania@debian7:/home/laura$ id uid=1006(tania) gid=1008(tania) groups=1008(tania) tania@debian7:/home/laura$ exit laura@debian7:~$ laura@debian7:~$ su - tania Password: tania@debian7:~$ pwd /home/tania tania@debian7:~$ logout laura@debian7:~$
7. Try to create a new user account (when using your normal user account). this should fail. (Details on adding user accounts are explained in the next chapter.) It is possible that useradd is located in /sbin/useradd on your computer. 8. Now try the same, but with sudo before your command. Notice that laura has no permission to use the sudo on this system. laura@debian7:~$ useradd valentina -su: useradd: command not found laura@debian7:~$ /usr/sbin/useradd valentina useradd: Permission denied. useradd: cannot lock /etc/passwd; try again later. laura@debian7:~$ sudo /usr/sbin/useradd valentina [sudo] password for laura: laura is not in the sudoers file. This incident will be reported. laura@debian7:~$
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Chapter 21. user management This chapter will teach you how to use useradd, usermod and userdel to create, modify and remove user accounts. You will need root access on a Linux computer to complete this chapter.
21.1. user management User management on Linux can be done in three complementary ways. You can use the graphical tools provided by your distribution. These tools have a look and feel that depends on the distribution. If you are a novice Linux user on your home system, then use the graphical tool that is provided by your distribution. This will make sure that you do not run into problems. Another option is to use command line tools like useradd, usermod, gpasswd, passwd and others. Server administrators are likely to use these tools, since they are familiar and very similar across many different distributions. This chapter will focus on these command line tools. A third and rather extremist way is to edit the local configuration files directly using vi (or vipw/vigr). Do not attempt this as a novice on production systems! 21.2. /etc/passwd The local user database on Linux (and on most Unixes) is /etc/passwd. As you can see, this file contains seven columns separated by a colon. The columns contain the username, an x, the user id, the primary group id, a description, the name of the home directory, and the login shell. More information can be found by typing man 5 passwd. [root@RHEL5 ~]# man 5 passwd 21.3. root The root user also called the superuser is the most powerful account on your Linux system. This user can do almost anything, including the creation of other users. The root user always has userid 0 (regardless of the name of the account). [root@RHEL5 ~]# tail /etc/passwd inge:x:518:524:art dealer:/home/inge:/bin/ksh ann:x:519:525:flute player:/home/ann:/bin/bash frederik:x:520:526:rubius poet:/home/frederik:/bin/bash steven:x:521:527:roman emperor:/home/steven:/bin/bash pascale:x:522:528:artist:/home/pascale:/bin/ksh geert:x:524:530:kernel developer:/home/geert:/bin/bash wim:x:525:531:master damuti:/home/wim:/bin/bash sandra:x:526:532:radish stresser:/home/sandra:/bin/bash annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh [root@RHEL5 ~]# head -1 /etc/passwd root:x:0:0:root:/root:/bin/bash
21.4. useradd You can add users with the useradd command. The example below shows how to add a user named yanina (last parameter) and at the same time forcing the creation of the home directory (-m), setting the name of the home directory (-d), and setting a description (-c). The user named yanina received userid 529 and primary group id 529. 21.5. /etc/default/useradd Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/useradd that contains some default user options. Besides using cat to display this file, you can also use useradd -D. 21.6. userdel You can delete the user yanina with userdel. The -r option of userdel will also remove the home directory. [root@RHEL5 ~]# userdel -r yanina 21.7. usermod You can modify the properties of a user with the usermod command. This example uses usermod to change the description of the user harry. [root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina [root@RHEL5 ~]# tail -1 /etc/passwd yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash [root@RHEL4 ~]# useradd -D GROUP=100 HOME=/home INACTIVE=-1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:harry potter:/home/harry:/bin/bash [root@RHEL4 ~]# usermod -c 'wizard' harry [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:wizard:/home/harry:/bin/bash
21.8. creating home directories The easiest way to create a home directory is to supply the -m option with useradd (it is likely set as a default option on Linux). A less easy way is to create a home directory manually with mkdir which also requires setting the owner and the permissions on the directory with chmod and chown (both commands are discussed in detail in another chapter). 21.9. /etc/skel/ When using useradd the -m option, the /etc/skel/ directory is copied to the newly created home directory. The /etc/skel/ directory contains some (usually hidden) files that contain profile settings and default values for applications. In this way /etc/skel/ serves as a default home directory and as a default user profile. [root@RHEL5 ~]# ls total 48 -la /etc/skel/ drwxr-xr-x 2 root root 4096 Apr 1 00:11 . drwxr-xr-x 97 root root 12288 Jun 24 15:36 .. -rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout -rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile -rw-r--r-- 1 root root 124 Jul 12 2006 .bashrc 21.10. deleting home directories The -r option of userdel will make sure that the home directory is deleted together with the user account. [root@RHEL5 ~]# mkdir /home/laura [root@RHEL5 ~]# chown laura:laura /home/laura [root@RHEL5 ~]# chmod 700 /home/laura [root@RHEL5 ~]# ls -ld /home/laura/ drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/ [root@RHEL5 ~]# ls -ld /home/wim/ drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/ [root@RHEL5 ~]# userdel -r wim [root@RHEL5 ~]# ls -ld /home/wim/ ls: /home/wim/: No such file or directory
21.11. login shell The /etc/passwd file specifies the login shell for the user. In the screenshot below you can see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh shell. You can use the usermod command to change the shell for a user. 21.12. chsh Users can change their login shell with the chsh command. First, user harry obtains a list of available shells (he could also have done a cat /etc/shells) and then changes his login shell to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash. Note that the -l option does not exist on Debian and that the above screenshot assumes that ksh and csh shells are installed. The screenshot below shows how laura can change her default shell (active on next login). 21.13. solution: user management 1. Create a user account named serena, including a home directory and a description (or comment) that reads Serena Williams. Do all this in one single command. root@debian7:~# useradd -m -c 'Serena Williams' serena 2. Create a user named venus, including home directory, bash shell, a description that reads Venus Williams all in one single command. root@debian7:~# useradd -m -c "Venus Williams" -s /bin/bash venus 3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group. [root@RHEL5 ~]# tail -2 /etc/passwd annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh [root@RHEL5 ~]# usermod -s /bin/bash laura [root@RHEL5 ~]# tail -1 /etc/passwd laura:x:528:534:art dealer:/home/laura:/bin/bash [laura@centos7 ~]$ chsh -l /bin/sh /bin/bash /sbin/nologin /usr/bin/sh /usr/bin/bash /usr/sbin/nologin /bin/ksh /bin/tcsh /bin/csh [laura@centos7 ~]$ [laura@centos7 ~]$ chsh -s /bin/ksh Changing shell for laura. Password: Shell changed.
4. Verify that their home directory was created. 5. Create a user named einstime with /bin/date as his default logon shell. root@debian7:~# useradd -s /bin/date einstime Or even better: root@debian7:~# useradd -s $(which date) einstime 7. What happens when you log on with the einstime user ? Can you think of a useful real world example for changing a user's login shell to an application ? It can be useful when users need to access only one application on the server. Just logging in opens the application for them, and closing the application automatically logs them out. root@debian7:~# tail -2 /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh venus:x:1009:1011:Venus Williams:/home/venus:/bin/bash root@debian7:~# tail -2 /etc/shadow serena:!:16358:0:99999:7::: venus:!:16358:0:99999:7::: root@debian7:~# tail -2 /etc/group serena:x:1010: venus:x:1011: root@debian7:~# ls -lrt /home | tail -2 drwxr-xr-x 2 serena drwxr-xr-x 2 venus root@debian7:~# serena venus 4096 Oct 15 10:50 serena 4096 Oct 15 10:59 venus root@debian7:~# su - einstime Wed Oct 15 11:05:56 UTC 2014 # You get the output of the date command root@debian7:~#
8. Create a file named welcome.txt and make sure every new user will see this file in their home directory. root@debian7:~# echo Hello > /etc/skel/welcome.txt 9. Verify this setup by creating (and deleting) a test user account. 10. Change the default login shell for the serena user to /bin/bash. Verify before and after you make this change. root@debian7:~# useradd -m test root@debian7:~# ls -l /home/test total 4 -rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt root@debian7:~# userdel -r test root@debian7:~# root@debian7:~# grep serena /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh root@debian7:~# usermod -s /bin/bash serena root@debian7:~# grep serena /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash root@debian7:~#
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Chapter 22. user passwords This chapter will tell you more about passwords for local users. Three methods for setting passwords are explained; using the passwd command, using openssel passwd, and using the crypt function in a C program. The chapter will also discuss password settings and disabling, suspending or locking accounts.
22.1. passwd Passwords of users can be set with the passwd command. Users will have to provide their old password before twice entering the new one. As you can see, the passwd tool will do some basic verification to prevent users from using too simple passwords. The root user does not have to follow these rules (there will be a warning though). The root user also does not have to provide the old password before entering the new password twice. 22.2. shadow file User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only and can only be read by root. We will see in the file permissions section how it is possible for users to change their password. For now, you will have to know that users can change their password with the /usr/bin/passwd command. The /etc/shadow file contains nine colon separated columns. The nine fields contain (from left to right) the user name, the encrypted password (note that only inge and laura have an encrypted password), the day the password was last changed (day 1 is January 1, 1970), number of days the password must be left unchanged, password expiry day, warning number of days before password expiry, number of days after expiry before disabling the account, and the day the account was disabled (again, since 1970). The last field has no meaning yet. All the passwords in the screenshot above are hashes of hunter2. [tania@centos7 ~]$ passwd Changing password for user tania. Changing password for tania. (current) UNIX password: New password: BAD PASSWORD: The password is shorter than 8 characters New password: BAD PASSWORD: The password is a palindrome New password: BAD PASSWORD: The password is too similar to the old one passwd: Have exhausted maximum number of retries for service root@debian7:~# passwd tania Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully [root@centos7 ~]# tail -4 /etc/shadow paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7::: tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7::: laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7::: valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7 GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7::: [root@centos7 ~]#
22.3. encryption with passwd Passwords are stored in an encrypted format. This encryption is done by the crypt function. The easiest (and recommended) way to add a user with a password to the system is to add the user with the useradd -m user command, and then set the user's password with passwd. 22.4. encryption with openssl Another way to create users with a password is to use the -p option of useradd, but that option requires an encrypted password. You can generate this encrypted password with the openssl passwd command. The openssl passwd command will generate several distinct hashes for the same password, for this it uses a salt. This salt can be chosen and is visible as the first two characters of the hash. This example shows how to create a user with password. root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed Note that this command puts the password in your command history! [root@RHEL4 ~]# useradd -m xavier [root@RHEL4 ~]# passwd xavier Changing password for user xavier. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL4 ~]# paul@rhel65:~$ openssl passwd hunter2 86jcUNlnGDFpY paul@rhel65:~$ openssl passwd hunter2 Yj7mDO9OAnvq6 paul@rhel65:~$ openssl passwd hunter2 YqDcJeGoDbzKA paul@rhel65:~$ paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$
22.5. encryption with crypt A third option is to create your own C program using the crypt function, and compile this into a command. This little program can be compiled with gcc like this. paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password, the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096 different ways. This variation prevents two users with the same password from having the same entry in /etc/shadow. Did you notice that the first two characters of the password are the salt? The standard output of the crypt function is using the DES algorithm which is old and can be cracked in minutes. A better method is to use md5 passwords which can be recognized by a salt starting with $1$. The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between the second and third $, so never use the password as the salt! paul@rhel65:~$ cat MyCrypt.c #include <stdio.h> #define USE_XOPEN #include <unistd.h> int main(int argc, char** argv) { if(argc==3) { printf("%sn", crypt(argv[1],argv[2])); } else { printf("Usage: MyCrypt $password $saltn" ); } return 0; } paul@rhel65:~$ ./MyCrypt hunter2 42 42ZrbtP1Ze8G. paul@rhel65:~$ ./MyCrypt hunter2 33 33d6taYSiEUXI paul@rhel65:~$ ./MyCrypt hunter2 '$1$42' $1$42$7l6Y3xT5282XmZrtDOF9f0 paul@rhel65:~$ ./MyCrypt hunter2 '$6$42' $6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i... paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2' $1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.
22.6. /etc/login.defs The /etc/login.defs file contains some default settings for user passwords like password aging and length settings. (You will also find the numerical limits of user ids and group ids and whether or not a home directory should be created by default). Debian also has this file. 22.7. chage The chage command can be used to set an expiration date for a user account (-E), set a minimum (-m) and maximum (-M) password age, a password expiration date, and set the number of warning days before the password expiration date. Much of this functionality is also available from the passwd command. The -l option of chage will list these settings for a user. root@rhel65:~# grep ^PASS /etc/login.defs PASS_MAX_DAYS 99999 PASS_MIN_DAYS 0 PASS_MIN_LEN 5 PASS_WARN_AGE 7 root@debian7:~# grep PASS /etc/login.defs # PASS_MAX_DAYS # PASS_MIN_DAYS # PASS_WARN_AGE PASS_MAX_DAYS PASS_MIN_DAYS PASS_WARN_AGE Maximum number of days a password may be used. Minimum number of days allowed between password changes. Number of days warning given before a password expires. 99999 #PASS_CHANGE_TRIES #PASS_ALWAYS_WARN #PASS_MIN_LEN #PASS_MAX_LEN # NO_PASSWORD_CONSOLE root@debian7:~# root@rhel65:~# chage -l paul Last password change Password expires Password inactive Account expires Minimum number of days between password change Maximum number of days between password change Number of days of warning before password expires root@rhel65:~# : Mar 27, 2014 : never : never : never : 0 : 99999 : 7
22.8. disabling a password Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field in /etc/passwd starts with an exclamation mark, then the password can not be used. Using this feature is often called locking, disabling, or suspending a user account. Besides vi (or vipw) you can also accomplish this with usermod. The first command in the next screenshot will show the hashed password of laura in /etc/ shadow. The next command disables the password of laura, making it impossible for Laura to authenticate using this password. As you can see below, the password hash is simply preceded with an exclamation mark. The root user (and users with sudo rights on su) still will be able to su into the laura account (because the password is not needed here). Also note that laura will still be able to login if she has set up passwordless ssh! You can unlock the account again with usermod -U. Watch out for tiny differences in the command line options of passwd, usermod, and useradd on different Linux distributions. Verify the local files when using features like "disabling, suspending, or locking" on user accounts and their passwords. 22.9. editing local files If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these commands for password management, then use vipw instead of vi(m) directly. The vipw tool will do proper locking of the file. root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV root@debian7:~# usermod -L laura root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:!$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJ root@debian7:~# root@debian7:~# su - laura laura@debian7:~$ root@debian7:~# usermod -U laura root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV [root@RHEL5 ~]# vipw /etc/passwd vipw: the password file is busy (/etc/ptmp present)
22.10. solution: user passwords 1. Set the password for serena to hunter2. 2. Also set a password for venus and then lock the venus user account with usermod. Verify the locking in /etc/shadow before and after you lock it. Note that usermod -L precedes the password hash with an exclamation mark (!). 3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow before and after disabling. 4. What is the difference between locking a user account and disabling a user account's password like we just did with usermod -L and passwd -d? Locking will prevent the user from logging on to the system with his password by putting a ! in front of the password in /etc/shadow. Disabling with passwd will erase the password from /etc/shadow. 5. Try changing the password of serena to serena as serena. log on as serena, then execute: passwd serena... it should fail! 6. Make sure serena has to change her password in 10 days. chage -M 10 serena 7. Make sure every new user needs to change their password every 10 days. vi /etc/login.defs (and change PASS_MAX_DAYS to 10) root@debian7:~# passwd serena Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@debian7:~# passwd venus Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@debian7:~# grep venus /etc/shadow | cut -c1-70 venus:$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM/ root@debian7:~# usermod -L venus root@debian7:~# grep venus /etc/shadow | cut -c1-70 venus:!$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM root@debian7:~# grep serena /etc/shadow | cut -c1-70 serena:$6$Es/omrPE$F2Ypu8kpLrfKdW0v/UIwA5jrYyBD2nwZ/dt.i/IypRgiPZSdB/B root@debian7:~# passwd -d serena passwd: password expiry information changed. root@debian7:~# grep serena /etc/shadow serena::16358:0:99999:7::: root@debian7:~#
8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from venus to serena. Can serena now log on with hunter2 as a password ? 9. Why use vipw instead of vi ? What could be the problem when using vi or vim ? 10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat / etc/shells. 11. Which useradd option allows you to name a home directory ? 12. How can you see whether the password of user serena is locked or unlocked ? Give a solution with grep and a solution with passwd. Yes. vipw will give a warning when someone else is already using that file (with vipw). chsh -l cat /etc/shells -d grep serena /etc/shadow passwd -S serena
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Chapter 23. user profiles Logged on users have a number of preset (and customized) aliases, variables, and functions, but where do they come from ? The shell uses a number of startup files that are executed (or rather sourced) whenever the shell is invoked. What follows is an overview of startup scripts.
23.1. system profile Both the bash and the ksh shell will verify the existence of /etc/profile and source it if it exists. When reading this script, you will notice (both on Debian and on Red Hat Enterprise Linux) that it builds the PATH environment variable (among others). The script might also change the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc This screenshot uses grep to show PATH manipulation in /etc/profile on Debian. This screenshot uses grep to show PATH manipulation in /etc/profile on RHEL7/CentOS7. The root user can use this script to set aliases, functions, and variables for every user on the system. 23.2. ~/.bash_profile When this file exists in the home directory, then bash will source it. On Debian Linux 5/6/7 this file does not exist by default. RHEL7/CentOS7 uses a small ~/.bash_profile where it checks for the existence of ~/.bashrc and then sources it. It also adds $HOME/bin to the $PATH variable. root@debian7:~# grep PATH /etc/profile PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games" export PATH root@debian7:~# [root@centos7 ~]# grep PATH /etc/profile case ":${PATH}:" in PATH=$PATH:$1 PATH=$1:$PATH export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL [root@centos7 ~]# [root@rhel7 ~]# cat /home/paul/.bash_profile # .bash_profile # Get the aliases and functions if [ -f ~/.bashrc ]; then . ~/.bashrc fi # User specific environment and startup programs PATH=$PATH:$HOME/.local/bin:$HOME/bin export PATH [root@rhel7 ~]#
23.3. ~/.bash_login When .bash_profile does not exist, then bash will check for ~/.bash_login and source it. Neither Debian nor Red Hat have this file by default. 23.4. ~/.profile When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence of ~/.profile and execute it. This file does not exist by default on Red Hat. On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH variable. RHEL/CentOS does not have this file by default. 23.5. ~/.bashrc The ~/.bashrc script is often sourced by other scripts. Let us take a look at what it does by default. Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and sourcing it. It also leaves room for custom aliases and functions. On Debian this script is quite a bit longer and configures $PS1, some history variables and a number af active and inactive aliases. root@debian7:~# tail -11 /home/paul/.profile if [ -n "$BASH_VERSION" ]; then # include .bashrc if it exists if [ -f "$HOME/.bashrc" ]; then . "$HOME/.bashrc" fi fi # set PATH so it includes user's private bin if it exists if [ -d "$HOME/bin" ] ; then PATH="$HOME/bin:$PATH" fi [root@rhel7 ~]# cat /home/paul/.bashrc # .bashrc # Source global definitions if [ -f /etc/bashrc ]; then . /etc/bashrc fi # Uncomment the following line if you don't like systemctl's auto-paging feature: # export SYSTEMD_PAGER= # User specific aliases and functions root@debian7:~# wc -l /home/paul/.bashrc 110 /home/paul/.bashrc
23.6. ~/.bash_logout When exiting bash, it can execute ~/.bash_logout. Debian use this opportunity to clear the console screen. Red Hat Enterprise Linux 5 will simple call the /usr/bin/clear command in this script. Red Hat Enterprise Linux 6 and 7 create this file, but leave it empty (except for a comment). serena@deb503:~$ cat .bash_logout # ~/.bash_logout: executed by bash(1) when login shell exits. # when leaving the console clear the screen to increase privacy if [ "$SHLVL" = 1 ]; then [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q fi [serena@rhel53 ~]$ cat .bash_logout # ~/.bash_logout /usr/bin/clear paul@rhel65:~$ cat .bash_logout # ~/.bash_logout
23.7. Debian overview Below is a table overview of when Debian is running any of these bash startup scripts. Table 30.1. Debian User Environment script su su - ssh gdm ~./bashrc no yes yes yes ~/.profile no yes yes yes /etc/profile no yes yes yes /etc/bash.bashrc yes no no yes 23.8. RHEL5 overview Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash startup scripts. Table 30.2. Red Hat User Environment script su su - ssh gdm ~./bashrc yes yes yes yes ~/.bash_profile no yes yes yes /etc/profile no yes yes yes /etc/bashrc yes yes yes yes
23.9. solution: user profiles 1. Make a list of all the profile files on your system. ls -a ~ ; ls -l /etc/pro* /etc/bash* 2. Read the contents of each of these, often they source extra scripts. 3. Put a unique variable, alias and function in each of those files. 4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrl- alt- F1, ...) and verify which of your custom variables, aliases and function are present in your environment. 5. Do you also know the order in which they are executed? same name aliases, functions and variables will overwrite each other 6. When an application depends on a setting in $HOME/.profile, does it matter whether $HOME/.bash_profile exists or not ? Yes it does matter. (man bash /INVOCATION)
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Chapter 24. groups Users can be listed in groups. Groups allow you to set permissions on the group level instead of having to set permissions for every individual user. Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users are advised to use this graphical tool. More experienced users can use command line tools to manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant files directly with vi or vigr.
24.1. groupadd Groups can be created with the groupadd command. The example below shows the creation of five (empty) groups. 24.2. group file Users can be a member of several groups. Group membership is defined by the /etc/group file. The first field is the group's name. The second field is the group's (encrypted) password (can be empty). The third field is the group identification or GID. The fourth field is the list of members, these groups have no members. 24.3. groups A user can type the groups command to see a list of groups where the user belongs to. root@laika:~# groupadd tennis root@laika:~# groupadd football root@laika:~# groupadd snooker root@laika:~# groupadd formula1 root@laika:~# groupadd salsa root@laika:~# tail -5 /etc/group tennis:x:1006: football:x:1007: snooker:x:1008: formula1:x:1009: salsa:x:1010: root@laika:~# [harry@RHEL4b ~]$ groups harry sports [harry@RHEL4b ~]$
24.4. usermod Group membership can be modified with the useradd or usermod command. Be careful when using usermod to add users to groups. By default, the usermod command will remove the user from every group of which he is a member if the group is not listed in the command! Using the -a (append) switch prevents this behaviour. 24.5. groupmod You can change the group name with the groupmod command. 24.6. groupdel You can permanently remove a group with the groupdel command. root@laika:~# usermod -a -G tennis inge root@laika:~# usermod -a -G tennis katrien root@laika:~# usermod -a -G salsa katrien root@laika:~# usermod -a -G snooker sandra root@laika:~# usermod -a -G formula1 annelies root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: snooker:x:1008:sandra formula1:x:1009:annelies salsa:x:1010:katrien root@laika:~# root@laika:~# groupmod -n darts snooker root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: formula1:x:1009:annelies salsa:x:1010:katrien darts:x:1008:sandra root@laika:~# groupdel tennis root@laika:~#
24.7. gpasswd You can delegate control of group membership to another user with the gpasswd command. In the example below we delegate permissions to add and remove group members to serena for the sports group. Then we su to serena and add harry to the sports group. Group administrators do not have to be a member of the group. They can remove themselves from a group, but this does not influence their ability to add or remove members. Information about group administrators is kept in the /etc/gshadow file. To remove all group administrators from a group, use the gpasswd command to set an empty administrators list. [root@RHEL4b ~]# gpasswd -A "" sports [root@RHEL4b ~]# gpasswd -A serena sports [root@RHEL4b ~]# su - serena [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry) [serena@RHEL4b ~]$ gpasswd -a harry sports Adding user harry to group sports [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry),522(sports) [serena@RHEL4b ~]$ tail -1 /etc/group sports:x:522:serena,venus,harry [serena@RHEL4b ~]$ [serena@RHEL4b ~]$ gpasswd -d serena sports Removing user serena from group sports [serena@RHEL4b ~]$ exit [root@RHEL4b ~]# tail -1 /etc/gshadow sports:!:serena:venus,harry [root@RHEL4b ~]#
24.8. newgrp You can start a child shell with a new temporary primary group using the newgrp command. 24.9. vigr Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since it will do proper locking of the file. Only experienced senior administrators should use vi or root@rhel65:~# mkdir prigroup root@rhel65:~# cd prigroup/ root@rhel65:~/prigroup# touch standard.txt root@rhel65:~/prigroup# ls -l total 0 -rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt root@rhel65:~/prigroup# echo $SHLVL 1 root@rhel65:~/prigroup# newgrp tennis root@rhel65:~/prigroup# echo $SHLVL 2 root@rhel65:~/prigroup# touch newgrp.txt root@rhel65:~/prigroup# ls -l total 0 -rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt -rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt root@rhel65:~/prigroup# exit exit root@rhel65:~/prigroup#
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24.10. solution: groups 1. Create the groups tennis, football and sports. 2. In one command, make venus a member of tennis and sports. 3. Rename the football group to foot. 4. Use vi to add serena to the tennis group. 5. Use the id command to verify that serena is a member of tennis. 6. Make someone responsible for managing group membership of foot and sports. Test that it works. groupadd tennis ; groupadd football ; groupadd sports usermod -a -G tennis,sports venus groupmod -n foot football vi /etc/group id (and after logoff logon serena should be member) gpasswd -A (to make manager) gpasswd -a (to add member)
Chapter 25. standard file permissions This chapter contains details about basic file security through file ownership and file permissions.
25.1. file ownership 25.1.1. user owner and group owner The users and groups of a system can be locally managed in /etc/passwd and /etc/group, or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files. Actually, every file has a user owner and a group owner, as can be seen in the following screenshot. paul@rhel65:~/owners$ ls -lh total 636K -rw-r--r--. 1 paul snooker 1.1K Apr 8 18:47 data.odt -rw-r--r--. 1 paul paul 626K Apr 8 18:46 file1 -rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2 -rw-rw-r--. 1 root root paul@rhel65:~/owners$ 0 Apr 8 18:47 stuff.txt User paul owns three files; file1 has paul as user owner and has the group paul as group owner, data.odt is group owned by the group snooker, file2 by the group tennis. The last file is called stuff.txt and is owned by the root user and the root group. 25.1.2. listing user accounts You can use the following command to list all local user accounts. paul@debian7~$ cut -d: -f1 /etc/passwd | column root ntp sam bert naomi daemon mysql tom rino matthias2 bin paul wouter antonio bram sys maarten robrecht simon fabrice sync kevin bilal sven chimene games yuri dimitri wouter2 messagebus man william ahmed tarik roger lp yves dylan jan frank mail kris robin ian toon news hamid matthias ivan rinus uucp vladimir ben azeddine eddy proxy abiy mike eric bram2 www-data david kevin2 kamel keith backup chahid kenzo ischa jesse list stef aaron bart frederick irc joeri lorenzo omer hans gnats glenn jens kurt dries nobody yannick ruben steve steve2 libuuid christof jelle constantin tomas Debian-exim george stefaan sam2 johan statd joost marc bjorn tom2 sshd arno thomas ronald
25.1.3. chgrp You can change the group owner of a file using the chgrp command. 25.1.4. chown The user owner of a file can be changed with chown command. You can also use chown to change both the user owner and the group owner. root@rhel65:/home/paul/owners# ls -l file2 -rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2 root@rhel65:/home/paul/owners# chgrp snooker file2 root@rhel65:/home/paul/owners# ls -l file2 -rw-r--r--. 1 root snooker 185 Apr 8 18:46 file2 root@rhel65:/home/paul/owners# root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown paul FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown root:project42 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul
25.2. list of special files When you use ls -l, for each file you can see ten characters before the user and group owner. The first character tells us the type of file. Regular files get a -, directories get a d, symbolic links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets an s. Table 32.1. Unix special files first character file type - normal file d directory l symbolic link p named pipe b block device c character device s socket Below a screenshot of a character device (the console) and a block device (the hard disk). And here you can see a directory, a regular file and a symbolic link. paul@debian6lt~$ ls -ld /dev/console /dev/sda crw------- 1 root root 5, 1 Mar 15 12:45 /dev/console brw-rw---- 1 root disk 8, 0 Mar 15 12:45 /dev/sda paul@debian6lt~$ ls -ld /etc /etc/hosts /etc/motd drwxr-xr-x 128 root root 12288 Mar 15 18:34 /etc -rw-r--r-- 1 root root 372 Dec 10 17:36 /etc/hosts lrwxrwxrwx 1 root root 13 Dec 5 10:36 /etc/motd -> /var/run/motd
25.3. permissions 25.3.1. rwx The nine characters following the file type denote the permissions in three triplets. A permission can be r for read access, w for write access, and x for execute. You need the r permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a directory. You need the w permission to create files in or remove files from a directory. Table 32.2. standard Unix file permissions permission on a file on a directory r (read) read file contents (cat) read directory contents (ls) w (write) change file contents (vi) create files in (touch) x (execute) execute the file enter the directory (cd) 25.3.2. three sets of rwx We already know that the output of ls -l starts with ten characters for each file. This screenshot shows a regular file (because the first character is a - ). Below is a table describing the function of all ten characters. Table 32.3. Unix file permissions position position characters function 1 - this is a regular file 2-4 rwx permissions for the user owner 5-7 r-x permissions for the group owner 8-10 r-- permissions for others When you are the user owner of a file, then the user owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you belong to the group that is the group owner of a file, then the group owner permissions apply to you. The rest of the permissions have no influence on your access to paul@RHELv4u4:~/test$ ls -l proc42.bash -rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash
the file. When you are not the user owner of a file and you do not belong to the group owner, then the others permissions apply to you. The rest of the permissions have no influence on your access to the file.
25.3.3. permission examples Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions. paul@laika:~/perms$ ls total 12K -lh drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete -rwxrwxrwx 1 paul paul 0 2007-02-07 22:21 EveryoneFullControl.txt -r--r----- 1 paul paul 0 2007-02-07 22:21 OnlyOwnersRead.txt -rwxrwx--- 1 paul paul 0 2007-02-07 22:21 OwnersAll_RestNothing.txt dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter dr-x------ 2 paul paul paul@laika:~/perms$ 4.0K 2007-02-07 22:25 OnlyUserEnter To summarise, the first rwx triplet represents the permissions for the user owner. The second triplet corresponds to the group owner; it specifies permissions for all members of that group. The third triplet defines permissions for all other users that are not the user owner and are not a member of the group owner.
25.3.4. setting permissions (chmod) Permissions can be changed with chmod. The first example gives the user owner execute permissions. This example removes the group owners read permission. This example removes the others read permission. This example gives all of them the write permission. You don't even have to type the a. You can also set explicit permissions. Feel free to make any kind of combination. Even fishy combinations are accepted by chmod. paul@laika:~/perms$ ls -l permissions.txt -rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u+x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod g-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod o-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod a+w permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod +x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rw permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
25.3.5. setting octal permissions Most Unix administrators will use the old school octal system to talk about and set permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1. Table 32.4. Octal permissions binary octal permission 000 0 --- 001 1 --x 010 2 -w- 011 3 -wx 100 4 r-- 101 5 r-x 110 6 rw- 111 7 rwx This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod command will accept these numbers. paul@laika:~/perms$ chmod 777 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 664 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 750 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
25.3.6. umask When creating a file or directory, a set of default permissions are applied. These default permissions are determined by the umask. The umask specifies permissions that you do not want set on by default. You can display the umask with the umask command. As you can also see, the file is also not executable by default. This is a general security feature among Unixes; newly created files are never executable by default. You have to explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the umask has no meaning--a umask of 0022 is the same as 0033. 25.3.7. mkdir -m When creating directories with mkdir you can use the -m option to set the mode. This screenshot explains. 25.3.8. cp -p To preserve permissions and time stamps from source files, use cp -p. paul@laika:~/perms$ cp file* cp paul@laika:~/perms$ cp -p file* cpp paul@laika:~/perms$ ll * -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 cp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:34 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42 cpp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 [Harry@RHEL4b ~]$ umask 0002 [Harry@RHEL4b ~]$ touch test [Harry@RHEL4b ~]$ ls -l test -rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test [Harry@RHEL4b ~]$ paul@debian5~$ mkdir -m 700 MyDir paul@debian5~$ mkdir -m 777 Public paul@debian5~$ ls -dl MyDir/ Public/ drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/ drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/
25.4. solution: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. mkdir ~/permissions ; touch ~/permissions/myfile.txt 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? cp /etc/hosts ~/permissions/ The copy is owned by you. 3. As root, create a file in the users ~/permissions directory. (become root)# touch /home/username/permissions/rootfile 4. As normal user, look at who owns this file created by root. ls -l ~/permissions The file created by root is owned by root. 5. Change the ownership of all files in ~/permissions to yourself. chown user ~/permissions/* You cannot become owner of the file that belongs to root. 6. Make sure you have all rights to these files, and others can only read. chmod 644 (on files) chmod 755 (on directories) 7. With chmod, is 770 the same as rwxrwx--- ? yes 8. With chmod, is 664 the same as r-xr-xr-- ? No 9. With chmod, is 400 the same as r-------- ? yes 10. With chmod, is 734 the same as rwxr-xr-- ? no 11a. Display the umask in octal and in symbolic form. umask ; umask -S 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. umask -S u=rwx,go=
12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. Yes, others can read this file 13b. Can root read this file ? Can root write to this file with vi ? Yes, root can read and write to this file. Permissions do not apply to root. 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. You can not yet do the last part of this exercise... (become root) # echo hello > /home/username/root.txt # chmod 744 /home/username/root.txt (become user) vi ~/root.txt echo hello > file ; chmod 744 file mkdir /home/project42 ; groupadd project42 chgrp project42 /home/project42 ; chmod 775 /home/project42
319 Chapter 26. advanced file permissions
26.1. sticky bit on directory You can set the sticky bit on a directory to prevent users from removing files that they do not own as a user owner. The sticky bit is displayed at the same location as the x permission for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there is no x for others). The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets. You will typically find the sticky bit on the /tmp directory. 26.2. setgid bit on directory setgid can be used on directories to make sure that all files inside the directory are owned by the group owner of the directory. The setgid bit is displayed at the same location as the x permission for group owner. The setgid bit is represented by an s (meaning x is also there) or a S (when there is no x for the group owner). As this example shows, even though root does not belong to the group proj55, the files created by root in /project55 will belong to proj55 since the setgid is set. You can use the find command to find all setgid directories. root@RHELv4u4:~# mkdir /project55 root@RHELv4u4:~# ls -ld /project55 drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# chmod +t /project55/ root@RHELv4u4:~# ls -ld /project55 drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# root@RHELv4u4:~# chmod 1775 /project55/ root@RHELv4u4:~# ls -ld /project55 drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# root@barry:~# ls -ld /tmp drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp root@RHELv4u4:~# groupadd proj55 root@RHELv4u4:~# chown root:proj55 /project55/ root@RHELv4u4:~# chmod 2775 /project55/ root@RHELv4u4:~# touch /project55/fromroot.txt root@RHELv4u4:~# ls -ld /project55/ drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/ root@RHELv4u4:~# ls -l /project55/ total 4 -rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt root@RHELv4u4:~# paul@laika:~$ find / -type d -perm -2000 2> /dev/null /var/log/mysql /var/log/news /var/local ...
26.3. setgid and setuid on regular files These two permissions cause an executable file to be executed with the permissions of the file owner instead of the executing owner. This means that if any user executes aprogram that belongs to the root user, and the setuid bit is set on that program, then the program runs as root. This can be dangerous, but sometimes this is good for security. Take the example of passwords; they are stored in /etc/shadow which is only readable by root. (The root user never needs permissions anyway.) Changing your password requires an update of this file, so how can normal non-root users do this? Let's take a look at the permissions on the /usr/bin/passwd. When running the passwd program, you are executing it with root credentials. You can use the find command to find all setuid programs. In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will result in these programs to run with the credentials of their group owner. 26.4. setuid on sudo The sudo binary has the setuid bit set, so any user can run it with the effective userid of root. paul@laika:~$ find /usr/bin -type f -perm -04000 /usr/bin/arping /usr/bin/kgrantpty /usr/bin/newgrp /usr/bin/chfn /usr/bin/sudo /usr/bin/fping6 /usr/bin/passwd /usr/bin/gpasswd ... root@RHELv4u4:~# ls -l /etc/shadow -r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow root@RHELv4u4:~# ls -l /usr/bin/passwd -r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd paul@rhel65:~$ ls -l $(which sudo) ---s--x--x. 1 root root 123832 Oct 7 2013 /usr/bin/sudo paul@rhel65:~$
26.5. solution: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. groupadd sports mkdir /home/sports chown root:sports /home/sports 1b. Members of the sports group should be able to create files in this directory. chmod 770 /home/sports 1c. All files created in this directory should be group-owned by the sports group. chmod 2770 /home/sports 1d. Users should be able to delete only their own user-owned files. chmod +t /home/sports 1e. Test that this works! Log in with different users (group members and others and root), create files and watch the permissions. Try changing and deleting files... 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. A normal user cannot change password now. 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 4755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd paul@laika:~$ sudo su - [sudo] password for paul: root@laika:~# mkdir attr root@laika:~# cd attr/ root@laika:~/attr# touch file42 root@laika:~/attr# lsattr ./file42 root@laika:~/attr# chattr +i file42
root@laika:~/attr# lsattr ----i------------- ./file42 root@laika:~/attr# rm -rf file42 rm: cannot remove `file42': Operation not permitted root@laika:~/attr# chattr -i file42 root@laika:~/attr# rm -rf file42 root@laika:~/attr#
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Chapter 27. access control lists Standard Unix permissions might not be enough for some organisations. This chapter introduces access control lists or acl's to further protect files and directories.
27.1. acl in /etc/fstab File systems that support access control lists, or acls, have to be mounted with the acl option listed in /etc/fstab. In the example below, you can see that the root file system has acl support, whereas /home/data does not. root@laika:~# tail -4 /etc/fstab /dev/sda1 / ext3 acl,relatime 0 1 /dev/sdb2 /home/data auto noacl,defaults 0 0 pasha:/home/r /home/pasha nfs defaults 0 0 wolf:/srv/data /home/wolf nfs defaults 0 0 27.2. getfacl Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl of file33 with getfacl. 27.3. setfacl Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how to change the acl of file33 with setfacl. First we add user sandra with octal permission 7 to the acl. paul@laika:~/test$ setfacl -m u:sandra:7 file33 Then we add the group tennis with octal permission 6 to the acl of the same file. paul@laika:~/test$ setfacl -m g:tennis:6 file33 The result is visible with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- mask::rwx other::r-- paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- user:sandra:rwx group::r-- group:tennis:rw- mask::rwx other::r--
27.4. remove an acl entry The -x option of the setfacl command will remove an acl entry from the targeted file. Note that omitting the u or g when defining the acl for an account will default it to a user account. 27.5. remove the complete acl The -b option of the setfacl command will remove the acl from the targeted file. 27.6. the acl mask The acl mask defines the maximum effective permissions for any entry in the acl. This mask is calculated every time you execute the setfacl or chmod commands. You can prevent the calculation by using the --no-mask switch. paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- user:sandra:rwx #effective:rw- group::r-- mask::rw- other::r-- paul@laika:~/test$ setfacl -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 | grep sandra user:sandra:rwx paul@laika:~/test$ setfacl -x sandra file33 paul@laika:~/test$ getfacl file33 | grep sandra paul@laika:~/test$ setfacl -b file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- other::r--
27.7. eiciel Desktop users might want to use eiciel to manage acls with a graphical tool. You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to manage acls. paul@laika:~$ sudo aptitude install eiciel nautilus-actions
Chapter 28. file links An average computer using Linux has a file system with many hard links and symbolic links. To understand links in a file system, you first have to understand what an inode is.
28.1. inodes 28.1.1. inode contents An inode is a data structure that contains metadata about a file. When the file system stores a new file on the hard disk, it stores not only the contents (data) of the file, but also extra properties like the name of the file, the creation date, its permissions, the owner of the file, and more. All this information (except the name of the file and the contents of the file) is stored in the inode of the file. The ls -l command will display some of the inode contents, as seen in this screenshot. 28.1.2. inode table The inode table contains all of the inodes and is created when you create the file system (with mkfs). You can use the df -i command to see how many inodes are used and free on mounted file systems. root@rhel53 ~# df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/mapper/VolGroup00-LogVol00 4947968 115326 4832642 3% / /dev/hda1 26104 45 26059 1% /boot tmpfs 64417 1 64416 1% /dev/shm /dev/sda1 262144 2207 259937 1% /home/project42 /dev/sdb1 74400 5519 68881 8% /home/project33 /dev/sdb5 0 0 0 - /home/sales /dev/sdb6 100744 11 100733 1% /home/research In the df -i screenshot above you can see the inode usage for several mounted file systems. You don't see numbers for /dev/sdb5 because it is a fat file system. 28.1.3. inode number Each inode has a unique number (the inode number). You can see the inode numbers with the ls -li command. These three files were created one after the other and got three different inodes (the first column). All the information you see with this ls command resides in the inode, except for the filename (which is contained in the directory). root@rhel53 ~# ls -ld /home/project42/ drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/ paul@RHELv4u4:~/test$ touch file1 paul@RHELv4u4:~/test$ touch file2 paul@RHELv4u4:~/test$ touch file3 paul@RHELv4u4:~/test$ ls -li total 12 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817267 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$
28.1.4. inode and file contents Let's put some data in one of the files. The data that is displayed by the cat command is not in the inode, but somewhere else on the disk. The inode contains a pointer to that data. 28.2. about directories 28.2.1. a directory is a table A directory is a special kind of file that contains a table which maps filenames to inodes. Listing our current directory with ls -ali will display the contents of the directory file. paul@RHELv4u4:~/test$ ls -ali total 32 817262 drwxrwxr-x 2 paul paul 4096 Feb 5 15:42 . 800768 drwx------ 16 paul paul 4096 Feb 5 15:42 .. 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul@RHELv4u4:~/test$ paul paul 0 Feb 5 15:38 file3 28.2.2. . and .. You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself, and the dotdot .. is a mapping to the parent directory. The three other names are mappings to different inodes. paul@RHELv4u4:~/test$ ls -li total 16 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$ cat file2 It is winter now and it is very cold. We do not like the cold, we prefer hot summer nights. paul@RHELv4u4:~/test$
28.3. hard links 28.3.1. creating hard links When we create a hard link to a file with ln, an extra entry is added in the directory. A new file name is mapped to an existing inode. Both files have the same inode, so they will always have the same permissions and the same owner. Both files will have the same content. Actually, both files are equal now, meaning you can safely remove the original file, the hardlinked file will remain. The inode contains a counter, counting the number of hard links to itself. When the counter drops to zero, then the inode is emptied. 28.3.2. finding hard links You can use the find command to look for files with a certain inode. The screenshot below shows how to search for all filenames that point to inode 817270. Remember that aninode number is unique to its partition. paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 24 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2 paul@RHELv4u4:~/test$ paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null /home/paul/test/file2 /home/paul/test/hardlink_to_file2
28.4. symbolic links Symbolic links (sometimes called soft links) do not link to inodes, but create a name to name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic link gets an inode of its own. paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 32 817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2 817267 lrwxrwxrwx 1 paul paul@RHELv4u4:~/test$ paul 5 Feb 5 16:55 symlink_to_file2 -> file2 Permissions on a symbolic link have no meaning, since the permissions of the target apply. Hard links are limited to their own partition (because they point to an inode), symbolic links can link anywhere (other file systems, even networked). 28.5. removing links Links can be removed with rm. 28.6. solution : links 1. Create two files named winter.txt and summer.txt, put some text in them. 2. Create a hard link to winter.txt named hlwinter.txt. 3. Display the inode numbers of these three files, the hard links should have the same inode. 4. Use the find command to list the two hardlinked files 5. Everything about a file is in the inode, except two things : name them! The name of the file is in a directory, and the contents is somewhere on the disk. 6. Create a symbolic link to summer.txt called slsummer.txt. paul@laika:~$ touch data.txt paul@laika:~$ ln -s data.txt sl_data.txt paul@laika:~$ ln data.txt hl_data.txt paul@laika:~$ rm sl_data.txt paul@laika:~$ rm hl_data.txt echo cold > winter.txt ; echo hot > summer.txt ln winter.txt hlwinter.txt ls -li winter.txt summer.txt hlwinter.txt find . -inum xyz #replace xyz with the inode number ln -s summer.txt slsummer.txt
7. Find all files with inode number 2. What does this information tell you ? It tells you there is more than one inode table (one for every formatted partition + virtual file systems) 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? 9. Look in /lib with ls -l... 10. Use find to look in your home directory for regular files that do not(!) have one hard link. ls -l /etc/init.d ls -l /etc/rc2.d ls -l /etc/rc3.d ls -l /lib find ~ ! -links 1 -type f
Appendix A. keyboard settings A.1. about keyboard layout Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when you are not from the USA and want a local keyboard layout on your system, then the best practice is to select this keyboard at installation time. Then the keyboard layout will always be correct. Also, whenever you use ssh to remotely manage a Linux system, your local keyboard layout will be used, independent of the server keyboard configuration. So you will not find much information on changing keyboard layout on the fly on linux, because not many people need it. Below are some tips to help you. A.2. X Keyboard Layout This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US- qwerty. When in Gnome or KDE or any other graphical environment, look in the graphical menu in preferences, there will be a keyboard section to choose your layout. Use the graphical menu instead of editing xorg.conf. A.3. shell keyboard layout When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US-qwerty configuration, followed by a Belgian azerty configuration. The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps. [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "be" [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="be-latin1" [paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/ total 52 drwxr-xr-x 2 root root 4096 Apr 1 00:14 amiga
linux.pdf
339 keyboard settings drwxr-xr-x 2 root root 4096 Apr 1 00:14 atari drwxr-xr-x 8 root root 4096 Apr 1 00:14 i386 drwxr-xr-x 2 root root 4096 Apr 1 00:14 include drwxr-xr-x 4 root root 4096 Apr 1 00:14 mac lrwxrwxrwx 1 root root 3 Apr 1 00:14 ppc -> mac drwxr-xr-x 2 root root 4096 Apr 1 00:14 sun
Appendix B. hardware B.1. buses B.1.1. about buses Hardware components communicate with the Central Processing Unit or cpu over a bus. The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card. These are all Plag and Play buses. Older x86 computers often had isa buses, which can be configured using jumpers or dip switches. B.1.2. /proc/bus To list the buses recognised by the Linux kernel on your computer, look at the contents of the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below). Can you guess which of these two screenshots was taken on a laptop ? B.1.3. /usr/sbin/lsusb To list all the usb devices connected to your system, you could read the contents of /proc/ bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is executed here on a SPARC system with Ubuntu. B.1.4. /var/lib/usbutils/usb.ids The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices. root@laika:~# ls /proc/bus/ input pccard pci usb [root@RHEL4b ~]# ls /proc/bus/ input pci usb root@shaka:~# lsusb Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage) root@shaka:~# paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head ------> /var/lib/usbutils/usb.ids <------ # # List of USB ID's # # Maintained by Vojtech Pavlik <vojtech@suse.cz>
hardware 341 B.1.5. /usr/sbin/lspci To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci (partial output below). B.2. interrupts B.2.1. about interrupts An interrupt request or IRQ is a request from a device to the CPU. A device raises an interrupt when it requires the attention of the CPU (could be because the device has data ready to be read by the CPU). Since the introduction of pci, irq's can be shared among devices. Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a channel for IRQ's 8 to 15, and thus is the same as IRQ 9. B.2.2. /proc/interrupts You can see a listing of interrupts on your system in /proc/interrupts. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 1320048 555 IO-APIC-edge timer 1: 10224 7 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 2 1 IO-APIC-edge rtc 10: 3062 21 IO-APIC-fasteoi acpi 12: 131 2 IO-APIC-edge i8042 15: 47073 0 IO-APIC-edge ide1 18: 0 1 IO-APIC-fasteoi yenta 19: 31056 1 IO-APIC-fasteoi libata, ohci1394 20: 19042 1 IO-APIC-fasteoi eth0 21: 44052 1 IO-APIC-fasteoi uhci_hcd:usb1, uhci_hcd:usb2,... 22: 188352 1 IO-APIC-fasteoi ra0 # If you have any new entries, send them to the maintainer. # The latest version can be obtained from # http://www.linux-usb.org/usb.ids # # $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $ paul@laika:~$ lspci ... 00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139... 00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816... 00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713... 00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI 00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ... 00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A... 00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... 00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... ...
23: 632444 1 IO-APIC-fasteoi nvidia 24: 1585 1 IO-APIC-fasteoi VIA82XX-MODEM, VIA8237 B.2.3. dmesg You can also use dmesg to find irq's allocated at boot time. B.3. io ports B.3.1. about io ports Communication in the other direction, from CPU to device, happens through IO ports. The CPU writes data or control codes to the IO port of the device. But this is not only a one way communication, the CPU can also use a device's IO port to read status information about the device. Unlike interrupts, ports cannot be shared! B.3.2. /proc/ioports You can see a listing of your system's IO ports via /proc/ioports. B.4. dma B.4.1. about dma A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With dma or Direct Memory Access a device can gain (temporary) access to a specific range of the ram memory. B.4.2. /proc/dma Looking at /proc/dma might not give you the information that you want, since it only contains currently assigned dma channels for isa devices. paul@laika:~$ dmesg | grep "irq 1[45]" [ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14 [ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15 [root@RHEL4b ~]# cat /proc/ioports 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 0060-006f : keyboard 0070-0077 : rtc 0080-008f : dma page reg 00a0-00a1 : pic2 00c0-00df : dma2 00f0-00ff : fpu 0170-0177 : ide1 02f8-02ff : serial ...
hardware 343 pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful. The screenshot below shows that during boot the parallel port received dma channel 1, and the Infrared port received dma channel 3. root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3' [ 20.576000] parport: PnPBIOS parport detected. [ 20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1... [ 20.764000] irda_init() -- [ 21.204000] pnp: Device 00:0b activated. [ 21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8... [ 21.204000] nsc-ircc, chip->init root@laika:~# cat /proc/dma 1: parport0 4: cascade
Appendix C. License GNU Free Documentation License Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The "Invariant Sections" are certain Secondary Sections whose titles
License 345 are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque". Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only. The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text. The "publisher" means any person or entity that distributes copies of the Document to the public. A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. 2. VERBATIM COPYING You may copy and distribute the Document in any medium, either
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License 347 * B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. * C. State on the Title page the name of the publisher of the Modified Version, as the publisher. * D. Preserve all the copyright notices of the Document. * E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. * F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. * G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. * H. Include an unaltered copy of this License. * I. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. * J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. * K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. * L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. * M. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version. * N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section. * O. Preserve any Warranty Disclaimers. If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles. You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties— for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. 5. COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements". 6. COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. 7. AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
License 349 8. TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. 9. TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. 10. FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/. Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies
that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document. 11. RELICENSING "Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site. "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. "Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

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linux.pdf

  • 2. Unix Vs Linux Today Linux is in great demand. You can see the use of Linux everywhere. It's dominating on our servers, desktop, smartphones and even used in some electrical devices like refrigerators. Some people think Unix and Linux as synonyms, but that's not true. Many operating systems were developed to be like Unix but none of them got the popularity as Linux. Linux is the clone of Unix. It has several features similar to Unix, still have some key differences. Before Linux and Windows, computer world was dominated by Unix. Unix is a copyrighted name and IBM AIX, HP-UX and Sun Solaris are only Unix operating system remained till date. Difference between Linux and Unix Comparison Linux Unix Definition It is an open-source operating system which is freely available to everyone. It is an operating system which can be only used by its copyrighters. Examples It has different distros like Ubuntu, Redhat, Fedora, etc IBM AIX, HP-UX and Sun Solaris. Users Nowadays, Linux is in great demand. Anyone can use Linux whether a home user, developer or a student. It was developed mainly for servers, workstations and mainframes.
  • 3. Usage Linux is used everywhere from servers, PC, smartphones, tablets to mainframes and supercomputers. It is used in servers, workstations and PCs. Cost Linux is freely distributed, downloaded, and distributed through magazines also. And priced distros of Linux are also cheaper than Windows. Unix copyright vendors decide different costs for their respective Unix Operating systems. Development As it is open source, it is developed by sharing and collaboration of codes by world-wide developers. Unix was developed by AT&T Labs, various commercial vendors and non- profit organizations. Manufacturer Linux kernel is developed by the community of developers from different parts of the world. Although the father of Linux, Linus Torvalds oversees things. Unix has three distributions IBM AIX, HP-UX and Sun Solaris. Apple also uses Unix to make OSX operating system. GUI Linux is command based but some distros provide GUI based Linux. Gnome and KDE are mostly used GUI. Initially it was command based OS, but later Common Desktop Environment was created. Most Unix distributions use Gnome. Interface The default interface is BASH (Bourne Again SHell). But some distros have developed their own interfaces. It originally used Bourne shell. But is also compatible with other GUIs. File system support Linux supports more file system than Unix. It also supports file system but lesser than Linux. Coding Linux is a Unix clone,behaves like Unix but doesn't contain its code. Unix contain a completely different coding developed by AT&T Labs. Operating system Linux is just the kernel. Unix is a complete package of Operating system. Security It provides higher security. Linux has about 60-100 viruses listed till date. Unix is also highly secured. It has about 85-120 viruses listed till date Error detection and solution As Linux is open-source,whenever a user post any kind of threat, developers from all over the world start working on it. And hence, it provides faster solution. In Unix, users have to wait for some time for the problem to be resolved.
  • 4. Linux Distributions (Distros) Choosing a Linux Distro Distribution Why To Use UBuntu It works like Mac OS and easy to use. Linux mint It works like windows and should be use by new comers. Debian It provides stability but not recommended to a new user. Fedora If you want to use red hat and latest software. Red hat enterprise To be used commercially. CentOS If you want to use red hat but without its trademark. OpenSUSE It works same as Fedora but slightly older and more stable. Arch Linux It is not for the beginners because every package has to be installed by yourself.
  • 6. Chapter 1. man pages This chapter will explain the use of man pages (also called manual pages) on your Unix or Linux computer. You will learn the man command together with related commands like whereis, whatis and mandb. Most Unix files and commands have pretty good man pages to explain their use. Man pages also come in handy when you are using multiple flavours of Unix or several Linux distributions since options and parameters sometimes vary.
  • 7. 1.1. man $command Type man followed by a command (for which you want help) and start reading. Press q to quit the manpage. Some man pages contain examples (near the end). 1.2. man $configfile Most configuration files have their own manual. 1.3. man $daemon This is also true for most daemons (background programs) on your system.. 1.4. man -k (apropos) man -k (or apropos) shows a list of man pages containing a string. 1.5. whatis To see just the description of a manual page, use whatis followed by a string. 1.6. whereis The location of a manpage can be revealed with whereis. This file is directly readable by man. paul@laika:~$ man /usr/share/man/man1/whois.1.gz paul@laika:~$ man whois Reformatting whois(1), please wait... paul@laika:~$ man syslog.conf Reformatting syslog.conf(5), please wait... paul@laika:~$ man syslogd Reformatting syslogd(8), please wait... paul@laika:~$ man -k syslog lm-syslog-setup (8) - configure laptop mode to switch syslog.conf ... logger (1) - a shell command interface to the syslog(3) ... syslog-facility (8) - Setup and remove LOCALx facility for sysklogd syslog.conf (5) - syslogd(8) configuration file syslogd (8) - Linux system logging utilities. syslogd-listfiles (8) - list system logfiles paul@u810:~$ whatis route route (8) - show / manipulate the IP routing table paul@laika:~$ whereis -m whois whois: /usr/share/man/man1/whois.1.gz
  • 8. 1.7. man sections By now you will have noticed the numbers between the round brackets. man man will explain to you that these are section numbers. Executable programs and shell commands reside in section one. 1.8. man $section $file Therefor, when referring to the man page of the passwd command, you will see it written as passwd(1); when referring to the passwd file, you will see it written as passwd(5). The screenshot explains how to open the man page in the correct section. 1.9. man man If you want to know more about man, then Read The Fantastic Manual (RTFM). Unfortunately, manual pages do not have the answer to everything... 1.10. mandb Should you be convinced that a man page exists, but you can't access it, then try running mandb on Debian/Mint. Or run makewhatis on CentOS/Redhat. [root@centos65 ~]# apropos scsi scsi: nothing appropriate [root@centos65 ~]# makewhatis [root@centos65 ~]# apropos scsi hpsa (4) - HP Smart Array SCSI driver lsscsi (8) - list SCSI devices (or hosts) and their attributes sd (4) - Driver for SCSI Disk Drives st (4) - SCSI tape device 1 Executable programs or shell commands 2 System calls (functions provided by the kernel) 3 Library calls (functions within program libraries) 4 Special files (usually found in /dev) 5 File formats and conventions eg /etc/passwd 6 Games 7 Miscellaneous (including macro packages and conventions), e.g. man(7) 8 System administration commands (usually only for root) 9 Kernel routines [Non standard] [paul@RHEL52 ~]$ man passwd [paul@RHEL52 ~]$ man 5 passwd # opens the first manual found # opens a page from section 5 paul@laika:~$ man woman No manual entry for woman root@laika:~# mandb 0 man subdirectories contained newer manual pages. 0 manual pages were added. 0 stray cats were added. 0 old database entries were purged.
  • 9. Chapter 2. working with directories This module is a brief overview of the most common commands to work with directories: pwd, cd, ls, mkdir and rmdir. These commands are available on any Linux (or Unix) system. This module also discusses absolute and relative paths and path completion in the bash shell.
  • 10. 2.1. pwd The you are here sign can be displayed with the pwd command (Print Working Directory). Go ahead, try it: Open a command line interface (also called a terminal, console or xterm) and type pwd. The tool displays your current directory. 2.2. cd You can change your current directory with the cd command (Change Directory). 2.2.1. cd ~ The cd is also a shortcut to get back into your home directory. Just typing cd without a target directory, will put you in your home directory. Typing cd ~ has the same effect. 2.2.2. cd .. To go to the parent directory (the one just above your current directory in the directory tree), type cd .. . To stay in the current directory, type cd . ;-) We will see useful use of the . character representing the current directory later. paul@debian8:~$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd /bin paul@debian8$ pwd /bin paul@debian8$ cd /home/paul/ paul@debian8$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd paul@debian8$ pwd /home/paul paul@debian8$ cd ~ paul@debian8$ pwd /home/paul paul@debian8$ pwd /usr/share/games paul@debian8$ cd .. paul@debian8$ pwd /usr/share
  • 11. 2.2.3. cd - Another useful shortcut with cd is to just type cd - to go to the previous directory. 2.3. absolute and relative paths You should be aware of absolute and relative paths in the file tree. When you type a path starting with a slash (/), then the root of the file tree is assumed. If you don't start your path with a slash, then the current directory is the assumed starting point. The screenshot below first shows the current directory /home/paul. From within this directory, you have to type cd /home instead of cd home to go to the /home directory. When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory paul of the current directory /home. In case your current directory is the root directory /, then both cd /home and cd home will get you in the /home directory. This was the last screenshot with pwd statements. From now on, the current directory will often be displayed in the prompt. Later in this book we will explain how the shell variable $PS1 can be configured to show this. paul@debian8$ pwd /home/paul paul@debian8$ cd /etc paul@debian8$ pwd /etc paul@debian8$ cd - /home/paul paul@debian8$ cd - /etc paul@debian8$ pwd /home/paul paul@debian8$ cd home bash: cd: home: No such file or directory paul@debian8$ cd /home paul@debian8$ pwd /home paul@debian8$ pwd /home paul@debian8$ cd /paul bash: cd: /paul: No such file or directory paul@debian8$ cd paul paul@debian8$ pwd /home/paul paul@debian8$ pwd / paul@debian8$ cd home paul@debian8$ pwd /home paul@debian8$ cd / paul@debian8$ cd /home paul@debian8$ pwd /home
  • 12. 2.4. path completion The tab key can help you in typing a path without errors. Typing cd /et followed by the tab key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key, nothing will happen because you typed the wrong path (upper case E). You will need fewer key strokes when using the tab key, and you will be sure your typed path is correct! 2.5. ls You can list the contents of a directory with ls. 2.5.1. ls -a A frequently used option with ls is -a to show all files. Showing all files means including the hidden files. When a file name on a Linux file system starts with a dot, it is considered a hidden file and it doesn't show up in regular file listings. 2.5.2. ls -l Many times you will be using options with ls to display the contents of the directory in different formats or to display different parts of the directory. Typing just ls gives you alist of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a long listing. paul@debian8:~$ ls -l total 17296 -rw-r--r-- 1 paul paul 17584442 Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 96650 Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 19558 Sep 17 00:04 services drwxr-xr-x 2 paul paul 4096 Sep 17 00:04 stuff -rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt paul@debian8:~$ ls allfiles.txt dmesg.txt services stuff summer.txt paul@debian8:~$ paul@debian8:~$ ls allfiles.txt dmesg.txt services paul@debian8:~$ ls -a . allfiles.txt .bash_profile .. .bash_history .bashrc paul@debian8:~$ stuff summer.txt dmesg.txt services .lesshst stuff .ssh summer.txt
  • 13. 2.5.3. ls -lh Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human readable format. Also shown below is some variation in the way you can give the options to ls. We will explain the details of the output later in this book. Note that we use the letter L as an option in this screenshot, not the number 1. paul@debian8:~$ ls -l -h total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul paul@debian8:~$ ls -lh 0 Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul paul@debian8:~$ ls -hl 0 Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul 0 paul@debian8:~$ ls -h -l Sep 17 00:04 summer.txt total 17M -rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt -rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt -rw-r--r-- 1 paul paul 20K Sep 17 00:04 services drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff -rw-r--r-- 1 paul paul@debian8:~$ paul 0 Sep 17 00:04 summer.txt
  • 14. 2.6. mkdir Walking around the Unix file tree is fun, but it is even more fun to create your own directories with mkdir. You have to give at least one parameter to mkdir, the name of the new directory to be created. Think before you type a leading / . 2.6.1. mkdir -p The following command will fail, because the parent directory of threedirsdeep does not exist. When given the option -p, then mkdir will create parent directories as needed. 2.7. rmdir When a directory is empty, you can use rmdir to remove the directory. paul@debian8:~$ mkdir mydir paul@debian8:~$ cd mydir paul@debian8:~/mydir$ ls -al total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:07 . drwxr-xr-x 48 paul paul 4096 Sep 17 00:07 .. paul@debian8:~/mydir$ mkdir stuff paul@debian8:~/mydir$ mkdir otherstuff paul@debian8:~/mydir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff paul@debian8:~/mydir$ paul@debian8:~$ mkdir mydir2/mysubdir2/threedirsdeep mkdir: cannot create directory ‘mydir2/mysubdir2/threedirsdeep’: No such fi le or directory paul@debian8:~$ mkdir -p mydir2/mysubdir2/threedirsdeep paul@debian8:~$ cd mydir2 paul@debian8:~/mydir2$ ls -l total 4 drwxr-xr-x 3 paul paul 4096 Sep 17 00:11 mysubdir2 paul@debian8:~/mydir2$ cd mysubdir2 paul@debian8:~/mydir2/mysubdir2$ ls -l total 4 drwxr-xr-x 2 paul paul 4096 Sep 17 00:11 threedirsdeep paul@debian8:~/mydir2/mysubdir2$ cd threedirsdeep/ paul@debian8:~/mydir2/mysubdir2/threedirsdeep$ pwd /home/paul/mydir2/mysubdir2/threedirsdeep paul@debian8:~/mydir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff paul@debian8:~/mydir$ rmdir otherstuff paul@debian8:~/mydir$ cd .. paul@debian8:~$ rmdir mydir rmdir: failed to remove ‘mydir’: Directory not empty paul@debian8:~$ rmdir mydir/stuff paul@debian8:~$ rmdir mydir paul@debian8:~$
  • 15. 2.7.1. rmdir -p And similar to the mkdir -p option, you can also use rmdir to recursively remove directories. 2.8. solution: working with directories 1. Display your current directory. 2. Change to the /etc directory. 3. Now change to your home directory using only three key presses. 4. Change to the /boot/grub directory using only eleven key presses. 5. Go to the parent directory of the current directory. 6. Go to the root directory. 7. List the contents of the root directory. 8. List a long listing of the root directory. 9. Stay where you are, and list the contents of /etc. 10. Stay where you are, and list the contents of /bin and /sbin. 11. Stay where you are, and list the contents of ~. 12. List all the files (including hidden files) in your home directory. 13. List the files in /boot in a human readable format. paul@debian8:~$ mkdir -p test42/subdir paul@debian8:~$ rmdir -p test42/subdir paul@debian8:~$ pwd cd /etc cd (and the enter key) cd /boot/grub (use the tab key) cd .. (with space between cd and ..) cd / ls ls -l ls /etc ls /bin /sbin ls ~ ls -al ~
  • 16. 14. Create a directory testdir in your home directory. 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. ls -lh /boot mkdir ~/testdir
  • 17. 16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). 17. Remove the directory testdir. 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. The Bash shell has two built-in commands called pushd and popd. Both commands work with a common stack of previous directories. Pushd adds a directory to the stack and changes to a new current directory, popd removes a directory from the stack and sets the current directory. cd /etc ; mkdir ~/newdir mkdir -p ~/dir1/dir2/dir3 rmdir testdir man bash /pushd n # opens the manual # searches for pushd # next (do this two/three times) paul@debian7:/etc$ cd /bin paul@debian7:/bin$ pushd /lib /lib /bin paul@debian7:/lib$ pushd /proc /proc /lib /bin paul@debian7:/proc$ popd /lib /bin paul@debian7:/lib$ popd /bin
  • 19. Chapter 3. working with files In this chapter we learn how to recognise, create, remove, copy and move files using commands like file, touch, rm, cp, mv and rename.
  • 20. 3.1. all files are case sensitive Files on Linux (or any Unix) are case sensitive. This means that FILE1 is different from file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical Linux computer). This screenshot shows the difference between two files, one with upper case W, the other with lower case w. 3.2. everything is a file A directory is a special kind of file, but it is still a (case sensitive!) file. Each terminal window (for example /dev/pts/4), any hard disk or partition (for example /dev/sdb1) and any process are all represented somewhere in the file system as a file. It will become clear throughout this course that everything on Linux is a file. 3.3. file The file utility determines the file type. Linux does not use extensions to determine the file type. The command line does not care whether a file ends in .txt or .pdf. As a system administrator, you should use the file command to determine the file type. Here are some examples on a typical Linux system. The file command uses a magic file that contains patterns to recognise file types. The magic file is located in /usr/share/file/magic. Type man 5 magic for more information. It is interesting to point out file -s for special files like those in /dev and /proc. paul@laika:~/Linux$ ls winter.txt Winter.txt paul@laika:~/Linux$ cat winter.txt It is cold. paul@laika:~/Linux$ cat Winter.txt It is very cold! paul@laika:~$ file pic33.png pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced paul@laika:~$ file /etc/passwd /etc/passwd: ASCII text paul@laika:~$ file HelloWorld.c HelloWorld.c: ASCII C program text root@debian6~# file /dev/sda /dev/sda: block special root@debian6~# file -s /dev/sda /dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead... root@debian6~# file /proc/cpuinfo /proc/cpuinfo: empty root@debian6~# file -s /proc/cpuinfo /proc/cpuinfo: ASCII C++ program text
  • 21. 3.4. touch 3.4.1. create an empty file One easy way to create an empty file is with touch. (We will see many other ways for creating files later in this book.) This screenshot starts with an empty directory, creates two files with touch and the lists those files. 3.4.2. touch -t The touch command can set some properties while creating empty files. Can you determine what is set by looking at the next screenshot? If not, check the manual for touch. paul@debian7:~$ touch -t 200505050000 SinkoDeMayo paul@debian7:~$ touch -t 130207111630 BigBattle.txt paul@debian7:~$ ls -l total 0 -rw-r--r-- 1 paul paul 0 Jul 11 1302 BigBattle.txt -rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33 -rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42 -rw-r--r-- 1 paul paul@debian7:~$ paul 0 May 5 2005 SinkoDeMayo paul@debian7:~$ ls -l total 0 paul@debian7:~$ touch file42 paul@debian7:~$ touch file33 paul@debian7:~$ ls -l total 0 -rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33 -rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42 paul@debian7:~$
  • 22. 3.5. rm 3.5.1. remove forever When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces, the command line in general does not have a waste bin or trash can to recover files. When you use rm to remove a file, the file is gone. Therefore, be careful when removing files! 3.5.2. rm -i To prevent yourself from accidentally removing a file, you can type rm -i. 3.5.3. rm -rf By default, rm -r will not remove non-empty directories. However rm accepts several options that will allow you to remove any directory. The rm -rf statement is famous because it will erase anything (providing that you have the permissions to do so). When you are logged on as root, be very careful with rm -rf (the f means force and the r means recursive) since being root implies that permissions don't apply to you. You can literally erase your entire file system by accident. paul@debian7:~$ ls BigBattle.txt file33 file42 SinkoDeMayo paul@debian7:~$ rm BigBattle.txt paul@debian7:~$ ls file33 file42 SinkoDeMayo paul@debian7:~$ paul@debian7:~$ ls file33 file42 SinkoDeMayo paul@debian7:~$ rm -i file33 rm: remove regular empty file `file33'? yes paul@debian7:~$ rm -i SinkoDeMayo rm: remove regular empty file `SinkoDeMayo'? n paul@debian7:~$ ls file42 SinkoDeMayo paul@debian7:~$ paul@debian7:~$ mkdir test paul@debian7:~$ rm test rm: cannot remove `test': Is a directory paul@debian7:~$ rm -rf test paul@debian7:~$ ls test ls: cannot access test: No such file or directory paul@debian7:~$
  • 23. 3.6. cp 3.6.1. copy one file To copy a file, use cp with a source and a target argument. 3.6.2. copy to another directory If the target is a directory, then the source files are copied to that target directory. 3.6.3. cp -r To copy complete directories, use cp -r (the -r option forces recursive copying of all files in all subdirectories). 3.6.4. copy multiple files to directory You can also use cp to copy multiple files into a directory. In this case, the last argument (a.k.a. the target) must be a directory. 3.6.5. cp -i To prevent cp from overwriting existing files, use the -i (for interactive) option. paul@debian7:~$ ls file42 SinkoDeMayo paul@debian7:~$ cp file42 file42.copy paul@debian7:~$ ls file42 file42.copy SinkoDeMayo paul@debian7:~$ mkdir dir42 paul@debian7:~$ cp SinkoDeMayo dir42 paul@debian7:~$ ls dir42/ SinkoDeMayo paul@debian7:~$ ls dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ cp -r dir42/ dir33 paul@debian7:~$ ls dir33 dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ ls dir33/ SinkoDeMayo paul@debian7:~$ cp file42 file42.copy SinkoDeMayo dir42/ paul@debian7:~$ ls dir42/ file42 file42.copy SinkoDeMayo paul@debian7:~$ cp SinkoDeMayo file42 paul@debian7:~$ cp SinkoDeMayo file42 paul@debian7:~$ cp -i SinkoDeMayo file42 cp: overwrite `file42'? n paul@debian7:~$
  • 24. 3.7. mv 3.7.1. rename files with mv Use mv to rename a file or to move the file to another directory. When you need to rename only one file then mv is the preferred command to use. 3.7.2. rename directories with mv The same mv command can be used to rename directories. paul@debian7:~$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir33 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42 -rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33 -rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy -rw-r--r-- 1 paul paul 0 May 5 2005 SinkoDeMayo paul@debian7:~$ mv dir33 backup paul@debian7:~$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 backup drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42 -rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33 -rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy -rw-r--r-- 1 paul paul@debian7:~$ paul 0 May 5 2005 SinkoDeMayo 3.7.3. mv -i The mv also has a -i switch similar to cp and rm. this screenshot shows that mv -i will ask permission to overwrite an existing file. paul@debian7:~$ ls dir33 dir42 file42 file42.copy SinkoDeMayo paul@debian7:~$ mv file42 file33 paul@debian7:~$ ls dir33 dir42 file33 file42.copy SinkoDeMayo paul@debian7:~$ paul@debian7:~$ mv -i file33 SinkoDeMayo mv: overwrite `SinkoDeMayo'? no paul@debian7:~$
  • 25. 3.8. rename 3.8.1. about rename The rename command is one of the rare occasions where the Linux Fundamentals book has to make a distinction between Linux distributions. Almost every command in the Fundamentals part of this book works on almost every Linux computer. But rename is different. Try to use mv whenever you need to rename only a couple of files. 3.8.2. rename on Debian/Ubuntu The rename command on Debian uses regular expressions (regular expression or shor regex are explained in a later chapter) to rename many files at once. Below a rename example that switches all occurrences of txt to png for all file names ending in .txt. This second example switches all (first) occurrences of file into document for all file names ending in .png. 3.8.3. rename on CentOS/RHEL/Fedora On Red Hat Enterprise Linux, the syntax of rename is a bit different. The first example below renames all *.conf files replacing any occurrence of .conf with .backup. The second example renames all (*) files replacing one with ONE. paul@debian7:~/test42$ ls abc.txt file33.txt file42.txt paul@debian7:~/test42$ rename 's/.txt/.png/' *.txt paul@debian7:~/test42$ ls abc.png file33.png file42.png paul@debian7:~/test42$ ls abc.png file33.png file42.png paul@debian7:~/test42$ rename 's/file/document/' *.png paul@debian7:~/test42$ ls abc.png document33.png document42.png paul@debian7:~/test42$ [paul@centos7 ~]$ touch one.conf two.conf three.conf [paul@centos7 ~]$ rename .conf .backup *.conf [paul@centos7 ~]$ ls one.backup three.backup two.backup [paul@centos7 ~]$ [paul@centos7 ~]$ ls one.backup three.backup two.backup [paul@centos7 ~]$ rename one ONE * [paul@centos7 ~]$ ls ONE.backup three.backup two.backup [paul@centos7 ~]$
  • 26. 3.9. practice: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux- training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/ LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. 10. Use one command to remove the directory ~/testbackup and all files into it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf
  • 27. 3.10. solution: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux- training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/ LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. 10. Use one command to remove the directory ~/testbackup and all files into it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? ls /bin file /bin/cat /etc/passwd /usr/bin/passwd wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf file wolf.jpg LinuxFun.pdf mv wolf.jpg wolf.pdf file wolf.pdf LinuxFun.pdf mkdir ~/touched ; cd ~/touched touch today.txt yesterday.txt touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday) cp yesterday.txt copy.yesterday.txt mv copy.yesterday.txt kim mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/ rm -rf ~/testbackup
  • 28. cp -r /etc/*.conf ~/etcbackup Only *.conf files that are directly in /etc/ are copied. 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf
  • 29. Chapter 4. working with file contents In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less and strings. We will also get a glimpse of the possibilities of tools like cat on the command line.
  • 30. 4.1. head You can use head to display the first ten lines of a file. The head command can also display the first n lines of a file. And head can also display the first n bytes. 4.2. tail Similar to head, the tail command will display the last ten lines of a file. You can give tail the number of lines you want to see. The tail command has other useful options, some of which we will use during this course. paul@debian7~$ head /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh root@debian7~# paul@debian7~$ head -4 /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh paul@debian7~$ paul@debian7~$ head -c14 /etc/passwd root:x:0:0:roopaul@debian7~$ paul@debian7~$ tail /etc/services vboxd 20012/udp binkp 24554/tcp asp 27374/tcp asp 27374/udp csync2 30865/tcp dircproxy 57000/tcp tfido 60177/tcp fido 60179/tcp # binkp fidonet protocol # Address Search Protocol # cluster synchronization tool # Detachable IRC Proxy # fidonet EMSI over telnet # fidonet EMSI over TCP # Local services paul@debian7~$ paul@debian7~$ tail -3 /etc/services fido 60179/tcp # fidonet EMSI over TCP # Local services paul@debian7~$
  • 31. 4.3. cat The cat command is one of the most universal tools, yet all it does is copy standard input to standard output. In combination with the shell this can be very powerful and diverse. Some examples will give a glimpse into the possibilities. The first example is simple, you can use cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end. 4.3.1. concatenate cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger (or complete) file. 4.3.2. create files You can use cat to create flat text files. Type the cat > winter.txt command as shown in the screenshot below. Then type one or more lines, finishing each line with the enter key. After the last line, type and hold the Control (Ctrl) key and press d. The Ctrl d key combination will send an EOF (End of File) to the running process ending the cat command. paul@debian8:~$ cat /etc/resolv.conf domain linux-training.be search linux-training.be nameserver 192.168.1.42 paul@debian8:~$ echo one >part1 paul@debian8:~$ echo two >part2 paul@debian8:~$ echo three >part3 paul@debian8:~$ cat part1 one paul@debian8:~$ cat part2 two paul@debian8:~$ cat part3 three paul@debian8:~$ cat part1 part2 part3 one two three paul@debian8:~$ cat part1 part2 part3 >all paul@debian8:~$ cat all one two three paul@debian8:~$ paul@debian8:~$ cat > winter.txt It is very cold today! paul@debian8:~$ cat winter.txt It is very cold today! paul@debian8:~$
  • 32. 4.3.3. custom end marker You can choose an end marker for cat with << as is shown in this screenshot. This construction is called a here directive and will end the cat command. 4.3.4. copy files In the third example you will see that cat can be used to copy files. We will explain in detail what happens here in the bash shell chapter. 4.4. tac Just one example will show you the purpose of tac (cat backwards). paul@debian8:~$ cat > hot.txt <<stop > It is hot today! > Yes it is summer. > stop paul@debian8:~$ cat hot.txt It is hot today! Yes it is summer. paul@debian8:~$ paul@debian8:~$ cat winter.txt It is very cold today! paul@debian8:~$ cat winter.txt > cold.txt paul@debian8:~$ cat cold.txt It is very cold today! paul@debian8:~$ paul@debian8:~$ cat count one two three four paul@debian8:~$ tac count four three two one
  • 33. 4.5. more and less The more command is useful for displaying files that take up more than one screen. More will allow you to see the contents of the file page by page. Use the space bar to see the next page, or q to quit. Some people prefer the less command to more. 4.6. strings With the strings command you can display readable ascii strings found in (binary) files. This example locates the ls binary then displays readable strings in the binary file (output is truncated). 4.7. solution: file contents 1. Display the first 12 lines of /etc/services. 2. Display the last line of /etc/passwd. 3. Use cat to create a file named count.txt that looks like this: 4. Use cp to make a backup of this file to cnt.txt. 5. Use cat to make a backup of this file to catcnt.txt. 6. Display catcnt.txt, but with all lines in reverse order (the last line first). 7. Use more to display /etc/services. paul@laika:~$ which ls /bin/ls paul@laika:~$ strings /bin/ls /lib/ld-linux.so.2 librt.so.1 gmon_start _Jv_RegisterClasses clock_gettime libacl.so.1 ... head -12 /etc/services tail -1 /etc/passwd cat > count.txt One Two Three Four Five (followed by Ctrl-d) cp count.txt cnt.txt cat count.txt > catcnt.txt tac catcnt.txt
  • 34. 8. Display the readable character strings from the /usr/bin/passwd command. 9. Use ls to find the biggest file in /etc. 10. Open two terminal windows (or tabs) and make sure you are in the same directory in both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f tailing.txt in the second terminal. Now go back to the first terminal and type echo This is another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal shows both lines. Stop the tail -f with Ctrl-C. 11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed by the contents of /etc/passwd. 12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded by the contents of /etc/passwd. more /etc/services strings /usr/bin/passwd ls -lrS /etc cat /etc/passwd >> tailing.txt mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt
  • 36. 101 Chapter 5. the Linux file tree This chapter takes a look at the most common directories in the Linux file tree. It also shows that on Unix everything is a file.
  • 37. 5.1. filesystem hierarchy standard Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS may help make more Unix/Linux file system trees conform better in the future. The FHS is available online at http://www.pathname.com/fhs/ where we read: "The filesystem hierarchy standard has been designed to be used by Unix distribution developers, package developers, and system implementers. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy." 5.2. man hier There are some differences in the filesystems between Linux distributions. For help about your machine, enter man hier to find information about the file system hierarchy. This manual will explain the directory structure on your computer. 5.3. the root directory / All Linux systems have a directory structure that starts at the root directory. The root directory is represented by a forward slash, like this: /. Everything that exists on your Linux system can be found below this root directory. Let's take a brief look at the contents of the root directory. [paul@RHELv4u3 ~]$ ls / bin dev home media mnt proc sbin srv tftpboot usr boot etc lib misc opt root selinux sys tmp var
  • 38. the Linux file tree 103 5.4. binary directories Binaries are files that contain compiled source code (or machine code). Binaries can be executed on the computer. Sometimes binaries are called executables. 5.4.1. /bin The /bin directory contains binaries for use by all users. According to the FHS the /bin directory should contain /bin/cat and /bin/date (among others). In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd, echo, grep, and so on. Many of these will be covered in this book. paul@laika:~$ ls /bin archdetect egrep mt setupcon autopartition false mt-gnu sh bash fgconsole mv sh.distrib bunzip2 fgrep nano sleep bzcat fuser nc stralign bzcmp fusermount nc.traditional stty bzdiff get_mountoptions netcat su bzegrep grep netstat sync bzexe gunzip ntfs-3g sysfs bzfgrep gzexe ntfs-3g.probe tailf bzgrep gzip parted_devices tar bzip2 hostname parted_server tempfile bzip2recover hw-detect partman touch bzless ip partman-commit true bzmore kbd_mode perform_recipe ulockmgr cat kill pidof umount ... 5.4.2. other /bin directories You can find a /bin subdirectory in many other directories. A user named serena could put her own programs in /home/serena/bin. Some applications, often when installed directly from source will put themselves in /opt. A samba server installation can use /opt/samba/bin to store its binaries. 5.4.3. /sbin /sbin contains binaries to configure the operating system. Many of the system binaries require root privilege to perform certain tasks. Below a screenshot containing system binaries to change the ip address, partition a disk and create an ext4 file system. paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4 -rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk -rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig -rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4
  • 39. 5.4.4. /lib Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a screenshot of the partial contents of /lib. /lib/modules Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel-version/. This directory is discussed in detail in the Linux kernel chapter. /lib32 and /lib64 We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may encounter directories named /lib32 and /lib64 which clarify the register size used during compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and libraries for compatibility with legacy applications. This screenshot uses the file utility to demonstrate the difference. The ELF (Executable and Linkable Format) is used in almost every Unix-like operating system since System V. 5.4.5. /opt The purpose of /opt is to store optional software. In many cases this is software from outside the distribution repository. You may find an empty /opt directory on many systems. A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/ $packagename/. If for example the package is called wp, then it installs in /opt/wp, putting binaries in /opt/wp/bin and manpages in /opt/wp/man. paul@laika:~$ ls /lib/libc* /lib/libc-2.5.so /lib/libcfont.so.0.0.0 /lib/libcom_err.so.2.1 /lib/libcap.so.1 /lib/libcidn-2.5.so /lib/libconsole.so.0 /lib/libcap.so.1.10 /lib/libcidn.so.1 /lib/libconsole.so.0.0.0 /lib/libcfont.so.0 /lib/libcom_err.so.2 /lib/libcrypt-2.5.so paul@laika:~$ file /lib32/libc-2.5.so /lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.0, stripped paul@laika:~$ file /lib64/libcap.so.1.10 /lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, version 1 (SYSV), stripped
  • 40. the Linux file tree 105 5.5. configuration directories 5.5.1. /boot The /boot directory contains all files needed to boot the computer. These files don't change very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which defines the boot menu that is displayed before the kernel starts. 5.5.2. /etc All of the machine-specific configuration files should be located in /etc. Historically /etc stood for etcetera, today people often use the Editable Text Configuration backronym. Many times the name of a configuration files is the same as the application, daemon, or protocol with .conf added as the extension. paul@laika:~$ ls /etc/*.conf /etc/adduser.conf /etc/ld.so.conf /etc/scrollkeeper.conf /etc/brltty.conf /etc/lftp.conf /etc/sysctl.conf /etc/ccertificates.conf /etc/libao.conf /etc/syslog.conf /etc/cvs-cron.conf /etc/logrotate.conf /etc/ucf.conf /etc/ddclient.conf /etc/ltrace.conf /etc/uniconf.conf /etc/debconf.conf /etc/mke2fs.conf /etc/updatedb.conf /etc/deluser.conf /etc/netscsid.conf /etc/usplash.conf /etc/fdmount.conf /etc/nsswitch.conf /etc/uswsusp.conf /etc/hdparm.conf /etc/pam.conf /etc/vnc.conf /etc/host.conf /etc/pnm2ppa.conf /etc/wodim.conf /etc/inetd.conf /etc/povray.conf /etc/wvdial.conf /etc/kernel-img.conf /etc/resolv.conf paul@laika:~$ There is much more to be found in /etc. /etc/init.d/ A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start and stop daemons. This directory could disappear as Linux migrates to systems that replace the old init way of starting all daemons. /etc/X11/ The graphical display (aka X Window System or just X) is driven by software from the X.org foundation. The configuration file for your graphical display is /etc/X11/xorg.conf. /etc/skel/ The skeleton directory /etc/skel is copied to the home directory of a newly created user. It usually contains hidden files like a .bashrc script. /etc/sysconfig/ This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise Linux configuration files. We will discuss some of them in greater detail. The screenshot below is the /etc/sysconfig directory from RHELv4u4 with everything installed.
  • 41. paul@RHELv4u4:~$ ls /etc/sysconfig/ apmd firstboot irda network saslauthd apm-scripts grub irqbalance networking selinux authconfig hidd keyboard ntpd spamassassin autofs httpd kudzu openib.conf squid bluetooth hwconf lm_sensors pand syslog clock i18n mouse pcmcia sys-config-sec console init mouse.B pgsql sys-config-users crond installinfo named prelink sys-logviewer desktop ipmi netdump rawdevices tux diskdump iptables netdump_id_dsa rhn vncservers dund iptables-cfg paul@RHELv4u4:~$ netdump_id_dsa.p samba xinetd The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file, and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install the latest updates, create a user account, join the Red Hat Network and more. It will then create the /etc/sysconfig/firstboot file again. The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file explains itself. You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from Red Hat for automatic discovery and configuration of hardware. The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more console keyboard information, check the manual pages of keymaps(5), dumpkeys(1), loadkeys(1) and the directory /lib/kbd/keymaps/. We will discuss networking files in this directory in the networking chapter. paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot RUN_FIRSTBOOT=NO root@RHELv4u4:/etc/sysconfig# cat keyboard KEYBOARDTYPE="pc" KEYTABLE="us"
  • 42. the Linux file tree 107 5.6. data directories 5.6.1. /home Users can store personal or project data under /home. It is common (but not mandatory by the fhs) practice to name the users home directory after the user name in the format /home/ $USERNAME. For example: Besides giving every user (or every project or group) a location to store personal files, the home directory of a user also serves as a location to store the user profile. A typical Unix user profile contains many hidden files (files whose file name starts with a dot). The hidden files of the Unix user profiles contain settings specific for that user. 5.6.2. /root On many systems /root is the default location for personal data and profile of the root user. If it does not exist by default, then some administrators create it. 5.6.3. /srv You may use /srv for data that is served by your system. The FHS allows locating cvs, rsync, ftp and www data in this location. The FHS also approves administrative naming in / srv, like /srv/project55/ftp and /srv/sales/www. On Sun Solaris (or Oracle Solaris) /export is used for this purpose. 5.6.4. /media The /media directory serves as a mount point for removable media devices such as CD- ROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the Unix world, you could very well encounter systems running without this directory. Solaris 9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable media in /media. 5.6.5. /mnt The /mnt directory should be empty and should only be used for temporary mount points (according to the FHS). paul@ubu606:~$ ls /home geert annik sandra paul tom paul@ubu606:~$ ls -d /home/paul/.* /home/paul/. /home/paul/.. /home/paul/.bash_history /home/paul/.bash_profile /home/paul/.ssh /home/paul/.bashrc /home/paul/.lesshst /home/paul/.viminfo paul@debian5:~$ ls /media/ cdrom cdrom0 usbdisk
  • 43. Unix and Linux administrators used to create many directories here, like /mnt/something/. You likely will encounter many systems with more than one directory created and/or mounted inside /mnt to be used for various local and remote filesystems. 5.6.6. /tmp Applications and users should use /tmp to store temporary data when needed. Data stored in /tmp may use either disk space or RAM. Both of which are managed by the operating system. Never use /tmp to store data that is important or which you wish to archive.
  • 44. the Linux file tree 109 5.7. in memory directories 5.7.1. /dev Device files in /dev appear to be ordinary files, but are not actually located on the hard disk. The /dev directory is populated with files as the kernel is recognising hardware. common physical devices Common hardware such as hard disk devices are represented by device files in /dev. Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop. (The detailed meaning of these devices will be discussed later.) Besides representing physical hardware, some device files are special. These special devices can be very useful. /dev/tty and /dev/pts For example, /dev/tty1 represents a terminal or console attached to the system. (Don't break your head on the exact terminology of 'terminal' or 'console', what we mean here is a command line interface.) When typing commands in a terminal that is part of a graphical interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can be another number). /dev/null On Linux you will find other special devices such as /dev/null which can be considered a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard unwanted output from commands. /dev/null is not a good location to store your backups ;-). 5.7.2. /proc conversation with the kernel /proc is another special directory, appearing to be ordinary files, but not taking up disk space. It is actually a view of the kernel, or better, what the kernel manages, and is a means to interact with it directly. /proc is a proc filesystem. # # SATA or SCSI or USB # paul@laika:~$ ls /dev/sd* /dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sdb /dev/sdb1 /dev/sdb2 # # IDE or ATAPI # paul@barry:~$ ls /dev/hd* /dev/hda /dev/hda1 /dev/hda2 /dev/hdb /dev/hdb1 /dev/hdb2 /dev/hdc paul@RHELv4u4:~$ mount -t proc
  • 45. When listing the /proc directory you will see many numbers (on any Unix) and some interesting files (on Linux) mul@laika:~$ ls /proc 1 2339 4724 5418 6587 7201 cmdline mounts 10175 2523 4729 5421 6596 7204 cpuinfo mtrr 10211 2783 4741 5658 6599 7206 crypto net 10239 2975 4873 5661 6638 7214 devices pagetypeinfo 141 29775 4874 5665 6652 7216 diskstats partitions 15045 29792 4878 5927 6719 7218 dma sched_debug 1519 2997 4879 6 6736 7223 driver scsi 1548 3 4881 6032 6737 7224 execdomains self 1551 30228 4882 6033 6755 7227 fb slabinfo 1554 3069 5 6145 6762 7260 filesystems stat 1557 31422 5073 6298 6774 7267 fs swaps 1606 3149 5147 6414 6816 7275 ide sys 180 31507 5203 6418 6991 7282 interrupts sysrq-trigger 181 3189 5206 6419 6993 7298 iomem sysvipc 182 3193 5228 6420 6996 7319 ioports timer_list 18898 3246 5272 6421 7157 7330 irq timer_stats 19799 3248 5291 6422 7163 7345 kallsyms tty 19803 3253 5294 6423 7164 7513 kcore uptime 19804 3372 5356 6424 7171 7525 key-users version 1987 4 5370 6425 7175 7529 kmsg version_signature 1989 42 5379 6426 7188 9964 loadavg vmcore 2 45 5380 6430 7189 acpi locks vmnet 20845 4542 5412 6450 7191 asound meminfo vmstat 221 46 5414 6551 7192 buddyinfo misc zoneinfo 2338 4704 5416 6568 7199 bus modules Let's investigate the file properties inside /proc. Looking at the date and time will display the current date and time showing the files are constantly updated (a view on the kernel). Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see this by executing cat on files like /proc/cpuinfo, which contains information about the CPU. none on /proc type proc (rw) paul@RHELv4u4:~$ date Mon Jan 29 18:06:32 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:06 /proc/cpuinfo paul@RHELv4u4:~$ paul@RHELv4u4:~$ ...time passes... paul@RHELv4u4:~$ paul@RHELv4u4:~$ date Mon Jan 29 18:10:00 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:10 /proc/cpuinfo paul@RHELv4u4:~$ file /proc/cpuinfo /proc/cpuinfo: empty paul@RHELv4u4:~$ cat /proc/cpuinfo processor : 0 vendor_id : AuthenticAMD cpu family : 15 model : 43
  • 46. the Linux file tree 111 Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000... Most of the files in /proc are read only, some require root privileges, some files are writable, and many files in /proc/sys are writable. Let's discuss some of the files in /proc. model name : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+ stepping : 1 cpu MHz : 2398.628 cache size : 512 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge... bogomips : 4803.54 paul@pasha:~$ cat /proc/cpuinfo cpu : TI UltraSparc III (Cheetah) fpu : UltraSparc III integrated FPU promlib : Version 3 Revision 2 prom : 4.2.2 type : sun4u ncpus probed : 2 ncpus active : 2 Cpu0Bogo : 498.68 Cpu0ClkTck : 000000002cb41780 Cpu1Bogo : 498.68 Cpu1ClkTck : 000000002cb41780 MMU Type : Cheetah State: CPU0: online CPU1: online
  • 47. /proc/interrupts On the x86 architecture, /proc/interrupts displays the interrupts. On a machine with two CPU's, the file looks like this. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 860013 0 IO-APIC-edge timer 1: 4533 0 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 6588227 0 IO-APIC-edge rtc 10: 2314 0 IO-APIC-fasteoi acpi 12: 133 0 IO-APIC-edge i8042 14: 0 0 IO-APIC-edge libata 15: 72269 0 IO-APIC-edge libata 18: 1 0 IO-APIC-fasteoi yenta 19: 115036 0 IO-APIC-fasteoi eth0 20: 126871 0 IO-APIC-fasteoi libata, ohci1394 21: 30204 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2 22: 1334 0 IO-APIC-fasteoi saa7133[0], saa7133[0] 24: 234739 0 IO-APIC-fasteoi nvidia NMI: 72 42 LOC: 860000 859994 ERR: 0 /proc/kcore The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes. paul@RHELv4u4:~$ cat /proc/interrupts CPU0 177: NMI: LOC: ERR: MIS: IO-APIC-edge timer IO-APIC-edge i8042 IO-APIC-edge rtc IO-APIC-level acpi IO-APIC-edge i8042 IO-APIC-edge ide0 IO-APIC-edge IO-APIC-level ioc0 IO-APIC-level eth0 paul@laika:~$ ls -lh /proc/kcore -r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore paul@laika:~$
  • 48. the Linux file tree 113 5.7.3. /sys Linux 2.6 hot plugging The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux- hotplug.sourceforge.net/ ). Basically the /sys directory contains kernel information about hardware.
  • 49. 5.8. /usr Unix System Resources Although /usr is pronounced like user, remember that it stands for Unix System Resources. The /usr hierarchy should contain shareable, read only data. Some people choose to mount /usr as read only. This can be done from its own partition or from a read only NFS share (NFS is discussed later). 5.8.1. /usr/bin The /usr/bin directory contains a lot of commands. (On Solaris the /bin directory is a symbolic link to /usr/bin.) 5.8.2. /usr/include The /usr/include directory contains general use include files for C. paul@ubu1010:~$ ls /usr/include/ aalib.h expat_config.h math.h search.h af_vfs.h expat_external.h mcheck.h semaphore.h aio.h expat.h memory.h setjmp.h AL fcntl.h menu.h sgtty.h aliases.h features.h mntent.h shadow.h ... 5.8.3. /usr/lib The /usr/lib directory contains libraries that are not directly executed by users or scripts. 5.8.4. /usr/local The /usr/local directory can be used by an administrator to install software locally. 5.8.5. /usr/share The /usr/share directory contains architecture independent data. As you can see, this is a fairly large directory. paul@deb508:~$ ls /usr/share/ | wc -l paul@deb508:~$ ls /usr/bin | wc -l 1395 paul@deb508:~$ ls /usr/lib | head -7 4Suite ao apt arj aspell avahi bonobo paul@deb508:~$ ls /usr/local/ bin etc games include lib man sbin share src paul@deb508:~$ du -sh /usr/local/ 128K /usr/local/
  • 50. the Linux file tree 115 This directory typically contains /usr/share/man for manual pages. And it contains /usr/share/games for all static game data (so no high-scores or play logs). 5.8.6. /usr/src The /usr/src directory is the recommended location for kernel source files. 263 paul@deb508:~$ du -sh /usr/share/ 1.3G /usr/share/ paul@deb508:~$ ls /usr/share/man cs de es fi fr hu it.UTF-8 man2 man6 pl.ISO8859-2 sv fr.ISO8859-1 id ja man3 man7 pl.UTF-8 tr fr.UTF-8 it ko man4 man8 pt_BR gl it.ISO8859-1 man1 man5 pl ru zh_CN zh_TW paul@ubu1010:~$ ls /usr/share/games/ openttd wesnoth paul@deb508:~$ ls -l /usr/src/ total 12 drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686 drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26
  • 51. 5.9. /var variable data Files that are unpredictable in size, such as log, cache and spool files, should be located in /var. 5.9.1. /var/log The /var/log directory serves as a central point to contain all log files. 5.9.2. /var/log/messages A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/ log/messages file. By default this file will contain information on what just happened to the system. The file is called /var/log/syslog on Debian and Ubuntu. 5.9.3. /var/cache The /var/cache directory can contain cache data for several applications. paul@ubu1010:~$ ls /var/cache/ apt dictionaries-common gdm man software-center binfmts flashplugin-installer hald pm-utils cups fontconfig jockey pppconfig debconf fonts ldconfig samba 5.9.4. /var/spool The /var/spool directory typically contains spool directories for mail and cron, but also serves as a parent directory for other spool files (for example print spool files). [paul@RHEL4b ~]$ ls /var/log acpid cron.2 amanda cron.3 anaconda.log cron.4 anaconda.syslog cups anaconda.xlog audit boot.log boot.log.1 boot.log.2 boot.log.3 boot.log.4 canna cron cron.1 dmesg exim gdm httpd iiim iptraf lastlog mail maillog maillog.2 maillog.3 maillog.4 mailman messages messages.1 messages.2 messages.3 messages.4 mysqld.log news pgsql ppp quagga radius rpmpkgs rpmpkgs.1 rpmpkgs.2 rpmpkgs.3 rpmpkgs.4 sa samba secure.4 spooler spooler.1 spooler.2 spooler.3 spooler.4 squid uucp vbox scrollkeeper.log vmware-tools-guestd secure secure.1 secure.2 maillog.1 prelink.log secure.3 wtmp wtmp.1 Xorg.0.log Xorg.0.log.old [root@RHEL4b ~]# tail /var/log/messages Jul 30 05:13:56 anacron: anacron startup succeeded Jul 30 05:13:56 atd: atd startup succeeded Jul 30 05:13:57 messagebus: messagebus startup succeeded Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded Jul 30 05:13:58 haldaemon: haldaemon startup succeeded Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for... Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for... Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by... Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul
  • 52. the Linux file tree 117 5.9.5. /var/lib The /var/lib directory contains application state information. Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/. 11.9.6. /var/... /var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary files that survive a reboot in /var/tmp and information about file locks in /var/lock. There
  • 53. 5.10. solution: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? 3. Create a directory ~/test. Then issue the following commands: dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? /dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send something to /dev/zero, but you can read zeroes from it. 4. Now issue the following command: dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/ random to ~/test/random.txt. Can you describe the functionality of /dev/random ? /dev/random acts as a random number generator on your Linux machine. 5. Issue the following two commands, and look at the first character of each output line. The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512 bytes are common. Character devices act as a stream of characters (or bytes). Mouse and keyboard are typical character devices. 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? ls /bin/cat ; file /bin/cat ls /bin/dd ; file /bin/dd ls /bin/echo ; file /bin/echo ls -lh /boot/vm* cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* /etc/hosts contains hostnames with their ip address /etc/resolv.conf should contain the ip address of a DNS name server.
  • 55. 7. Are there any files in /etc/skel/ ? Check also for hidden files. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? The size is zero, yet the file contains data. It is not stored anywhere because /proc is a virtual file system that allows you to talk with the kernel. (If you answered "stored in RAM- memory, that is also correct...). 10. Can you enter the /root directory ? Are there (hidden) files ? Try "cd /root". The /root directory is not accessible for normal users on most modern Linux sy 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? 12. Is /var/log a file or a directory ? What about /var/spool ? 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. 14. Read the man page of random and explain the difference between /dev/random and / dev/urandom. Issue "ls -al /etc/skel/". Yes, there should be hidden files there. The file should contain at least one line with Intel or other cpu. Because those files are only meant for system administrators. Both are directories. tty-terminal: echo Hello > /dev/tty1 pts-terminal: echo Hello > /dev/pts/1 man 4 random
  • 56. 125 Chapter 6. commands and arguments This chapter introduces you to shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed. The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell). This chapter frequently uses the echo command to demonstrate shell features. The echo command is very simple: it echoes the input that it receives. paul@laika:~$ echo Burtonville Burtonville paul@laika:~$ echo Smurfs are blue Smurfs are blue
  • 57. 6.1. arguments One of the primary features of a shell is to perform a command line scan. When you enter a command at the shell's command prompt and press the enter key, then the shell will start scanning that line, cutting it up in arguments. While scanning the line, the shell maymake many changes to the arguments you typed. This process is called shell expansion. When the shell has finished scanning and modifying that line, then it will be executed. 6.2. white space removal Parts that are separated by one or more consecutive white spaces (or tabs) are considered separate arguments, any white space is removed. The first argument is the command to be executed, the other arguments are given to the command. The shell effectively cuts your command into one or more arguments. This explains why the following four different command lines are the same after shell expansion. The echo command will display each argument it receives from the shell. The echo command will also add a new white space between the arguments it received. [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ Hello World echo Hello World
  • 58. commands and arguments 127 6.3. single quotes You can prevent the removal of white spaces by quoting the spaces. The contents of the quoted string are considered as one argument. In the screenshot below the echo receives only one argument. 6.4. double quotes You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument. Later in this book, when discussing variables we will see important differences between single and double quotes. 6.5. echo and quotes Quoted lines can include special escaped characters recognised by the echo command (when using echo -e). The screenshot below shows how to use n for a newline and t for a tab (usually eight white spaces). The echo command can generate more than white spaces, tabs and newlines. Look in the man page for a list of options. [paul@RHEL4b ~]$ echo 'A line with single quotes' A line with [paul@RHEL4b ~]$ single quotes [paul@RHEL4b ~]$ echo "A line with double quotes" A line with [paul@RHEL4b ~]$ double quotes [paul@RHEL4b ~]$ echo -e "A line with na newline" A line with a newline [paul@RHEL4b ~]$ echo -e 'A line with na newline' A line with a newline [paul@RHEL4b ~]$ echo -e "A line with ta tab" A line with a tab [paul@RHEL4b ~]$ echo -e 'A line with ta tab' A line with a tab [paul@RHEL4b ~]$
  • 59. 6.6. commands 6.6.1. external or built-in commands? Not all commands are external to the shell, some are builtin. External commands are programs that have their own binary and reside somewhere in the file system. Many external commands are located in /bin or /sbin. Builtin commands are an integral part of the shell program itself. 6.6.2. type To find out whether a command given to the shell will be executed as an external command or as a builtin command, use the type command. As you can see, the cd command is builtin and the cat command is external. You can also use this command to show you whether the command is aliased or not. 6.6.3. running external commands Some commands have both builtin and external versions. When one of these commands is executed, the builtin version takes priority. To run the external version, you must enter the full path to the command. 6.6.4. which The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands. paul@laika:~$ type cd cd is a shell builtin paul@laika:~$ type cat cat is /bin/cat paul@laika:~$ type ls ls is aliased to `ls --color=auto' paul@laika:~$ type -a echo echo is a shell builtin echo is /bin/echo paul@laika:~$ /bin/echo Running the external echo command... Running the external echo command... [root@RHEL4b ~]# which cp ls cd mkdir pwd /bin/cp /bin/ls /usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:... /bin/mkdir /bin/pwd
  • 60. commands and arguments 129 6.7. aliases 6.7.1. create an alias The shell allows you to create aliases. Aliases are often used to create an easier to remember name for an existing command or to easily supply parameters. 6.7.2. abbreviate commands An alias can also be useful to abbreviate an existing command. 6.7.3. default options Aliases can be used to supply commands with default options. The example below shows how to set the -i option default when typing rm. Some distributions enable default aliases to protect users from accidentally erasing files ('rm -i', 'mv -i', 'cp -i') 6.7.4. viewing aliases You can provide one or more aliases as arguments to the alias command to get their definitions. Providing no arguments gives a complete list of current aliases. [paul@RHELv4u3 ~]$ cat count.txt one two three [paul@RHELv4u3 ~]$ alias dog=tac [paul@RHELv4u3 ~]$ dog count.txt three two one paul@laika:~$ alias ll='ls -lh --color=auto' paul@laika:~$ alias c='clear' paul@laika:~$ [paul@RHELv4u3 ~]$ rm -i winter.txt rm: remove regular file `winter.txt'? no [paul@RHELv4u3 ~]$ rm winter.txt [paul@RHELv4u3 ~]$ ls winter.txt ls: winter.txt: No such file or directory [paul@RHELv4u3 ~]$ touch winter.txt [paul@RHELv4u3 ~]$ alias rm='rm -i' [paul@RHELv4u3 ~]$ rm winter.txt rm: remove regular empty file `winter.txt'? no [paul@RHELv4u3 ~]$ paul@laika:~$ alias c ll alias c='clear' alias ll='ls -lh --color=auto'
  • 61. 6.7.5. unalias You can undo an alias with the unalias command. 6.8. displaying shell expansion You can display shell expansion with set -x, and stop displaying it with set +x. You might want to use this further on in this course, or when in doubt about exactly what the shell is doing with your command. [paul@RHEL4b ~]$ which rm /bin/rm [paul@RHEL4b ~]$ alias rm='rm -i' [paul@RHEL4b ~]$ which rm alias rm='rm -i' /bin/rm [paul@RHEL4b ~]$ unalias rm [paul@RHEL4b ~]$ which rm /bin/rm [paul@RHEL4b ~]$ [paul@RHELv4u3 ~]$ set -x ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo paul paul ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo '$USER' $USER ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ set +x + set +x [paul@RHELv4u3 ~]$ echo $USER paul
  • 62. commands and arguments 131 6.9. solution: commands and arguments 1. How many arguments are in this line (not counting the command itself). 2. Is tac a shell builtin command ? 3. Is there an existing alias for rm ? 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? 6. List all current aliases. 7a. Create an alias called 'city' that echoes your hometown. 7b. Use your alias to test that it works. 8. Execute set -x to display shell expansion for every command. 9. Test the functionality of set -x by executing your city and rm aliases. 10 Execute set +x to stop displaying shell expansion. 11. Remove your city alias. touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" answer: three type tac alias rm man rm touch testfile rm -i testfile touch testfile rm testfile (should ask for confirmation) alias alias city='echo Antwerp' city (it should display Antwerp) set -x shell should display the resolved aliases and then execute the command: paul@deb503:~$ set -x paul@deb503:~$ city + echo antwerp antwerp set +x
  • 63. 12. What is the location of the cat and the passwd commands ? 13. Explain the difference between the following commands: The echo command will be interpreted by the shell as the built-in echo command. The /bin/ echo command will make the shell execute the echo binary located in the /bin directory. 14. Explain the difference between the following commands: The -n option of the echo command will prevent echo from echoing a trailing newline. echo Hello will echo six characters in total, echo -n hello only echoes five characters. (The -n option might not work in the Korn shell.) 15. Display A B C with two spaces between B and C. 16. Complete the following command (do not use spaces) to display exactly the following output: The solution is to use tabs with t. 17. Use echo to display the following exactly: Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra). 18. Use one echo command to display three words on three lines. unalias city which cat (probably /bin/cat) which passwd (probably /usr/bin/passwd) echo /bin/echo echo Hello echo -n Hello echo "A B C" 4+4 =8 10+14 =24 echo -e "4+4t=8" ; echo -e "10+14t=24" ?? echo '??' echo -e '??' echo "??" echo -e "??" echo ?? echo -e "one ntwo nthree"
  • 64. 135 Chapter 7. control operators In this chapter we put more than one command on the command line using control operators. We also briefly discuss related parameters ($?) and similar special characters(&).
  • 65. 7.1. ; semicolon You can put two or more commands on the same line separated by a semicolon ; . The shell will scan the line until it reaches the semicolon. All the arguments before this semicolon will be considered a separate command from all the arguments after the semicolon. Both series will be executed sequentially with the shell waiting for each command to finish before starting the next one. 7.2. & ampersand When a line ends with an ampersand &, the shell will not wait for the command to finish. You will get your shell prompt back, and the command is executed in background. You will get a message when this command has finished executing in background. The technical explanation of what happens in this case is explained in the chapter about processes. 7.3. $? dollar question mark The exit code of the previous command is stored in the shell variable $?. Actually $? is a shell parameter and not a variable, since you cannot assign a value to $?. [paul@RHELv4u3 ~]$ echo Hello Hello [paul@RHELv4u3 ~]$ echo World World [paul@RHELv4u3 ~]$ echo Hello ; echo World Hello World [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ sleep 20 & [1] 7925 [paul@RHELv4u3 ~]$ ...wait 20 seconds... [paul@RHELv4u3 ~]$ [1]+ Done sleep 20 paul@debian5:~/test$ touch file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 rm: cannot remove `file1': No such file or directory paul@debian5:~/test$ echo $? 1 paul@debian5:~/test$
  • 66. control operators 137 7.4. && double ampersand The shell will interpret && as a logical AND. When using && the second command is executed only if the first one succeeds (returns a zero exit status). Another example of the same logical AND principle. This example starts with a working cd followed by ls, then a non-working cd which is not followed by ls. 7.5. || double vertical bar The || represents a logical OR. The second command is executed only when the first command fails (returns a non-zero exit status). Another example of the same logical OR principle. 7.6. combining && and || You can use this logical AND and logical OR to write an if-then-else structure on the command line. This example uses echo to display whether the rm command was successful. paul@barry:~$ echo first && echo second first second paul@barry:~$ zecho first && echo second -bash: zecho: command not found [paul@RHELv4u3 ~]$ cd gen && ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ cd gen && ls -bash: cd: gen: No such file or directory paul@barry:~$ echo first || echo second ; echo third first third paul@barry:~$ zecho first || echo second ; echo third -bash: zecho: command not found second third paul@barry:~$ [paul@RHELv4u3 ~]$ cd gen || ls [paul@RHELv4u3 gen]$ cd gen || ls -bash: cd: gen: No such file or directory file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! It worked! paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! rm: cannot remove `file1': No such file or directory It failed! paul@laika:~/test$
  • 67. 7.7. # pound sign Everything written after a pound sign (#) is ignored by the shell. This is useful to write a shell comment, but has no influence on the command execution or shell expansion. 7.8. escaping special characters The backslash character enables the use of control characters, but without the shell interpreting it, this is called escaping characters. 7.8.1. end of line backslash Lines ending in a backslash are continued on the next line. The shell does not interpret the newline character and will wait on shell expansion and execution of the command line until a newline without backslash is encountered. paul@debian4:~$ mkdir test paul@debian4:~$ cd test paul@debian4:~/test$ ls paul@debian4:~/test$ # we create a directory #### we enter the directory # is it empty ? [paul@RHELv4u3 ~]$ echo hello ; world hello ; world [paul@RHELv4u3 ~]$ echo hello world hello world [paul@RHELv4u3 ~]$ echo escaping # & " ' escaping # & " ' [paul@RHELv4u3 ~]$ echo escaping ?*"' escaping ?*"' [paul@RHEL4b ~]$ echo This command line > is split in three > parts This command line is split in three parts [paul@RHEL4b ~]$
  • 68. control operators 139 7.9. practice: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes)
  • 69. 7.10. solution: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) which passwd file /usr/bin/passwd pwd ; pwd cd /etc && ls cd etc || cd /etc paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed it worked paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed touch: cannot touch `test42': Permission denied it failed pausing for six seconds sleep 200 & rm file55 && echo success || echo failed echo "Hello World with strange' characters * [ } ~ . " or echo ""Hello World with strange' characters * [ } ~ . ""
  • 70. 141 Chapter 8. shell variables In this chapter we learn to manage environment variables in the shell. These variables are often needed by applications.
  • 71. 8.1. $ dollar sign Another important character interpreted by the shell is the dollar sign $. The shell willlook for an environment variable named like the string following the dollar sign and replaceit with the value of the variable (or with nothing if the variable does not exist). These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME. 8.2. case sensitive This example shows that shell variables are case sensitive! 8.3. creating variables This example creates the variable $MyVar and sets its value. It then uses echo to verify the value. [paul@RHELv4u3 ~]$ echo This is the $SHELL shell This is the /bin/bash shell [paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME This is /bin/bash on computer RHELv4u3.localdomain [paul@RHELv4u3 ~]$ echo The userid of $USER is $UID The userid of paul is 500 [paul@RHELv4u3 ~]$ echo My homedir is $HOME My homedir is /home/paul [paul@RHELv4u3 ~]$ echo Hello $USER Hello paul [paul@RHELv4u3 ~]$ echo Hello $user Hello [paul@RHELv4u3 gen]$ MyVar=555 [paul@RHELv4u3 gen]$ echo $MyVar 555 [paul@RHELv4u3 gen]$
  • 72. shell variables 143 8.4. quotes Notice that double quotes still allow the parsing of variables, whereas single quotes prevent this. The bash shell will replace variables with their value in double quoted lines, but not in single quoted lines. 8.5. set You can use the set command to display a list of environment variables. On Ubuntu and Debian systems, the set command will also list shell functions after the shell variables.Use set | more to see the variables then. 8.6. unset Use the unset command to remove a variable from your shell environment. [paul@RHELv4u3 ~]$ MyVar=555 [paul@RHELv4u3 ~]$ echo $MyVar 555 [paul@RHELv4u3 ~]$ echo "$MyVar" 555 [paul@RHELv4u3 ~]$ echo '$MyVar' $MyVar paul@laika:~$ city=Burtonville paul@laika:~$ echo "We are in $city today." We are in Burtonville today. paul@laika:~$ echo 'We are in $city today.' We are in $city today. [paul@RHEL4b ~]$ MyVar=8472 [paul@RHEL4b ~]$ echo $MyVar 8472 [paul@RHEL4b ~]$ unset MyVar [paul@RHEL4b ~]$ echo $MyVar [paul@RHEL4b ~]$
  • 73. 8.7. $PS1 The $PS1 variable determines your shell prompt. You can use backslash escaped special characters like u for the username or w for the working directory. The bash manual has a complete reference. In this example we change the value of $PS1 a couple of times. To avoid unrecoverable mistakes, you can set normal user prompts to green and the root prompt to red. Add the following to your .bashrc for a green user prompt: paul@deb503:~$ PS1=prompt prompt promptPS1='prompt ' prompt prompt PS1='> ' > > PS1='u@h$ ' paul@deb503$ paul@deb503$ PS1='u@h:W$' paul@deb503:~$ # color prompt by paul RED='[033[01;31m]' WHITE='[033[01;00m]' GREEN='[033[01;32m]' BLUE='[033[01;34m]' export PS1="${debian_chroot:+($debian_chroot)}$GREENu$WHITE@$BLUEh$WHITEw$ "
  • 74. shell variables 145 8.8. $PATH The $PATH variable is determines where the shell is looking for commands to execute (unless the command is builtin or aliased). This variable contains a list of directories, separated by colons. The shell will not look in the current directory for commands to execute! (Looking for executables in the current directory provided an easy way to hack PC-DOS computers). If you want the shell to look in the current directory, then add a . at the end of your $PATH. Your path might be different when using su instead of su - because the latter will take on the environment of the target user. The root user typically has /sbin directories added to the $PATH variable. [[paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin: [paul@RHEL4b ~]$ PATH=$PATH:. [paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:. [paul@RHEL4b ~]$ [paul@RHEL3 ~]$ su Password: [root@RHEL3 paul]# echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin [root@RHEL3 paul]# exit [paul@RHEL3 ~]$ su - Password: [root@RHEL3 ~]# echo $PATH /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: [root@RHEL3 ~]#
  • 75. 8.9. env The env command without options will display a list of exported variables. The difference with set with options is that set lists all variables, including those not exported to child shells. But env can also be used to start a clean shell (a shell without any inherited environment). The env -i command clears the environment for the subshell. Notice in this screenshot that bash will set the $SHELL variable on startup. You can use the env command to set the $LANG, or any other, variable for just one instance of bash with one command. The example below uses this to show the influence of the $LANG variable on file globbing (see the chapter on file globbing). 8.10. export You can export shell variables to other shells with the export command. This will export the variable to child shells. But it will not export to the parent shell (previous screenshot continued). [paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER' /bin/bash /home/paul paul [paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER' /bin/bash [paul@RHEL4b ~]$ [paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]' Filea Fileb [paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]' Filea FileA Fileb FileB [paul@RHEL4b test]$ [paul@RHEL4b ~]$ var3=three [paul@RHEL4b ~]$ var4=four [paul@RHEL4b ~]$ export var4 [paul@RHEL4b ~]$ echo $var3 $var4 three four [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $var3 $var4 four [paul@RHEL4b ~]$ export var5=five [paul@RHEL4b ~]$ echo $var3 $var4 $var5 four five [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ echo $var3 $var4 $var5 three four [paul@RHEL4b ~]$
  • 76. shell variables 147 8.11. delineate variables Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing until the first occurrence of a non-alphanumeric character that is not an underscore. In some situations, this can be a problem. This issue can be resolved with curly braces like in this example. 8.12. unbound variables The example below tries to display the value of the $MyVar variable, but it fails because the variable does not exist. By default the shell will display nothing when a variable is unbound (does not exist). There is, however, the nounset shell option that you can use to generate an error when a variable does not exist. In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set +o nounset. [paul@RHEL4b ~]$ prefix=Super [paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl Hello and [paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl Hello Superman and Supergirl [paul@RHEL4b ~]$ [paul@RHELv4u3 gen]$ echo $MyVar [paul@RHELv4u3 gen]$ paul@laika:~$ set -u paul@laika:~$ echo $Myvar bash: Myvar: unbound variable paul@laika:~$ set +u paul@laika:~$ echo $Myvar paul@laika:~$
  • 77. 8.13. practice: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Create two variables, and export one of them. 8. Display the exported variable in an interactive child shell. 9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt.
  • 78. shell variables 149 8.14. solution: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Create two variables, and export one of them. 8. Display the exported variable in an interactive child shell. 9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. echo Hello $USER answer=42 MyLANG=$LANG set set|more on Ubuntu/Debian env export declare -x env | more set | more unset answer var1=1; export var2=2 bash echo $var2 varx=Dumb; vary=do echo ${varx}le${vary}re solution by Yves from Dexia : echo $varx'le'$vary're' solution by Erwin from Telenet : echo "$varx"le"$vary"re PS1='t u@h W$ '
  • 79. Chapter 9. shell embedding and options This chapter takes a brief look at child shells, embedded shells and shell options.
  • 80. shell embedding and options 151 9.1. shell embedding Shells can be embedded on the command line, or in other words, the command line scan can spawn new processes containing a fork of the current shell. You can use variables to prove that new shells are created. In the screenshot below, the variable $var1 only exists in the (temporary) sub shell. You can embed a shell in an embedded shell, this is called nested embedding of shells. This screenshot shows an embedded shell inside an embedded shell. 9.1.1. backticks Single embedding can be useful to avoid changing your current directory. The screenshot below uses backticks instead of dollar-bracket to embed. You can only use the $() notation to nest embedded shells, backticks cannot do this. 9.1.2. backticks or single quotes Placing the embedding between backticks uses one character less than the dollar and parenthesis combo. Be careful however, backticks are often confused with single quotes. The technical difference between ' and ` is significant! paul@deb503:~$ A=shell paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(C=sub;echo $C$B$A)) shell subshell subsubshell [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1) 5 [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ [paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass` passwd passwd- passwd.OLD [paul@RHELv4u3 ~]$ [paul@RHELv4u3 gen]$ echo `var1=5;echo $var1` 5 [paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1' var1=5;echo $var1 [paul@RHELv4u3 gen]$
  • 81. 9.2. shell options Both set and unset are builtin shell commands. They can be used to set options of the bash shell itself. The next example will clarify this. By default, the shell will treat unset variables as a variable having no value. By setting the -u option, the shell will treat any reference to unset variables as an error. See the man page of bash for more information. To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be explained later in this book (in the I/O redirection chapter). When typing set without options, you get a list of all variables without function when the shell is on posix mode. You can set bash in posix mode typing set -o posix. 9.3. solution: shell embedding 1. Find the list of shell options in the man page of bash. What is the difference betweenset -u and set -o nounset? read the manual of bash (man bash), search for nounset -- both mean the same thing. 2. Activate nounset in your shell. Test that it shows an error message when using non- existing variables. Both these lines have the same effect. 3. Deactivate nounset. 4. Execute cd /var and ls in an embedded shell. echo $(cd /var ; ls) [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ set -u [paul@RHEL4b ~]$ echo $var123 -bash: var123: unbound variable [paul@RHEL4b ~]$ set +u [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ set -C ; set -u [paul@RHEL4b ~]$ echo $- himuBCH [paul@RHEL4b ~]$ set +C ; set +u [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ set -u OR set -o nounset set +u OR set +o nounset
  • 82. shell embedding and options 153 The echo command is only needed to show the result of the ls command. Omitting will result in the shell trying to execute the first file as a command. 5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? echo $(embvar=emb;echo $embvar) ; echo $embvar #the last echo fails $embvar does not exist in your current shell 6. Explain what "set -x" does. Can this be useful ? It displays shell expansion for troubleshooting your command. (optional)7. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last echo -n First; echo -n Middle; echo Last 8. Display a long listing (ls -l) of the passwd command using the which command inside an embedded shell. ls -l $(which passwd)
  • 84. 155 Chapter 10. shell history The shell makes it easy for us to repeat commands, this chapter explains how.
  • 85. 10.1. repeating the last command To repeat the last command in bash, type !!. This is pronounced as bang bang. 10.2. repeating other commands You can repeat other commands using one bang followed by one or more characters. The shell will repeat the last command that started with those characters. 10.3. history To see older commands, use history to display the shell command history (or use history n to see the last n commands). 10.4. !n When typing ! followed by the number preceding the command you want repeated, then the shell will echo the command and execute it. paul@debian5:~/test42$ echo this will be repeated > file42.txt paul@debian5:~/test42$ !! echo this will be repeated > file42.txt paul@debian5:~/test42$ paul@debian5:~/test42$ touch file42 paul@debian5:~/test42$ cat file42 paul@debian5:~/test42$ !to touch file42 paul@debian5:~/test42$ paul@debian5:~/test$ history 10 38 mkdir test 39 cd test 40 touch file1 41 echo hello > file2 42 echo It is very cold today > winter.txt 43 ls 44 ls -l 45 cp winter.txt summer.txt 46 ls -l 47 history 10 paul@debian5:~/test$ !43 ls file1 file2 summer.txt winter.txt
  • 86. shell history 157 10.5. Ctrl-r Another option is to use ctrl-r to search in the history. In the screenshot below i only typed ctrl-r followed by four characters apti and it finds the last command containing these four consecutive characters. 10.6. $HISTSIZE The $HISTSIZE variable determines the number of commands that will be remembered in your current environment. Most distributions default this variable to 500 or 1000. You can change it to any value you like. 10.7. $HISTFILE The $HISTFILE variable points to the file that contains your history. The bash shell defaults this value to ~/.bash_history. A session history is saved to this file when you exit the session! Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your terminal's history. 10.8. $HISTFILESIZE The number of commands kept in your history file can be set using $HISTFILESIZE. paul@debian5:~$ (reverse-i-search)`apti': sudo aptitude install screen paul@debian5:~$ echo $HISTSIZE 500 paul@debian5:~$ HISTSIZE=15000 paul@debian5:~$ echo $HISTSIZE 15000 paul@debian5:~$ echo $HISTFILE /home/paul/.bash_history paul@debian5:~$ echo $HISTFILESIZE 15000
  • 87. 10.9. prevent recording a command You can prevent a command from being recorded in history using a space prefix. paul@debian8:~/github$ echo abc abc paul@debian8:~/github$ echo def def paul@debian8:~/github$ echo ghi ghi paul@debian8:~/github$ history 3 9501 echo abc 9502 echo ghi 9503 history 3 10.10. (optional)regular expressions It is possible to use regular expressions when using the bang to repeat commands. The screenshot below switches 1 into 2. 10.11. (optional) Korn shell history Repeating a command in the Korn shell is very similar. The Korn shell also has the history command, but uses the letter r to recall lines from history. This screenshot shows the history command. Note the different meaning of the parameter. Repeating with r can be combined with the line numbers given by the history command, or with the first few letters of the command. paul@debian5:~/test$ cat file1 paul@debian5:~/test$ !c:s/1/2 cat file2 hello paul@debian5:~/test$ $ history 17 17 clear 18 echo hoi 19 history 12 20 echo world 21 history 17 $ r e echo world world $ cd /etc $ r cd /etc $
  • 88. shell history 159 10.12. practice: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. 2. Repeat the previous command using only two characters (there are two solutions!) 3. Display the last 5 commands you typed. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? 6. Where are these commands stored when exiting the shell ? 7. How many commands can be written to the history file when exiting your current shell session ? 8. Make sure your current bash shell remembers the next 5000 commands you type. 9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ?
  • 89. 10.13. solution: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. echo The answer to the meaning of life, the universe and everything is 42 2. Repeat the previous command using only two characters (there are two solutions!) 3. Display the last 5 commands you typed. You will receive different line numbers. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? echo $HISTSIZE 6. Where are these commands stored when exiting the shell ? echo $HISTFILE 7. How many commands can be written to the history file when exiting your current shell session ? echo $HISTFILESIZE 8. Make sure your current bash shell remembers the next 5000 commands you type. HISTSIZE=5000 9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ? when you type exit !! OR !e paul@ubu1010:~$ history 5 52 ls -l 53 ls 54 df -h | grep sda 55 echo The answer to the meaning of life, the universe and everything is 42 56 history 5 paul@ubu1010:~$ !55 echo The answer to the meaning of life, the universe and everything is 42 The answer to the meaning of life, the universe and everything is 42
  • 90. 161 Chapter 11. file globbing The shell is also responsible for file globbing (or dynamic filename generation). This chapter will explain file globbing.
  • 91. 11.1. * asterisk The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk to any combination of characters (even none). When no path is given, the shell will use filenames in the current directory. See the man page of glob(7) for more information.(This is part of LPI topic 1.103.3.) 11.2. ? question mark Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate filenames, matching the question mark with exactly one character. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File* File4 File55 FileA Fileab FileAB [paul@RHELv4u3 gen]$ ls file* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls *ile55 File55 [paul@RHELv4u3 gen]$ ls F*ile55 File55 [paul@RHELv4u3 gen]$ ls F*55 File55 [paul@RHELv4u3 gen]$ [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File? File4 FileA [paul@RHELv4u3 gen]$ ls Fil?4 File4 [paul@RHELv4u3 gen]$ ls Fil?? File4 FileA [paul@RHELv4u3 gen]$ ls File?? File55 Fileab FileAB [paul@RHELv4u3 gen]$
  • 92. file globbing 163 11.3. [] square brackets The square bracket [ is interpreted by the shell as a sign to generate filenames, matching any of the characters between [ and the first subsequent ]. The order in this list between the brackets is not important. Each pair of brackets is replaced by exactly one character. You can also exclude characters from a list between square brackets with the exclamation mark !. And you are allowed to make combinations of these wild cards. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File[5A] FileA [paul@RHELv4u3 gen]$ ls File[A5] FileA [paul@RHELv4u3 gen]$ ls File[A5][5b] File55 [paul@RHELv4u3 gen]$ ls File[a5][5b] File55 Fileab [paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm] ls: File[a5][5b][abcdefghijklm]: No such file or directory [paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm] fileabc [paul@RHELv4u3 gen]$ [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls file[a5][!Z] fileab [paul@RHELv4u3 gen]$ ls file[!5]* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls file[!5]? fileab [paul@RHELv4u3 gen]$
  • 93. 11.4. a-z and 0-9 ranges The bash shell will also understand ranges of characters between brackets. 11.5. $LANG and square brackets But, don't forget the influence of the LANG variable. Some languages include lower case letters in an upper case range (and vice versa). If $LC_ALL is set, then this will also need to be reset to prevent file globbing. [paul@RHELv4u3 gen]$ ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ ls file[a-z]* fileab fileab2 fileabc [paul@RHELv4u3 gen]$ ls file[0-9] file1 file2 file3 [paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]* fileab2 [paul@RHELv4u3 gen]$ paul@RHELv4u4:~/test$ ls [A-Z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ echo $LANG en_US.UTF-8 paul@RHELv4u4:~/test$ LANG=C paul@RHELv4u4:~/test$ echo $LANG C paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 paul@RHELv4u4:~/test$ ls [A-Z]ile? File4 paul@RHELv4u4:~/test$
  • 94. file globbing 165 11.6. preventing file globbing The screenshot below should be no surprise. The echo * will echo a * when in an empty directory. And it will echo the names of all files when the directory is not empty. Globbing can be prevented using quotes or by escaping the special characters, as shown in this screenshot. paul@ubu1010:~$ mkdir test42 paul@ubu1010:~$ cd test42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ touch file42 file33 paul@ubu1010:~/test42$ echo * file33 file42 paul@ubu1010:~/test42$ echo * file33 file42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ echo '*' * paul@ubu1010:~/test42$ echo "*" *
  • 95. 11.7. solution: shell globbing 1. Create a test directory and enter it. mkdir testdir; cd testdir 2. Create the following files : (the last one has 6 characters including a space) 3. List (with ls) all files starting with file ls file* 4. List (with ls) all files starting with File ls File* 5. List (with ls) all files starting with file and ending in a number. ls file*[0-9] 6. List (with ls) all files starting with file and ending with a letter ls file*[a-z] 7. List (with ls) all files starting with File and having a digit as fifth character. ls File[0-9]* 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. ls File[0-9] 9. List (with ls) all files starting with a letter and ending in a number. ls [a-z]*[0-9] 10. List (with ls) all files that have exactly five characters. file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 touch file1 file10 file11 file2 File2 File3 touch file33 fileAB filea fileA fileAAA touch "file(" touch "file 2"
  • 97. 11. List (with ls) all files that start with f or F and end with 3 or A. 12. List (with ls) all files that start with f have i or R as second character and end in a number. 13. List all files that do not start with the letter F. 14. Copy the value of $LANG to $MyLANG. 15. Show the influence of $LANG in listing A-Z or a-z ranges. 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? 17. Is there another command besides cd to change directories ? ls ????? ls [fF]*[3A] ls f[iR]*[0-9] ls [!F]* MyLANG=$LANG see example in book echo * pushd popd
  • 98. 171 Chapter 12. I/O redirection One of the powers of the Unix command line is the use of input/output redirection and pipes. This chapter explains redirection of input, output and error streams.
  • 99. 12.1. stdin, stdout, and stderr The bash shell has three basic streams; it takes input from stdin (stream 0), it sends output to stdout (stream 1) and it sends error messages to stderr (stream 2) . The drawing below has a graphical interpretation of these three streams. The keyboard often serves as stdin, whereas stdout and stderr both go to the display. This can be confusing to new Linux users because there is no obvious way to recognize stdout from stderr. Experienced users know that separating output from errors can be very useful. The next sections will explain how to redirect these streams. stdout (1) stdin (0) bash stderr (2)
  • 100. I/O redirection 173 12.2. output redirection 12.2.1. > stdout stdout can be redirected with a greater than sign. While scanning the line, the shell will see the > sign and will clear the file. The > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1). Note that the bash shell effectively removes the redirection from the command line before argument 0 is executed. This means that in the case of this command: echo hello > greetings.txt the shell only counts two arguments (echo = argument 0, hello = argument 1). The redirection is removed before the argument counting takes place. 12.2.2. output file is erased While scanning the line, the shell will see the > sign and will clear the file! Since this happens before resolving argument 0, this means that even when the command fails, the file will have been cleared! [paul@RHELv4u3 ~]$ echo It is cold today! It is cold today! [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt -bash: zcho: command not found [paul@RHELv4u3 ~]$ cat winter.txt [paul@RHELv4u3 ~]$
  • 101. 12.2.3. noclobber Erasing a file while using > can be prevented by setting the noclobber option. 12.2.4. overruling noclobber The noclobber can be overruled with >|. 12.2.5. >> append Use >> to append output to a file. [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ set +o noclobber [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is very cold today! [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! Where is the summer ? [paul@RHELv4u3 ~]$
  • 102. I/O redirection 175 12.3. error redirection 12.3.1. 2> stderr Redirecting stderr is done with 2>. This can be very useful to prevent error messages from cluttering your screen. The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing 1> is the same as >. 18.3.2. 2>&1 To redirect both stdout and stderr to the same file, use 2>&1. Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error made a copy of the standard output before the standard output was redirected to dirlist. [paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1 [paul@RHELv4u3 ~]$
  • 103. 12.4. output redirection and pipes By default you cannot grep inside stderr when using pipes on the command line, because only stdout is passed. With 2>&1 you can force stderr to go to stdout. This enables the next command in the pipe to act on both streams. You cannot use both 1>&2 and 2>&1 to switch stdout and stderr. You need a third stream to switch stdout and stderr after a pipe symbol. 12.5. joining stdout and stderr The &> construction will put both stdout and stderr in one stream (to a file). paul@debian7:~$ rm file42 file33 file1201 | grep file42 rm: cannot remove ‘file42’: No such file or directory rm: cannot remove ‘file33’: No such file or directory rm: cannot remove ‘file1201’: No such file or directory paul@debian7:~$ rm file42 file33 file1201 2>&1 | grep file42 rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ rm file42 file33 file1201 2>&1 1>&2 | grep file42 rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ echo file42 2>&1 1>&2 | sed 's/file42/FILE42/' FILE42 paul@debian7:~$ echo file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' file42 paul@debian7:~$ rm file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' rm: cannot remove ‘FILE42’: No such file or directory paul@debian7:~$ rm file42 &> out_and_err paul@debian7:~$ cat out_and_err rm: cannot remove ‘file42’: No such file or directory paul@debian7:~$ echo file42 &> out_and_err paul@debian7:~$ cat out_and_err file42 paul@debian7:~$
  • 104. I/O redirection 177 12.6. input redirection 12.6.1. < stdin Redirecting stdin is done with < (short for 0<). 12.6.2. << here document The here document (sometimes called here-is-document) is a way to append input until a certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or can be called with Ctrl-D. 12.6.3. <<< here string The here string can be used to directly pass strings to a command. The result is the same as using echo string | command (but you have one less process running). See rfc 3548 for more information about base64. [paul@RHEL4b ~]$ cat < text.txt one two [paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt ONE ZZO [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ cat <<EOF > text.txt > one > two > EOF [paul@RHEL4b ~]$ cat text.txt one two [paul@RHEL4b ~]$ cat <<brol > text.txt > brel > brol [paul@RHEL4b ~]$ cat text.txt brel [paul@RHEL4b ~]$ paul@ubu1110~$ base64 <<< linux-training.be bGludXgtdHJhaW5pbmcuYmUK paul@ubu1110~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK linux-training.be
  • 105. 12.7. confusing redirection The shell will scan the whole line before applying redirection. The following command line is very readable and is correct. cat winter.txt > snow.txt 2> errors.txt But this one is also correct, but less readable. 2> errors.txt cat winter.txt > snow.txt Even this will be understood perfectly by the shell. < winter.txt > snow.txt 2> errors.txt cat 12.8. quick file clear So what is the quickest way to clear a file ? >foo And what is the quickest way to clear a file when the noclobber option is set ?
  • 106. I/O redirection 179 12.9. solution: input/output redirection 1. Activate the noclobber shell option. 2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file. 4. When listing all shell options, which character represents the noclobber option ? echo $- (noclobber is visible as C) 5. Deactivate the noclobber option. set +o noclobber 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. set -o noclobber set -C ls /etc > etc.txt ls /etc > etc.txt (should not work) paul@deb503:~$ > tailing.txt paul@deb503:~$ tail -f tailing.txt hello world in the other shell: paul@deb503:~$ echo hello >> tailing.txt paul@deb503:~$ echo world >> tailing.txt paul@deb503:~$ cat > tennis.txt << ace > Justine Henin > Venus Williams > Serena Williams > Martina Hingis > Kim Clijsters > ace paul@deb503:~$ cat tennis.txt Justine Henin Venus Williams Serena Williams Martina Hingis Kim Clijsters paul@deb503:~$
  • 108. 181 Chapter 13. filters Commands that are created to be used with a pipe are often called filters. These filters are very small programs that do one specific thing very efficiently. They can be used as building blocks. This chapter will introduce you to the most common filters. The combination of simple commands and filters in a long pipe allows you to design elegant solutions.
  • 109. 13.1. cat When between two pipes, the cat command does nothing (except putting stdin on stdout). 13.2. tee Writing long pipes in Unix is fun, but sometimes you may want intermediate results. This is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is almost the same as cat, except that it has two identical outputs. 13.3. grep The grep filter is famous among Unix users. The most common use of grep is to filter lines of text containing (or not containing) a certain string. You can write this without the cat. One of the most useful options of grep is grep -i which filters in a case insensitive way. [paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat five four three two one [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac one two three four five [paul@RHEL4b pipes]$ cat temp.txt five four three two one [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | grep Williams Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ grep Williams tennis.txt Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ grep Bel tennis.txt Justine Henin, Bel [paul@RHEL4b pipes]$ grep -i Bel tennis.txt
  • 110. filters 183 Another very useful option is grep -v which outputs lines not matching the string. And of course, both options can be combined to filter all lines not containing a case insensitive string. With grep -A1 one line after the result is also displayed. With grep -B1 one line before the result is also displayed. With grep -C1 (context) one line before and one after are also displayed. All three options (A,B, and C) can display any number of lines (using e.g. A2, B4 or C20). Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ grep -v Fra tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ grep -vi usa tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ paul@debian5:~/pipes$ grep -A1 Henin tennis.txt Justine Henin, Bel Serena Williams, usa paul@debian5:~/pipes$ grep -B1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel paul@debian5:~/pipes$ grep -C1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa
  • 111. 13.4. cut The cut filter can select columns from files, depending on a delimiter or a count of bytes. The screenshot below uses cut to filter for the username and userid in the /etc/passwd file. It uses the colon as a delimiter, and selects fields 1 and 3. When using a space as the delimiter for cut, you have to quote the space. This example uses cut to display the second to the seventh character of /etc/passwd. 13.5. tr You can translate characters with tr. The screenshot shows the translation of all occurrences of e to E. Here we set all letters to uppercase by defining two ranges. Here we translate all newlines to spaces. [[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4 Figo:510 Pfaff:511 Harry:516 Hermione:517 [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt Amelie Kim Justine Serena Venus [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4 igo:x: faff:x arry:x ermion [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E' AmEliE MaurEsmo, Fra Kim ClijstErs, BEL JustinE HEnin, BEl SErEna Williams, usa VEnus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z' AMELIE MAURESMO, FRA KIM CLIJSTERS, BEL JUSTINE HENIN, BEL SERENA WILLIAMS, USA VENUS WILLIAMS, USA [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat count.txt one two
  • 112. filters 185 The tr -s filter can also be used to squeeze multiple occurrences of a character to one. You can also use tr to 'encrypt' texts with rot13. This last example uses tr -d to delete characters. 13.6. wc Counting words, lines and characters is easy with wc. three four five [paul@RHEL4b pipes]$ cat count.txt | tr 'n' ' ' one two three four five [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat spaces.txt one two three four five six [paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' ' one two three four five six [paul@RHEL4b pipes]$ [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ paul@debian5:~/pipes$ cat tennis.txt | tr -d e Amli Maursmo, Fra Kim Clijstrs, BEL Justin Hnin, Bl Srna Williams, usa Vnus Williams, USA [paul@RHEL4b pipes]$ wc tennis.txt 5 15 100 tennis.txt [paul@RHEL4b pipes]$ wc -l tennis.txt 5 tennis.txt [paul@RHEL4b pipes]$ wc -w tennis.txt 15 tennis.txt [paul@RHEL4b pipes]$ wc -c tennis.txt 100 tennis.txt [paul@RHEL4b pipes]$
  • 113. 13.7. sort The sort filter will default to an alphabetical sort. But the sort filter has many options to tweak its usage. This example shows sorting different columns (column 1 or column 2). The screenshot below shows the difference between an alphabetical sort and a numerical sort (both on the third column). paul@debian5:~/pipes$ cat music.txt Queen Brel Led Zeppelin Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Led Zeppelin Queen [paul@RHEL4b pipes]$ sort -k1 country.txt Belgium, Brussels, 10 France, Paris, 60 Germany, Berlin, 100 Iran, Teheran, 70 Italy, Rome, 50 [paul@RHEL4b pipes]$ sort -k2 country.txt Germany, Berlin, 100 Belgium, Brussels, 10 France, Paris, 60 Italy, Rome, 50 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -k3 country.txt Belgium, Brussels, 10 Germany, Berlin, 100 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -n -k3 country.txt Belgium, Brussels, 10 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 Germany, Berlin, 100
  • 114. filters 187 13.8. uniq With uniq you can remove duplicates from a sorted list. uniq can also count occurrences with the -c option. paul@debian5:~/pipes$ cat music.txt Queen Brel Queen Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Queen Queen paul@debian5:~/pipes$ sort music.txt |uniq Abba Brel Queen paul@debian5:~/pipes$ sort music.txt |uniq -c 1 Abba 1 Brel 2 Queen
  • 115. 13.9. comm Comparing streams (or files) can be done with the comm. By default comm will output three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet are only in the first file, Turner is only in the second. The output of comm can be easier to read when outputting only a single column. The digits point out which output columns should not be displayed. paul@debian5:~/pipes$ cat > list1.txt Abba Bowie Cure Queen Sweet paul@debian5:~/pipes$ cat > list2.txt Abba Cure Queen Turner paul@debian5:~/pipes$ comm list1.txt list2.txt Abba Bowie Cure Queen Sweet Turner paul@debian5:~/pipes$ comm -12 list1.txt list2.txt Abba Cure Queen paul@debian5:~/pipes$ comm -13 list1.txt list2.txt Turner paul@debian5:~/pipes$ comm -23 list1.txt list2.txt Bowie Sweet
  • 116. filters 189 13.10. od European humans like to work with ascii characters, but computers store files in bytes.The example below creates a simple file, and then uses od to show the contents of the file in hexadecimal bytes The same file can also be displayed in octal bytes. And here is the file in ascii (or backslashed) characters. paul@laika:~/test$ cat > text.txt abcdefg 1234567 paul@laika:~/test$ od -t x1 text.txt 0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a 0000020 paul@laika:~/test$ od -b text.txt 0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012 0000020 paul@laika:~/test$ od -c text.txt 0000000 0000020 b c d e f g n 1 2 3 4 5 6 7 n
  • 117. 13.11. sed The stream editor sed can perform editing functions in the stream, using regular expressions. Add g for global replacements (all occurrences of the string per line). With d you can remove lines from a stream containing a character. paul@debian5:~/pipes$ echo level5 | sed 's/5/42/' level42 paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/' jump5 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/' jump5 level7 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g' jump5 jump7 paul@debian5:~/test42$ cat tennis.txt Venus Williams, USA Martina Hingis, SUI Justine Henin, BE Serena williams, USA Kim Clijsters, BE Yanina Wickmayer, BE paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d' Venus Williams, USA Martina Hingis, SUI Serena williams, USA
  • 118. filters 191 13.12. pipe examples 13.12.1. who | wc How many users are logged on to this system ? 13.12.2. who | cut | sort Display a sorted list of logged on users. Display a sorted list of logged on users, but every user only once . 13.12.3. grep | cut Display a list of all bash user accounts on this computer. Users accounts are explained in detail later. [paul@RHEL4b pipes]$ who root tty1 Jul 25 10:50 paul pts/0 Jul 25 09:29 (laika) Harry pts/1 Jul 25 12:26 (barry) paul pts/2 Jul 25 12:26 (pasha) [paul@RHEL4b pipes]$ who | wc -l 4 [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort Harry paul paul root [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq Harry paul root paul@debian5:~$ grep bash /etc/passwd root:x:0:0:root:/root:/bin/bash paul:x:1000:1000:paul,,,:/home/paul:/bin/bash serena:x:1001:1001::/home/serena:/bin/bash paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1 root paul serena
  • 119. 13.13. solution: filters 1. Put a sorted list of all bash users in bashusers.txt. grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt 2. Put a sorted list of all logged on users in onlineusers.txt. who | cut -d' ' -f1 | sort > onlineusers.txt 3. Make a list of all filenames in /etc that contain the string conf in their filename. ls /etc | grep conf 4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their filename. ls /etc | grep -i conf | sort 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. /sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5 6. Write a line that removes all non-letters from a stream. 7. Write a line that receives a text file, and outputs all words on a separate line. 8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/ dict/ .) paul@deb503:~$ cat text This is, yes really! , a text with ?&* too many str$ange# characters ;-) paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-' This is yes really a text with too many strange characters paul@deb503:~$ cat text2 it is very cold today without the sun paul@deb503:~$ cat text2 | tr ' ' 'n' it is very cold today without the sun paul@rhel ~$ echo "The zun is shining today" > text paul@rhel ~$ cat > DICT is shining sun the
  • 120. filters 193 You could also add the solution from question number 6 to remove non-letters, and tr -s ' ' to remove redundant spaces. today paul@rhel ~$ cat text | tr 'A-Z ' 'a-zn' | sort | uniq | comm -23 - DICT zun
  • 122. 195 Chapter 14. basic Unix tools This chapter introduces commands to find or locate files and to compress files, together with other common tools that were not discussed before. While the tools discussed hereare technically not considered filters, they can be used in pipes.
  • 123. 14.1. find The find command can be very useful at the start of a pipe to search for files. Here are some examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your screen with error messages. Find all files in /etc and put the list in etcfiles.txt Find all files of the entire system and put the list in allfiles.txt Find files that end in .conf in the current directory (and all subdirs). Find files of type file (not directory, pipe or etc.) that end in .conf. Find files of type directory that end in .bak . Find files that are newer than file42.txt Find can also execute another command on every file found. This example will look for *.odf files and copy them to /backup/. Find can also execute, after your confirmation, another command on every file found. This example will remove *.odf files if you approve of it for every file found. find /etc > etcfiles.txt find / > allfiles.txt find . -name "*.conf" find . -type f -name "*.conf" find /data -type d -name "*.bak" find . -newer file42.txt find /data -name "*.odf" -exec cp {} /backup/ ; find /data -name "*.odf" -ok rm {} ;
  • 124. basic Unix tools 197 14.2. locate The locate tool is very different from find in that it uses an index to locate files. This is a lot faster than traversing all the directories, but it also means that it is always outdated. If the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux) with the updatedb command. Most Linux distributions will schedule the updatedb to run once every day. 14.3. date The date command can display the date, time, time zone and more. A date string can be customised to display the format of your choice. Check the man page for more options. Time on any Unix is calculated in number of seconds since 1969 (the first second being the first second of the first of January 1970). Use date +%s to display Unix time in seconds. When will this seconds counter reach two thousand million ? [paul@RHEL4b ~]$ locate Samba warning: locate: could not open database: /var/lib/slocate/slocate.db:... warning: You need to run the 'updatedb' command (as root) to create th... Please have a look at /etc/updatedb.conf to enable the daily cron job. [paul@RHEL4b ~]$ updatedb fatal error: updatedb: You are not authorized to create a default sloc... [paul@RHEL4b ~]$ su - Password: [root@RHEL4b ~]# updatedb [root@RHEL4b ~]# paul@rhel55 ~$ date Sat Apr 17 12:44:30 CEST 2010 paul@rhel55 ~$ date +'%A %d-%m-%Y' Saturday 17-04-2010 paul@rhel55 ~$ date +%s 1271501080 paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds' Wed May 18 04:33:20 CEST 2033
  • 125. 14.4. cal The cal command displays the current month, with the current day highlighted. paul@rhel55 ~$ cal April 2010 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 You can select any month in the past or the future. paul@rhel55 ~$ cal 2 1970 February 1970 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 14.5. sleep The sleep command is sometimes used in scripts to wait a number of seconds. This example shows a five second sleep. paul@rhel55 ~$ sleep 5 paul@rhel55 ~$
  • 126. basic Unix tools 199 14.6. time The time command can display how long it takes to execute a command. The date command takes only a little time. The sleep 5 command takes five real seconds to execute, but consumes little cpu time. This bzip2 command compresses a file and uses a lot of cpu time. paul@rhel55 ~$ time date Sat Apr 17 13:08:27 CEST 2010 real user sys 0m0.014s 0m0.008s 0m0.006s paul@rhel55 ~$ time sleep 5 real user sys 0m5.018s 0m0.005s 0m0.011s paul@rhel55 ~$ time bzip2 text.txt real user sys 0m2.368s 0m0.847s 0m0.539s
  • 127. 14.7. gzip - gunzip Users never have enough disk space, so compression comes in handy. The gzip command can make files take up less space. You can get the original back with gunzip. 14.8. zcat - zmore Text files that are compressed with gzip can be viewed with zcat and zmore. paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ ls -lh text.txt.gz -rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz paul@rhel55 ~$ gunzip text.txt.gz paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ head -4 text.txt / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ zcat text.txt.gz | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh
  • 128. basic Unix tools 201 14.9. bzip2 - bunzip2 Files can also be compressed with bzip2 which takes a little more time than gzip, but compresses better. Files can be uncompressed again with bunzip2. 14.10. bzcat - bzmore And in the same way bzcat and bzmore can display files compressed with bzip2. paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ ls -lh text.txt.bz2 -rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2 paul@rhel55 ~$ bunzip2 text.txt.bz2 paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ bzcat text.txt.bz2 | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh
  • 129. 14.11. solution: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt When *.txt is quoted then the shell will not touch it. The find tool will look in the /data for all files ending in .txt. When *.txt is not quoted then the shell might expand this (when one or more files that ends in .txt exist in the current directory). The find might show a different result, or can result in a syntax error. 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt The first find will output all .odf filenames in /data and all subdirectories. The shell will redirect this to a file. The second find will output all files named .odf in /data and will also output all files that exist in directories named *.odf (in /data). With two million files the command line would be expanded beyond the maximum that the shell can accept. The last part of the command line would be lost. 3. Write a find command that finds all files created after January 30th 2010. 4. Write a find command that finds all *.odf files created in September 2009. The exclamation mark ! -newer can be read as not newer. 5. Count the number of *.conf files in /etc and all its subdirs. find /etc -type f -name '*.conf' | wc -l 6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ touch -t 201001302359 marker_date find . -type f -newer marker_date There is another solution : find . -type f -newerat "20100130 23:59:59" touch -t 200908312359 marker_start touch -t 200910010000 marker_end find . -type f -name "*.odf" -newer marker_start ! -newer marker_end
  • 130. basic Unix tools 203 find /data -name "*.odf" -exec cp {} /backup/ ; The first might fail when there are too many files to fit on one command line. 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? You cannot locate this with locate because it is not yet in the index. updatedb 8. Use find and -exec to rename all .htm files to .html. 9. Issue the date command. Now display the date in YYYY/MM/DD format. date +%Y/%m/%d 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? cal 1582 The calendars are different depending on the country. Check http://linux-training.be/files/ studentfiles/dates.txt paul@rhel55 ~$ find . -name '*.htm' ./one.htm ./two.htm paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l ; paul@rhel55 ~$ find . -name '*.htm*' ./one.html ./two.html
  • 132. 205 Chapter 15. regular expressions Regular expressions are a very powerful tool in Linux. They can be used with a variety of programs like bash, vi, rename, grep, sed, and more. This chapter introduces you to the basics of regular expressions.
  • 133. 15.1. regex versions There are three different versions of regular expression syntax: Depending on the tool being used, one or more of these syntaxes can be used. For example the grep tool has the -E option to force a string to be read as ERE while -G forces BRE and -P forces PRCE. Note that grep also has -F to force the string to be read literally. The sed tool also has options to choose a regex syntax. Read the manual of the tools you use! BRE: Basic Regular Expressions ERE: Extended Regular Expressions PRCE: Perl Regular Expressions
  • 134. regular expressions 207 15.2. grep 15.2.1. print lines matching a pattern grep is a popular Linux tool to search for lines that match a certain pattern. Below are some examples of the simplest regular expressions. This is the contents of the test file. This file contains three lines (or three newline characters). When grepping for a single character, only the lines containing that character are returned. The pattern matching in this example should be very straightforward; if the given character occurs on a line, then grep will return that line. 15.2.2. concatenating characters Two concatenated characters will have to be concatenated in the same way to have a match. This example demonstrates that ia will match Tania but not Valentina and in will match Valentina but not Tania. paul@rhel65:~$ cat names Tania Laura Valentina paul@rhel65:~$ grep u names Laura paul@rhel65:~$ grep e names Valentina paul@rhel65:~$ grep i names Tania Valentina paul@rhel65:~$ grep a names Tania Laura Valentina paul@rhel65:~$ grep ia names Tania paul@rhel65:~$ grep in names Valentina paul@rhel65:~$
  • 135. 15.2.3. one or the other PRCE and ERE both use the pipe symbol to signify OR. In this example we grep for lines containing the letter i or the letter a. Note that we use the -E switch of grep to force interpretion of our string as an ERE. We need to escape the pipe symbol in a BRE to get the same logical OR. 15.2.4. one or more The * signifies zero, one or more occurences of the previous and the + signifies one or more of the previous. paul@debian7:~$ grep -G 'i|a' list paul@debian7:~$ grep -G 'i|a' list Tania Laura paul@debian7:~$ cat list Tania Laura paul@debian7:~$ grep -E 'i|a' list Tania Laura paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o*' list2 ll lol lool loool paul@debian7:~$ grep -E 'o+' list2 lol lool loool paul@debian7:~$
  • 136. regular expressions 209 15.2.5. match the end of a string For the following examples, we will use this file. The two examples below show how to use the dollar character to match the end of a string. 15.2.6. match the start of a string The caret character (^) will match a string at the start (or the beginning) of a line. Given the same file as above, here are two examples. Both the dollar sign and the little hat are called anchors in a regex. paul@debian7:~$ grep ^Val names Valentina paul@debian7:~$ grep ^F names Fleur Floor paul@debian7:~$ cat names Tania Laura Valentina Fleur Floor paul@debian7:~$ grep a$ names Tania Laura Valentina paul@debian7:~$ grep r$ names Fleur Floor
  • 137. 15.2.7. separating words Regular expressions use a b sequence to reference a word separator. Take for example this file: Simply grepping for over will give too many results. Surrounding the searched word with spaces is not a good solution (because other characters can be word separators). This screenshot below show how to use b to find only the searched word: Note that grep also has a -w option to grep for words. paul@debian7:~$ cat text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep over text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep 'boverb' text The winter is over. Can you get over there? paul@debian7:~$ paul@debian7:~$ cat text The governer is governing. The winter is over. Can you get over there? paul@debian7:~$ grep -w over text The winter is over. Can you get over there? paul@debian7:~$
  • 138. regular expressions 211 15.2.8. grep features Sometimes it is easier to combine a simple regex with grep options, than it is to write a more complex regex. These options where discussed before: 15.2.9. preventing shell expansion of a regex The dollar sign is a special character, both for the regex and also for the shell (remember variables and embedded shells). Therefore it is advised to always quote the regex, this prevents shell expansion. grep -i grep -v grep -w grep -A5 grep -B5 grep -C5 paul@debian7:~$ grep 'r$' names Fleur Floor
  • 139. 15.3. rename 15.3.1. the rename command On Debian Linux the /usr/bin/rename command is a link to /usr/bin/prename installed by the perl package. Red Hat derived systems do not install the same rename command, so this section does not describe rename on Red Hat (unless you copy the perl script manually). There is often confusion on the internet about the rename command because solutions that work fine in Debian (and Ubuntu, xubuntu, Mint, ...) cannot be used in Red Hat (and CentOS, Fedora, ...). 15.3.2. perl The rename command is actually a perl script that uses perl regular expressions. The complete manual for these can be found by typing perldoc perlrequick (after installing perldoc). paul@pi ~ $ dpkg -S $(readlink -f $(which rename)) perl: /usr/bin/prename root@pi:~# aptitude install perl-doc The following NEW packages will be installed: perl-doc 0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded. Need to get 8,170 kB of archives. After unpacking 13.2 MB will be used. Get: 1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main perl-do... Fetched 8,170 kB in 19s (412 kB/s) Selecting previously unselected package perl-doc. (Reading database ... 67121 files and directories currently installed.) Unpacking perl-doc (from .../perl-doc_5.14.2-21+rpi2_all.deb) ... Adding 'diversion of /usr/bin/perldoc to /usr/bin/perldoc.stub by perl-doc' Processing triggers for man-db ... Setting up perl-doc (5.14.2-21+rpi2) ... root@pi:~# perldoc perlrequick
  • 140. regular expressions 213 15.3.3. well known syntax The most common use of the rename is to search for filenames matching a certain string and replacing this string with an other string. This is often presented as s/string/other string/ as seen in this example: And here is another example that uses rename with the well know syntax to change the extensions of the same files once more: These two examples appear to work because the strings we used only exist at the end of the filename. Remember that file extensions have no meaning in the bash shell. The next example shows what can go wrong with this syntax. Only the first occurrence of the searched string is replaced. paul@pi ~ $ ls abc allfiles.TXT bllfiles.TXT Scratch abc.conf backup cllfiles.TXT temp.TXT tennis2.TXT tennis.TXT paul@pi ~ $ rename 's/TXT/text/' * paul@pi ~ $ ls abc allfiles.text bllfiles.text Scratch tennis2.text abc.conf backup cllfiles.text temp.text tennis.text paul@pi ~ $ ls abc allfiles.text bllfiles.text Scratch tennis2.text abc.conf backup cllfiles.text temp.text tennis.text paul@pi ~ $ rename 's/text/txt/' *.text paul@pi ~ $ ls abc allfiles.txt bllfiles.txt Scratch tennis2.txt abc.conf backup cllfiles.txt temp.txt tennis.txt paul@pi ~ $ paul@pi ~ $ touch atxt.txt paul@pi ~ $ rename 's/txt/problem/' atxt.txt paul@pi ~ $ ls abc allfiles.txt backup cllfiles.txt temp.txt tennis.txt abc.conf aproblem.txt bllfiles.txt Scratch tennis2.txt paul@pi ~ $
  • 141. 15.3.4. a global replace The syntax used in the previous example can be described as s/regex/replacement/. This is simple and straightforward, you enter a regex between the first two slashes and a replacement string between the last two. This example expands this syntax only a little, by adding a modifier. The syntax we use now can be described as s/regex/replacement/g where s signifies switch and g stands for global. Note that this example used the -n switch to show what is being done (instead of actually renaming the file). 15.3.5. case insensitive replace Another modifier that can be useful is i. this example shows how to replace a case insensitive string with another string. 15.3.6. renaming extensions Command line Linux has no knowledge of MS-DOS like extensions, but many end users and graphical application do use them. Here is an example on how to use rename to only rename the file extension. It uses the dollar sign to mark the ending of the filename. Note that the dollar sign in the regex means at the end. Without the dollar sign this command would fail on the really.txt.txt file. paul@pi ~ $ rename -n 's/TXT/txt/g' aTXT.TXT aTXT.TXT renamed as atxt.txt paul@pi ~ $ paul@debian7:~/files$ ls file1.text file2.TEXT file3.txt paul@debian7:~/files$ rename 's/.text/.txt/i' * paul@debian7:~/files$ ls file1.txt file2.txt file3.txt paul@debian7:~/files$ paul@pi ~ $ ls *.txt allfiles.txt bllfiles.txt cllfiles.txt really.txt.txt temp.txt tennis.txt paul@pi ~ $ rename 's/.txt$/.TXT/' *.txt paul@pi ~ $ ls *.TXT allfiles.TXT bllfiles.TXT cllfiles.TXT really.txt.TXT temp.TXT tennis.TXT paul@pi ~ $
  • 142. regular expressions 215 15.4. sed 15.4.1. stream editor The stream editor or short sed uses regex for stream editing. In this example sed is used to replace a string. The slashes can be replaced by a couple of other characters, which can be handy in some cases to improve readability. 15.4.2. interactive editor While sed is meant to be used in a stream, it can also be used interactively on a file. echo Sunday | sed 's/Sun/Mon/' Monday echo Sunday | sed 's:Sun:Mon:' Monday echo Sunday | sed 's_Sun_Mon_' Monday echo Sunday | sed 's|Sun|Mon|' Monday paul@debian7:~/files$ echo Sunday > today paul@debian7:~/files$ cat today Sunday paul@debian7:~/files$ sed -i 's/Sun/Mon/' today paul@debian7:~/files$ cat today Monday
  • 143. 15.4.3. simple back referencing The ampersand character can be used to reference the searched (and found) string. In this example the ampersand is used to double the occurence of the found string. 15.4.4. back referencing Parentheses (often called round brackets) are used to group sections of the regex so they can leter be referenced. Consider this simple example: 15.4.5. a dot for any character In a regex a simple dot can signify any character. 15.4.6. multiple back referencing When more than one pair of parentheses is used, each of them can be referenced separately by consecutive numbers. This feature is called grouping. echo Sunday | sed 's/Sun/&&/' SunSunday echo Sunday | sed 's/day/&&/' Sundayday paul@debian7:~$ echo Sunday | sed 's_(Sun)_1ny_' Sunnyday paul@debian7:~$ echo Sunday | sed 's_(Sun)_1ny 1_' Sunny Sunday paul@debian7:~$ echo 2014-04-01 | sed 's/....-.... /YYYY-MM-DD/' YYYY-MM-DD paul@debian7:~$ echo abcd-ef-gh | sed 's/....-.... /YYYY-MM-DD/' YYYY-MM-DD paul@debian7:~$ echo 2014-04-01 | sed 's/(... )-(..)-(..)/1+2+3/' 2014+04+01 paul@debian7:~$ echo 2014-04-01 | sed 's/(....)-(..)-(..)/3:2:1/' 01:04:2014
  • 144. regular expressions 217 15.4.7. white space The s can refer to white space such as a space or a tab. This example looks for white spaces (s) globally and replaces them with 1 space. 15.4.8. optional occurrence A question mark signifies that the previous is optional. The example below searches for three consecutive letter o, but the third o is optional. paul@debian7:~$ echo -e 'todaytistwarm' today is warm paul@debian7:~$ echo -e 'todaytistwarm' | sed 's_s_ _g' today is warm paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'ooo?' list2 lool loool paul@debian7:~$ cat list2 | sed 's/ooo?/A/' ll lol lAl lAl
  • 145. 15.4.9. exactly n times You can demand an exact number of times the oprevious has to occur. This example wants exactly three o's. 15.4.10. between n and m times And here we demand exactly from minimum 2 to maximum 3 times. paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o{3}' list2 loool paul@debian7:~$ cat list2 | sed 's/o{3}/A/' ll lol lool lAl paul@debian7:~$ paul@debian7:~$ cat list2 ll lol lool loool paul@debian7:~$ grep -E 'o{2,3}' list2 lool loool paul@debian7:~$ grep 'o{2,3}' list2 lool loool paul@debian7:~$ cat list2 | sed 's/o{2,3}/A/' ll lol lAl lAl paul@debian7:~$
  • 146. 233 paul@debian7:~/hist$ history 6 paul@debian7:~/hist$ !2092 ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !2092:s/1/2 ls -l file2 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file2 paul@debian7:~/hist$ 15.5. bash history The bash shell can also interprete some regular expressions. This example shows how to manipulate the exclamation mask history feature of the bash shell. This also works with the history numbers in bash. 2089 mkdir hist 2090 cd hist/ 2091 touch file1 file2 file3 2092 ls -l file1 2093 ls -l file3 2094 history 6 paul@debian7:~$ mkdir hist paul@debian7:~$ cd hist/ paul@debian7:~/hist$ touch file1 file2 file3 paul@debian7:~/hist$ ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !l ls -l file1 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1 paul@debian7:~/hist$ !l:s/1/3 ls -l file3 -rw-r--r-- 1 paul paul 0 Apr 15 22:07 file3 paul@debian7:~/hist$
  • 147. Chapter 16. scripting introduction Shells like bash and Korn have support for programming constructs that can be saved as scripts. These scripts in turn then become more shell commands. Many Linux commands are scripts. User profile scripts are run when a user logs on and init scripts are run when a daemon is stopped or started. This means that system administrators also need basic knowledge of scripting to understand how their servers and their applications are started, updated, upgraded, patched, maintained, configured and removed, and also to understand how a user environment is built. The goal of this chapter is to give you enough information to be able to read and understand scripts. Not to become a writer of complex scripts.
  • 148. scripting introduction 235 16.1. prerequisites You should have read and understood part III shell expansion and part IV pipes and commands before starting this chapter. 16.2. hello world Just like in every programming course, we start with a simple hello_world script. The following script will output Hello World. echo Hello World After creating this simple script in vi or with echo, you'll have to chmod +x hello_world to make it executable. And unless you add the scripts directory to your path, you'll have to type the path to the script for the shell to be able to find it. 16.3. she-bang Let's expand our example a little further by putting #!/bin/bash on the first line of the script. The #! is called a she-bang (sometimes called sha-bang), where the she-bang is the first two characters of the script. You can never be sure which shell a user is running. A script that works flawlessly in bash might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell, you can start your script with a she-bang followed by the shell it is supposed to run in. This script will run in a bash shell. This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/ shells file contains a list of shells on your system. [paul@RHEL4a ~]$ echo echo Hello World > hello_world [paul@RHEL4a ~]$ chmod +x hello_world [paul@RHEL4a ~]$ ./hello_world Hello World [paul@RHEL4a ~]$ #!/bin/bash echo Hello World #!/bin/bash echo -n hello echo A bash subshell `echo -n hello` #!/bin/ksh echo -n hello echo a Korn subshell `echo -n hello`
  • 149. 16.4. comment Let's expand our example a little further by adding comment lines. 16.5. variables Here is a simple example of a variable inside a script. Scripts can contain variables, but since scripts are run in their own shell, the variables do not survive the end of the script. 16.6. sourcing a script Luckily, you can force a script to run in the same shell; this is called sourcing a script. The above is identical to the below. #!/bin/bash # # Hello World Script # echo Hello World #!/bin/bash # # simple variable in script # var1=4 echo var1 = $var1 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ [paul@RHEL4a ~]$ source ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$ [paul@RHEL4a ~]$ . ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$
  • 150. scripting introduction 237 16.7. troubleshooting a script Another way to run a script in a separate shell is by typing bash with the name of the script as a parameter. Expanding this to bash -x allows you to see the commands that the shell is executing (after shell expansion). Notice the absence of the commented (#) line, and the replacement of the variable before execution of echo. 16.8. prevent setuid root spoofing Some user may try to perform setuid based script root spoofing. This is a rare but possible attack. To improve script security and to avoid interpreter spoofing, you need to add -- after the #!/bin/bash, which disables further option processing so the shell will not accept any options. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --. paul@debian6~/test$ bash runme 42 paul@debian6~/test$ bash -x runme + var4=42 + echo 42 42 paul@debian6~/test$ cat runme # the runme script var4=42 echo $var4 paul@debian6~/test$ #!/bin/bash - or #!/bin/bash --
  • 151. 16.9. solution: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. 2. Make sure the script runs in the bash shell. 3. Make sure the script runs in the Korn shell. Note that while first.bash will technically work as a Korn shell script, the name ending in .bash is confusing. 4. Create a script that defines two variables, and outputs their value. 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. source second.bash 6. Is there a shorter way to source the script ? . ./second.bash 7. Comment your scripts so that you know what they are doing. $ echo 'echo Antwerp' > first.bash $ chmod +x first.bash $ ./first.bash Antwerp $ cat first.bash #!/bin/bash echo Antwerp $ cat first.bash #!/bin/ksh echo Antwerp $ cat second.bash #!/bin/bash var33=300 var42=400 echo $var33 $var42 $ cat second.bash #!/bin/bash # script to test variables and sourcing # define two variables var33=300 var42=400 # output the value of these variables echo $var33 $var42
  • 154. scripting loops 241 17.1. test [ ] The test command can test whether something is true or false. Let's start by testing whether 10 is greater than 55. The test command returns 1 if the test fails. And as you see in the next screenshot, test returns 0 when a test succeeds. If you prefer true and false, then write the test like this. The test command can also be written as square brackets, the screenshot below is identical to the one above. Below are some example tests. Take a look at man test to see more options for tests. Tests can be combined with logical AND and OR. [paul@RHEL4b ~]$ test 10 -gt 55 ; echo $? 1 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ test 56 -gt 55 ; echo $? 0 [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false true [paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false false [paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false true [paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false false [ -d foo ] [ -e bar ] [ '/etc' = $PWD ] [ $1 != 'secret' ] [ 55 -lt $bar ] [ $foo -ge 1000 ] [ "abc" < $bar ] [ -f foo ] [ -r bar ] [ foo -nt bar ] [ -o nounset ] Does the directory foo exist ? Does the file bar exist ? Is the string /etc equal to the variable $PWD ? Is the first parameter different from secret ? Is 55 less than the value of $bar ? Is the value of $foo greater or equal to 1000 ? Does abc sort before the value of $bar ? Is foo a regular file ? Is bar a readable file ? Is file foo newer than file bar ? Is the shell option nounset set ? paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false true paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false false paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false true
  • 155. 17.2. if then else The if then else construction is about choice. If a certain condition is met, then execute something, else execute something else. The example below tests whether a file exists, and if the file exists then a proper message is echoed. If we name the above script 'choice', then it executes like this. 17.3. if then elif You can nest a new if inside an else with elif. This is a simple example. 17.4. for loop The example below shows the syntax of a classical for loop in bash. An example of a for loop combined with an embedded shell. The same example as above can be written without the embedded shell using the bash {from..to} shorthand. #!/bin/bash if [ -f isit.txt ] then echo isit.txt exists! else echo isit.txt not found! fi [paul@RHEL4a scripts]$ ./choice isit.txt not found! [paul@RHEL4a scripts]$ touch isit.txt [paul@RHEL4a scripts]$ ./choice isit.txt exists! [paul@RHEL4a scripts]$ #!/bin/bash count=42 if [ $count -eq 42 ] then echo "42 is correct." elif [ $count -gt 42 ] then echo "Too much." else echo "Not enough." fi for i in 1 2 4 do echo $i done #!/bin/ksh for counter in `seq 1 20` do echo counting from 1 to 20, now at $counter sleep 1 done
  • 156. scripting loops 243 This for loop uses file globbing (from the shell expansion). Putting the instruction on the command line has identical functionality. 17.5. while loop Below a simple example of a while loop. Endless loops can be made with while true or while : , where the colon is the equivalent of no operation in the Korn and bash shells. 17.6. until loop Below a simple example of an until loop. #!/bin/bash for counter in {1..20} do echo counting from 1 to 20, now at $counter sleep 1 done kahlan@solexp11$ ls count.ksh go.ksh kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done kahlan@solexp11$ ls count.ksh count.ksh.backup go.ksh go.ksh.backup i=100; while [ $i -ge 0 ] ; do echo Counting down, from 100 to 0, now at $i; let i--; done #!/bin/ksh # endless loop while : do echo hello sleep 1 done let i=100; until [ $i -le 0 ] ; do echo Counting down, from 100 to 1, now at $i; let i--; done
  • 157. 17.7. solution: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. 2. Write a script that uses a for loop to count from 1 to 17000. 3. Write a script that uses a while loop to count from 3 to 7. 4. Write a script that uses an until loop to count down from 8 to 4. 5. Write a script that counts the number of files ending in .txt in the current directory. 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. #!/bin/bash for i in 3 4 5 6 7 do echo Counting from 3 to 7, now at $i done #!/bin/bash for i in `seq 1 17000` do echo Counting from 1 to 17000, now at $i done #!/bin/bash i=3 while [ $i -le 7 ] do echo Counting from 3 to 7, now at $i let i=i+1 done #!/bin/bash i=8 until [ $i -lt 4 ] do echo Counting down from 8 to 4, now at $i let i=i-1 done #!/bin/bash let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" #!/bin/bash ls *.txt > /dev/null 2>&1 if [ $? -ne 0 ]
  • 158. scripting loops 245 then echo "There are 0 files ending in .txt" else let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" fi
  • 161. 18.1. script parameters A bash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list. Below is the output of the script above in action. Once more the same script, but with only two parameters. Here is another example, where we use $0. The $0 parameter contains the name of the script. #!/bin/bash echo The first argument is $1 echo The second argument is $2 echo The third argument is $3 echo $ $$ PID of the script echo # $# count arguments echo ? $? last return code echo * $* all the arguments [paul@RHEL4a scripts]$ ./pars one two three The first argument is one The second argument is two The third argument is three $ 5610 PID of the script # 3 count arguments ? 0 last return code * one two three all the arguments [paul@RHEL4a scripts]$ ./pars 1 2 The first argument is 1 The second argument is 2 The third argument is $ 5612 PID of the script # 2 count arguments ? 0 last return code * 1 2 all the arguments [paul@RHEL4a scripts]$ paul@debian6~$ cat myname echo this script is called $0 paul@debian6~$ ./myname this script is called ./myname paul@debian6~$ mv myname test42 paul@debian6~$ ./test42 this script is called ./test42
  • 162. scripting parameters 249 18.2. shift through parameters The shift statement can parse all parameters one by one. This is a sample script. Below is some sample output of the script above. 18.3. runtime input You can ask the user for input with the read command in a script. kahlan@solexp11$ cat shift.ksh #!/bin/ksh if [ "$#" == "0" ] then echo You have to give at least one parameter. exit 1 fi while (( $# )) do echo You gave me $1 shift done kahlan@solexp11$ ./shift.ksh one You gave me one kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42" You gave me one You gave me two You gave me three You gave me 1201 You gave me 33 42 kahlan@solexp11$ ./shift.ksh You have to give at least one parameter. #!/bin/bash echo -n Enter a number: read number
  • 163. 18.4. sourcing a config file The source (as seen in the shell chapters) can be used to source a configuration file. Below a sample configuration file for an application. And here an application that uses this file. The running application can use the values inside the sourced configuration file. [paul@RHEL4a scripts]$ cat myApp.conf # The config file of myApp # Enter the path here myAppPath=/var/myApp # Enter the number of quines here quines=5 [paul@RHEL4a scripts]$ cat myApp.bash #!/bin/bash # # Welcome to the myApp application # . ./myApp.conf echo There are $quines quines [paul@RHEL4a scripts]$ ./myApp.bash There are 5 quines [paul@RHEL4a scripts]$
  • 164. scripting parameters 251 18.5. get script options with getopts The getopts function allows you to parse options given to a command. The following script allows for any combination of the options a, f and z. This is sample output from the script above. First we use correct options, then we enter twice an invalid option. kahlan@solexp11$ cat options.ksh #!/bin/ksh while getopts ":afz" option; do case $option in a) echo received -a ;; f) echo received -f ;; z) echo received -z ;; *) echo "invalid option -$OPTARG" ;; esac done kahlan@solexp11$ ./options.ksh kahlan@solexp11$ ./options.ksh -af received -a received -f kahlan@solexp11$ ./options.ksh -zfg received -z received -f invalid option -g kahlan@solexp11$ ./options.ksh -a -b -z received -a invalid option -b received -z
  • 165. You can also check for options that need an argument, as this example shows. This is sample output from the script above. 18.6. get shell options with shopt You can toggle the values of variables controlling optional shell behaviour with the shopt built-in shell command. The example below first verifies whether the cdspell option is set; it is not. The next shopt command sets the value, and the third shopt command verifies that the option really is set. You can now use minor spelling mistakes in the cd command. The man page of bash has a complete list of options. kahlan@solexp11$ cat argoptions.ksh #!/bin/ksh while getopts ":af:z" option; do case $option in a) echo received -a ;; f) echo received -f with $OPTARG ;; z) echo received -z ;; :) echo "option -$OPTARG needs an argument" ;; *) echo "invalid option -$OPTARG" ;; esac done kahlan@solexp11$ ./argoptions.ksh -a -f hello -z received -a received -f with hello received -z kahlan@solexp11$ ./argoptions.ksh -zaf 42 received -z received -a received -f with 42 kahlan@solexp11$ ./argoptions.ksh -zf received -z option -f needs an argument paul@laika:~$ shopt -q cdspell ; echo $? 1 paul@laika:~$ shopt -s cdspell paul@laika:~$ shopt -q cdspell ; echo $? 0 paul@laika:~$ cd /Etc /etc
  • 166. scripting parameters 253 18.7. solution: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. echo $4 $3 $2 $1 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. 3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. #!/bin/bash if [ -f $1 ] then echo $1 exists! else echo $1 not found! fi if [ -f $2 ] then echo $2 exists! else echo $2 not found! fi
  • 168. 255 Chapter 19. more scripting
  • 169. 19.1. eval eval reads arguments as input to the shell (the resulting commands are executed). This allows using the value of a variable as a variable. Both in bash and Korn the arguments can be quoted. Sometimes the eval is needed to have correct parsing of arguments. Consider this example where the date command receives one parameter 1 week ago. When we set this command in a variable, then executing that variable fails unless we use eval. 19.2. (( )) The (( )) allows for evaluation of numerical expressions. paul@deb503:~/test42$ answer=42 paul@deb503:~/test42$ word=answer paul@deb503:~/test42$ eval x=$$word ; echo $x 42 kahlan@solexp11$ answer=42 kahlan@solexp11$ word=answer kahlan@solexp11$ eval "y=$$word" ; echo $y 42 paul@debian6~$ date --date="1 week ago" Thu Mar 8 21:36:25 CET 2012 paul@debian6~$ lastweek='date --date="1 week ago"' paul@debian6~$ $lastweek date: extra operand `ago"' Try `date --help' for more information. paul@debian6~$ eval $lastweek Thu Mar 8 21:36:39 CET 2012 paul@deb503:~/test42$ (( 42 > 33 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false false paul@deb503:~/test42$ var42=42 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false true paul@deb503:~/test42$ var42=33 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false false
  • 170. more scripting 257 19.3. let The let built-in shell function instructs the shell to perform an evaluation of arithmetic expressions. It will return 0 unless the last arithmetic expression evaluates to 0. The shell can also convert between different bases. There is a difference between assigning a variable directly, or using let to evaluate the arithmetic expressions (even if it is just assigning a value). [paul@RHEL4b ~]$ let x="3 + 4" ; echo $x 7 [paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x 20 [paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x 18 [paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x 30 [paul@RHEL4b ~]$ let x="0xFF" ; echo $x 255 [paul@RHEL4b ~]$ let x="0xC0" ; echo $x 192 [paul@RHEL4b ~]$ let x="0xA8" ; echo $x 168 [paul@RHEL4b ~]$ let x="8#70" ; echo $x 56 [paul@RHEL4b ~]$ let x="8#77" ; echo $x 63 [paul@RHEL4b ~]$ let x="16#c0" ; echo $x 192 kahlan@solexp11$ dec=15 ; oct=017 ; hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 017 0x0f kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 15 15
  • 171. 19.4. case You can sometimes simplify nested if statements with a case construct. [paul@RHEL4b ~]$ ./help What animal did you see ? lion You better start running fast! [paul@RHEL4b ~]$ ./help What animal did you see ? dog Don't worry, give it a cookie. [paul@RHEL4b ~]$ cat help #!/bin/bash # # Wild Animals Helpdesk Advice # echo -n "What animal did you see ? " read animal case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty...'." ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac [paul@RHEL4b ~]$
  • 172. more scripting 259 19.5. shell functions Shell functions can be used to group commands in a logical way. This is sample output from this script with a function. A shell function can also receive parameters. kahlan@solexp11$ cat addfunc.ksh #!/bin/ksh function plus { let result="$1 + $2" echo } $1 + $2 = $result plus 3 10 plus 20 13 plus 20 22 This script produces the following output. kahlan@solexp11$ cat funcs.ksh #!/bin/ksh function greetings { echo Hello World! echo and hello to $USER to! } echo We will now call a function greetings echo The end kahlan@solexp11$ ./funcs.ksh We will now call a function Hello World! and hello to kahlan to! The end kahlan@solexp11$ ./addfunc.ksh 3 + 10 = 13 20 + 13 = 33 20 + 22 = 42
  • 173. 19.6. solution : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. 3. Improve the previous script to congratulate the user if the sum equals the product. 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. Enter a number: 5 Enter another number: 2 Sum: 5 + 2 = 7 Product: 5 x 2 = 10 #!/bin/bash echo -n "Enter a number : " read n1 echo -n "Enter another number : " read n2 let sum="$n1+$n2" let pro="$n1*$n2" echo -e "Sumt: $n1 + $n2 = $sum" echo -e "Productt: $n1 * $n2 = $pro" echo -n "Enter a number between 1 and 100 : " read n1 if [ $n1 -lt 1 -o $n1 -gt 100 ] then echo Wrong number... exit 1 fi if [ $sum -eq $pro ] then echo Congratulations $sum == $pro fi #!/bin/bash # # Wild Animals Case Insensitive Helpdesk Advice # if shopt -q nocasematch; then nocase=yes; else nocase=no; shopt -s nocasematch; fi echo -n "What animal did you see ? " read animal case $animal in
  • 174. 265 5. If time permits (or if you are waiting for other students to finish this practice), take a look at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty.'" ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac if [ nocase = yes ] ; then shopt -s nocasematch; else shopt -u nocasematch; fi
  • 175. Chapter 20. introduction to users This little chapter will teach you how to identify your user account on a Unix computer using commands like who am i, id, and more. In a second part you will learn how to become another user with the su command. And you will learn how to run a program as another user with sudo.
  • 176. introduction to users 267 01, 0.02 20.1. whoami The whoami command tells you your username. 20.2. who The who command will give you information about who is logged on the system. [paul@centos7 ~]$ who root pts/0 2014-10-10 23:07 (10.104.33.101) paul pts/1 2014-10-10 23:30 (10.104.33.101) laura pts/2 2014-10-10 23:34 (10.104.33.96) tania pts/3 [paul@centos7 ~]$ 2014-10-10 23:39 (10.104.33.91) 20.3. who am i With who am i the who command will display only the line pointing to your current session. 20.4. w The w command shows you who is logged on and what they are doing. [paul@centos7 ~]$ w 23:34:07 up 31 min, 2 users, load average: 0.00, 0. USER TTY LOGIN@ IDLE JCPU PCPU WHAT root pts/0 23:07 15.00s 0.01s 0.01s top paul pts/1 23:30 [paul@centos7 ~]$ 7.00s 0.00s 0.00s w 20.5. id The id command will give you your user id, primary group id, and a list of the groups that you belong to. On RHEL/CentOS you will also get SELinux context information with this command. [paul@centos7 ~]$ whoami paul [paul@centos7 ~]$ [paul@centos7 ~]$ who am i paul pts/1 2014-10-10 23:30 (10.104.33.101) [paul@centos7 ~]$ paul@debian7:~$ id uid=1000(paul) gid=1000(paul) groups=1000(paul) [root@centos7 ~]# id uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r :unconfined_t:s0-s0:c0.c1023
  • 177. 20.6. su to another user The su command allows a user to run a shell as another user. 20.7. su to root Yes you can also su to become root, when you know the root password. 20.8. su as root You need to know the password of the user you want to substitute to, unless your are logged in as root. The root user can become any existing user without knowing that user's password. 20.9. su - $username By default, the su command maintains the same shell environment. To become another user and also get the target user's environment, issue the su - command followed by the target username. 20.10. su - When no username is provided to su or su -, the command will assume root is the target. laura@debian7:~$ su tania Password: tania@debian7:/home/laura$ laura@debian7:~$ su root Password: root@debian7:/home/laura# root@debian7:~# id uid=0(root) gid=0(root) groups=0(root) root@debian7:~# su - valentina valentina@debian7:~$ root@debian7:~# su laura laura@debian7:/root$ exit exit root@debian7:~# su - laura laura@debian7:~$ pwd /home/laura tania@debian7:~$ su - Password: root@debian7:~#
  • 178. introduction to users 269 20.11. run a program as another user The sudo program allows a user to start a program with the credentials of another user. Before this works, the system administrator has to set up the /etc/sudoers file. This can be useful to delegate administrative tasks to another user (without giving the root password). The screenshot below shows the usage of sudo. User paul received the right to run useradd with the credentials of root. This allows paul to create new users on the system without becoming root and without knowing the root password. First the command fails for paul. But with sudo it works. 20.12. visudo Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers is out of scope for this fundamentals book. paul@debian7:~$ /usr/sbin/useradd -m valentina useradd: Permission denied. useradd: cannot lock /etc/passwd; try again later. paul@debian7:~$ sudo /usr/sbin/useradd -m valentina [sudo] password for paul: paul@debian7:~$ paul@rhel65:~$ apropos visudo visudo (8) - edit the sudoers file paul@rhel65:~$
  • 179. 20.13. sudo su - On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password set. This means that it is not possible to login as root (extra security). To perform tasks as root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are members of the admin group can use sudo to run all commands as root. The end result of this is that the user can type sudo su - and become root without having to enter the root password. The sudo command does require you to enter your own password. Thus the password prompt in the screenshot below is for sudo, not for su. 20.14. sudo logging Using sudo without authorization will result in a severe warning: The root user can see this in the /var/log/secure on Red Hat and in /var/log/auth.log on Debian). root@laika:~# grep admin /etc/sudoers # Members of the admin group may gain root privileges %admin ALL=(ALL) ALL paul@laika:~$ sudo su - Password: root@laika:~# paul@rhel65:~$ sudo su - We trust you have received the usual lecture from the local System Administrator. It usually boils down to these three things: #1) Respect the privacy of others. #2) Think before you type. #3) With great power comes great responsibility. [sudo] password for paul: paul is not in the sudoers file. This incident will be reported. paul@rhel65:~$ root@rhel65:~# tail /var/log/secure | grep sudo | tr -s ' ' Apr 13 16:03:42 rhel65 sudo: paul : user NOT in sudoers ; TTY=pts/0 ; PWD= /home/paul ; USER=root ; COMMAND=/bin/su - root@rhel65:~#
  • 180. introduction to users 271 20.15. solution: introduction to users 1. Run a command that displays only your currently logged on user name. 2. Display a list of all logged on users. 3. Display a list of all logged on users including the command they are running at this very moment. laura@debian7:~$ w 07:47:02 up 16 min, 2 users, USER TTY FROM load average: 0.00, 0.00, LOGIN@ IDLE JCPU 0.00 PCPU WHAT root pts/0 10.104.33.101 07:30 6.00s 0.04s 0.00s w root pts/1 10.104.33.101 laura@debian7:~$ 07:46 6.00s 0.01s 0.00s sleep 42 4. Display your user name and your unique user identification (userid). 5. Use su to switch to another user account (unless you are root, you will need the password of the other account). And get back to the previous account. 6. Now use su - to switch to another user and notice the difference. Note that su - gets you into the home directory of Tania. laura@debian7:~$ whoami laura laura@debian7:~$ echo $USER laura laura@debian7:~$ who laura pts/0 2014-10-13 07:22 (10.104.33.101) laura@debian7:~$ laura@debian7:~$ id uid=1005(laura) gid=1007(laura) groups=1007(laura) laura@debian7:~$ laura@debian7:~$ su tania Password: tania@debian7:/home/laura$ id uid=1006(tania) gid=1008(tania) groups=1008(tania) tania@debian7:/home/laura$ exit laura@debian7:~$ laura@debian7:~$ su - tania Password: tania@debian7:~$ pwd /home/tania tania@debian7:~$ logout laura@debian7:~$
  • 181. 7. Try to create a new user account (when using your normal user account). this should fail. (Details on adding user accounts are explained in the next chapter.) It is possible that useradd is located in /sbin/useradd on your computer. 8. Now try the same, but with sudo before your command. Notice that laura has no permission to use the sudo on this system. laura@debian7:~$ useradd valentina -su: useradd: command not found laura@debian7:~$ /usr/sbin/useradd valentina useradd: Permission denied. useradd: cannot lock /etc/passwd; try again later. laura@debian7:~$ sudo /usr/sbin/useradd valentina [sudo] password for laura: laura is not in the sudoers file. This incident will be reported. laura@debian7:~$
  • 183. Chapter 21. user management This chapter will teach you how to use useradd, usermod and userdel to create, modify and remove user accounts. You will need root access on a Linux computer to complete this chapter.
  • 184. 21.1. user management User management on Linux can be done in three complementary ways. You can use the graphical tools provided by your distribution. These tools have a look and feel that depends on the distribution. If you are a novice Linux user on your home system, then use the graphical tool that is provided by your distribution. This will make sure that you do not run into problems. Another option is to use command line tools like useradd, usermod, gpasswd, passwd and others. Server administrators are likely to use these tools, since they are familiar and very similar across many different distributions. This chapter will focus on these command line tools. A third and rather extremist way is to edit the local configuration files directly using vi (or vipw/vigr). Do not attempt this as a novice on production systems! 21.2. /etc/passwd The local user database on Linux (and on most Unixes) is /etc/passwd. As you can see, this file contains seven columns separated by a colon. The columns contain the username, an x, the user id, the primary group id, a description, the name of the home directory, and the login shell. More information can be found by typing man 5 passwd. [root@RHEL5 ~]# man 5 passwd 21.3. root The root user also called the superuser is the most powerful account on your Linux system. This user can do almost anything, including the creation of other users. The root user always has userid 0 (regardless of the name of the account). [root@RHEL5 ~]# tail /etc/passwd inge:x:518:524:art dealer:/home/inge:/bin/ksh ann:x:519:525:flute player:/home/ann:/bin/bash frederik:x:520:526:rubius poet:/home/frederik:/bin/bash steven:x:521:527:roman emperor:/home/steven:/bin/bash pascale:x:522:528:artist:/home/pascale:/bin/ksh geert:x:524:530:kernel developer:/home/geert:/bin/bash wim:x:525:531:master damuti:/home/wim:/bin/bash sandra:x:526:532:radish stresser:/home/sandra:/bin/bash annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh [root@RHEL5 ~]# head -1 /etc/passwd root:x:0:0:root:/root:/bin/bash
  • 185. 21.4. useradd You can add users with the useradd command. The example below shows how to add a user named yanina (last parameter) and at the same time forcing the creation of the home directory (-m), setting the name of the home directory (-d), and setting a description (-c). The user named yanina received userid 529 and primary group id 529. 21.5. /etc/default/useradd Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/useradd that contains some default user options. Besides using cat to display this file, you can also use useradd -D. 21.6. userdel You can delete the user yanina with userdel. The -r option of userdel will also remove the home directory. [root@RHEL5 ~]# userdel -r yanina 21.7. usermod You can modify the properties of a user with the usermod command. This example uses usermod to change the description of the user harry. [root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina [root@RHEL5 ~]# tail -1 /etc/passwd yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash [root@RHEL4 ~]# useradd -D GROUP=100 HOME=/home INACTIVE=-1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:harry potter:/home/harry:/bin/bash [root@RHEL4 ~]# usermod -c 'wizard' harry [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:wizard:/home/harry:/bin/bash
  • 186. 21.8. creating home directories The easiest way to create a home directory is to supply the -m option with useradd (it is likely set as a default option on Linux). A less easy way is to create a home directory manually with mkdir which also requires setting the owner and the permissions on the directory with chmod and chown (both commands are discussed in detail in another chapter). 21.9. /etc/skel/ When using useradd the -m option, the /etc/skel/ directory is copied to the newly created home directory. The /etc/skel/ directory contains some (usually hidden) files that contain profile settings and default values for applications. In this way /etc/skel/ serves as a default home directory and as a default user profile. [root@RHEL5 ~]# ls total 48 -la /etc/skel/ drwxr-xr-x 2 root root 4096 Apr 1 00:11 . drwxr-xr-x 97 root root 12288 Jun 24 15:36 .. -rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout -rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile -rw-r--r-- 1 root root 124 Jul 12 2006 .bashrc 21.10. deleting home directories The -r option of userdel will make sure that the home directory is deleted together with the user account. [root@RHEL5 ~]# mkdir /home/laura [root@RHEL5 ~]# chown laura:laura /home/laura [root@RHEL5 ~]# chmod 700 /home/laura [root@RHEL5 ~]# ls -ld /home/laura/ drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/ [root@RHEL5 ~]# ls -ld /home/wim/ drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/ [root@RHEL5 ~]# userdel -r wim [root@RHEL5 ~]# ls -ld /home/wim/ ls: /home/wim/: No such file or directory
  • 187. 21.11. login shell The /etc/passwd file specifies the login shell for the user. In the screenshot below you can see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh shell. You can use the usermod command to change the shell for a user. 21.12. chsh Users can change their login shell with the chsh command. First, user harry obtains a list of available shells (he could also have done a cat /etc/shells) and then changes his login shell to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash. Note that the -l option does not exist on Debian and that the above screenshot assumes that ksh and csh shells are installed. The screenshot below shows how laura can change her default shell (active on next login). 21.13. solution: user management 1. Create a user account named serena, including a home directory and a description (or comment) that reads Serena Williams. Do all this in one single command. root@debian7:~# useradd -m -c 'Serena Williams' serena 2. Create a user named venus, including home directory, bash shell, a description that reads Venus Williams all in one single command. root@debian7:~# useradd -m -c "Venus Williams" -s /bin/bash venus 3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group. [root@RHEL5 ~]# tail -2 /etc/passwd annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh [root@RHEL5 ~]# usermod -s /bin/bash laura [root@RHEL5 ~]# tail -1 /etc/passwd laura:x:528:534:art dealer:/home/laura:/bin/bash [laura@centos7 ~]$ chsh -l /bin/sh /bin/bash /sbin/nologin /usr/bin/sh /usr/bin/bash /usr/sbin/nologin /bin/ksh /bin/tcsh /bin/csh [laura@centos7 ~]$ [laura@centos7 ~]$ chsh -s /bin/ksh Changing shell for laura. Password: Shell changed.
  • 188. 4. Verify that their home directory was created. 5. Create a user named einstime with /bin/date as his default logon shell. root@debian7:~# useradd -s /bin/date einstime Or even better: root@debian7:~# useradd -s $(which date) einstime 7. What happens when you log on with the einstime user ? Can you think of a useful real world example for changing a user's login shell to an application ? It can be useful when users need to access only one application on the server. Just logging in opens the application for them, and closing the application automatically logs them out. root@debian7:~# tail -2 /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh venus:x:1009:1011:Venus Williams:/home/venus:/bin/bash root@debian7:~# tail -2 /etc/shadow serena:!:16358:0:99999:7::: venus:!:16358:0:99999:7::: root@debian7:~# tail -2 /etc/group serena:x:1010: venus:x:1011: root@debian7:~# ls -lrt /home | tail -2 drwxr-xr-x 2 serena drwxr-xr-x 2 venus root@debian7:~# serena venus 4096 Oct 15 10:50 serena 4096 Oct 15 10:59 venus root@debian7:~# su - einstime Wed Oct 15 11:05:56 UTC 2014 # You get the output of the date command root@debian7:~#
  • 189. 8. Create a file named welcome.txt and make sure every new user will see this file in their home directory. root@debian7:~# echo Hello > /etc/skel/welcome.txt 9. Verify this setup by creating (and deleting) a test user account. 10. Change the default login shell for the serena user to /bin/bash. Verify before and after you make this change. root@debian7:~# useradd -m test root@debian7:~# ls -l /home/test total 4 -rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt root@debian7:~# userdel -r test root@debian7:~# root@debian7:~# grep serena /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh root@debian7:~# usermod -s /bin/bash serena root@debian7:~# grep serena /etc/passwd serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash root@debian7:~#
  • 191. Chapter 22. user passwords This chapter will tell you more about passwords for local users. Three methods for setting passwords are explained; using the passwd command, using openssel passwd, and using the crypt function in a C program. The chapter will also discuss password settings and disabling, suspending or locking accounts.
  • 192. 22.1. passwd Passwords of users can be set with the passwd command. Users will have to provide their old password before twice entering the new one. As you can see, the passwd tool will do some basic verification to prevent users from using too simple passwords. The root user does not have to follow these rules (there will be a warning though). The root user also does not have to provide the old password before entering the new password twice. 22.2. shadow file User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only and can only be read by root. We will see in the file permissions section how it is possible for users to change their password. For now, you will have to know that users can change their password with the /usr/bin/passwd command. The /etc/shadow file contains nine colon separated columns. The nine fields contain (from left to right) the user name, the encrypted password (note that only inge and laura have an encrypted password), the day the password was last changed (day 1 is January 1, 1970), number of days the password must be left unchanged, password expiry day, warning number of days before password expiry, number of days after expiry before disabling the account, and the day the account was disabled (again, since 1970). The last field has no meaning yet. All the passwords in the screenshot above are hashes of hunter2. [tania@centos7 ~]$ passwd Changing password for user tania. Changing password for tania. (current) UNIX password: New password: BAD PASSWORD: The password is shorter than 8 characters New password: BAD PASSWORD: The password is a palindrome New password: BAD PASSWORD: The password is too similar to the old one passwd: Have exhausted maximum number of retries for service root@debian7:~# passwd tania Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully [root@centos7 ~]# tail -4 /etc/shadow paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7::: tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7::: laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7::: valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7 GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7::: [root@centos7 ~]#
  • 193. 22.3. encryption with passwd Passwords are stored in an encrypted format. This encryption is done by the crypt function. The easiest (and recommended) way to add a user with a password to the system is to add the user with the useradd -m user command, and then set the user's password with passwd. 22.4. encryption with openssl Another way to create users with a password is to use the -p option of useradd, but that option requires an encrypted password. You can generate this encrypted password with the openssl passwd command. The openssl passwd command will generate several distinct hashes for the same password, for this it uses a salt. This salt can be chosen and is visible as the first two characters of the hash. This example shows how to create a user with password. root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed Note that this command puts the password in your command history! [root@RHEL4 ~]# useradd -m xavier [root@RHEL4 ~]# passwd xavier Changing password for user xavier. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL4 ~]# paul@rhel65:~$ openssl passwd hunter2 86jcUNlnGDFpY paul@rhel65:~$ openssl passwd hunter2 Yj7mDO9OAnvq6 paul@rhel65:~$ openssl passwd hunter2 YqDcJeGoDbzKA paul@rhel65:~$ paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$ openssl passwd -salt 42 hunter2 42ZrbtP1Ze8G. paul@rhel65:~$
  • 194. 22.5. encryption with crypt A third option is to create your own C program using the crypt function, and compile this into a command. This little program can be compiled with gcc like this. paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password, the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096 different ways. This variation prevents two users with the same password from having the same entry in /etc/shadow. Did you notice that the first two characters of the password are the salt? The standard output of the crypt function is using the DES algorithm which is old and can be cracked in minutes. A better method is to use md5 passwords which can be recognized by a salt starting with $1$. The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between the second and third $, so never use the password as the salt! paul@rhel65:~$ cat MyCrypt.c #include <stdio.h> #define USE_XOPEN #include <unistd.h> int main(int argc, char** argv) { if(argc==3) { printf("%sn", crypt(argv[1],argv[2])); } else { printf("Usage: MyCrypt $password $saltn" ); } return 0; } paul@rhel65:~$ ./MyCrypt hunter2 42 42ZrbtP1Ze8G. paul@rhel65:~$ ./MyCrypt hunter2 33 33d6taYSiEUXI paul@rhel65:~$ ./MyCrypt hunter2 '$1$42' $1$42$7l6Y3xT5282XmZrtDOF9f0 paul@rhel65:~$ ./MyCrypt hunter2 '$6$42' $6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i... paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2' $1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.
  • 195. 22.6. /etc/login.defs The /etc/login.defs file contains some default settings for user passwords like password aging and length settings. (You will also find the numerical limits of user ids and group ids and whether or not a home directory should be created by default). Debian also has this file. 22.7. chage The chage command can be used to set an expiration date for a user account (-E), set a minimum (-m) and maximum (-M) password age, a password expiration date, and set the number of warning days before the password expiration date. Much of this functionality is also available from the passwd command. The -l option of chage will list these settings for a user. root@rhel65:~# grep ^PASS /etc/login.defs PASS_MAX_DAYS 99999 PASS_MIN_DAYS 0 PASS_MIN_LEN 5 PASS_WARN_AGE 7 root@debian7:~# grep PASS /etc/login.defs # PASS_MAX_DAYS # PASS_MIN_DAYS # PASS_WARN_AGE PASS_MAX_DAYS PASS_MIN_DAYS PASS_WARN_AGE Maximum number of days a password may be used. Minimum number of days allowed between password changes. Number of days warning given before a password expires. 99999 #PASS_CHANGE_TRIES #PASS_ALWAYS_WARN #PASS_MIN_LEN #PASS_MAX_LEN # NO_PASSWORD_CONSOLE root@debian7:~# root@rhel65:~# chage -l paul Last password change Password expires Password inactive Account expires Minimum number of days between password change Maximum number of days between password change Number of days of warning before password expires root@rhel65:~# : Mar 27, 2014 : never : never : never : 0 : 99999 : 7
  • 196. 22.8. disabling a password Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field in /etc/passwd starts with an exclamation mark, then the password can not be used. Using this feature is often called locking, disabling, or suspending a user account. Besides vi (or vipw) you can also accomplish this with usermod. The first command in the next screenshot will show the hashed password of laura in /etc/ shadow. The next command disables the password of laura, making it impossible for Laura to authenticate using this password. As you can see below, the password hash is simply preceded with an exclamation mark. The root user (and users with sudo rights on su) still will be able to su into the laura account (because the password is not needed here). Also note that laura will still be able to login if she has set up passwordless ssh! You can unlock the account again with usermod -U. Watch out for tiny differences in the command line options of passwd, usermod, and useradd on different Linux distributions. Verify the local files when using features like "disabling, suspending, or locking" on user accounts and their passwords. 22.9. editing local files If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these commands for password management, then use vipw instead of vi(m) directly. The vipw tool will do proper locking of the file. root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV root@debian7:~# usermod -L laura root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:!$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJ root@debian7:~# root@debian7:~# su - laura laura@debian7:~$ root@debian7:~# usermod -U laura root@debian7:~# grep laura /etc/shadow | cut -c1-70 laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV [root@RHEL5 ~]# vipw /etc/passwd vipw: the password file is busy (/etc/ptmp present)
  • 197. 22.10. solution: user passwords 1. Set the password for serena to hunter2. 2. Also set a password for venus and then lock the venus user account with usermod. Verify the locking in /etc/shadow before and after you lock it. Note that usermod -L precedes the password hash with an exclamation mark (!). 3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow before and after disabling. 4. What is the difference between locking a user account and disabling a user account's password like we just did with usermod -L and passwd -d? Locking will prevent the user from logging on to the system with his password by putting a ! in front of the password in /etc/shadow. Disabling with passwd will erase the password from /etc/shadow. 5. Try changing the password of serena to serena as serena. log on as serena, then execute: passwd serena... it should fail! 6. Make sure serena has to change her password in 10 days. chage -M 10 serena 7. Make sure every new user needs to change their password every 10 days. vi /etc/login.defs (and change PASS_MAX_DAYS to 10) root@debian7:~# passwd serena Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@debian7:~# passwd venus Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@debian7:~# grep venus /etc/shadow | cut -c1-70 venus:$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM/ root@debian7:~# usermod -L venus root@debian7:~# grep venus /etc/shadow | cut -c1-70 venus:!$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM root@debian7:~# grep serena /etc/shadow | cut -c1-70 serena:$6$Es/omrPE$F2Ypu8kpLrfKdW0v/UIwA5jrYyBD2nwZ/dt.i/IypRgiPZSdB/B root@debian7:~# passwd -d serena passwd: password expiry information changed. root@debian7:~# grep serena /etc/shadow serena::16358:0:99999:7::: root@debian7:~#
  • 198. 8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from venus to serena. Can serena now log on with hunter2 as a password ? 9. Why use vipw instead of vi ? What could be the problem when using vi or vim ? 10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat / etc/shells. 11. Which useradd option allows you to name a home directory ? 12. How can you see whether the password of user serena is locked or unlocked ? Give a solution with grep and a solution with passwd. Yes. vipw will give a warning when someone else is already using that file (with vipw). chsh -l cat /etc/shells -d grep serena /etc/shadow passwd -S serena
  • 200. Chapter 23. user profiles Logged on users have a number of preset (and customized) aliases, variables, and functions, but where do they come from ? The shell uses a number of startup files that are executed (or rather sourced) whenever the shell is invoked. What follows is an overview of startup scripts.
  • 201. 23.1. system profile Both the bash and the ksh shell will verify the existence of /etc/profile and source it if it exists. When reading this script, you will notice (both on Debian and on Red Hat Enterprise Linux) that it builds the PATH environment variable (among others). The script might also change the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc This screenshot uses grep to show PATH manipulation in /etc/profile on Debian. This screenshot uses grep to show PATH manipulation in /etc/profile on RHEL7/CentOS7. The root user can use this script to set aliases, functions, and variables for every user on the system. 23.2. ~/.bash_profile When this file exists in the home directory, then bash will source it. On Debian Linux 5/6/7 this file does not exist by default. RHEL7/CentOS7 uses a small ~/.bash_profile where it checks for the existence of ~/.bashrc and then sources it. It also adds $HOME/bin to the $PATH variable. root@debian7:~# grep PATH /etc/profile PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games" export PATH root@debian7:~# [root@centos7 ~]# grep PATH /etc/profile case ":${PATH}:" in PATH=$PATH:$1 PATH=$1:$PATH export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL [root@centos7 ~]# [root@rhel7 ~]# cat /home/paul/.bash_profile # .bash_profile # Get the aliases and functions if [ -f ~/.bashrc ]; then . ~/.bashrc fi # User specific environment and startup programs PATH=$PATH:$HOME/.local/bin:$HOME/bin export PATH [root@rhel7 ~]#
  • 202. 23.3. ~/.bash_login When .bash_profile does not exist, then bash will check for ~/.bash_login and source it. Neither Debian nor Red Hat have this file by default. 23.4. ~/.profile When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence of ~/.profile and execute it. This file does not exist by default on Red Hat. On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH variable. RHEL/CentOS does not have this file by default. 23.5. ~/.bashrc The ~/.bashrc script is often sourced by other scripts. Let us take a look at what it does by default. Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and sourcing it. It also leaves room for custom aliases and functions. On Debian this script is quite a bit longer and configures $PS1, some history variables and a number af active and inactive aliases. root@debian7:~# tail -11 /home/paul/.profile if [ -n "$BASH_VERSION" ]; then # include .bashrc if it exists if [ -f "$HOME/.bashrc" ]; then . "$HOME/.bashrc" fi fi # set PATH so it includes user's private bin if it exists if [ -d "$HOME/bin" ] ; then PATH="$HOME/bin:$PATH" fi [root@rhel7 ~]# cat /home/paul/.bashrc # .bashrc # Source global definitions if [ -f /etc/bashrc ]; then . /etc/bashrc fi # Uncomment the following line if you don't like systemctl's auto-paging feature: # export SYSTEMD_PAGER= # User specific aliases and functions root@debian7:~# wc -l /home/paul/.bashrc 110 /home/paul/.bashrc
  • 203. 23.6. ~/.bash_logout When exiting bash, it can execute ~/.bash_logout. Debian use this opportunity to clear the console screen. Red Hat Enterprise Linux 5 will simple call the /usr/bin/clear command in this script. Red Hat Enterprise Linux 6 and 7 create this file, but leave it empty (except for a comment). serena@deb503:~$ cat .bash_logout # ~/.bash_logout: executed by bash(1) when login shell exits. # when leaving the console clear the screen to increase privacy if [ "$SHLVL" = 1 ]; then [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q fi [serena@rhel53 ~]$ cat .bash_logout # ~/.bash_logout /usr/bin/clear paul@rhel65:~$ cat .bash_logout # ~/.bash_logout
  • 204. 23.7. Debian overview Below is a table overview of when Debian is running any of these bash startup scripts. Table 30.1. Debian User Environment script su su - ssh gdm ~./bashrc no yes yes yes ~/.profile no yes yes yes /etc/profile no yes yes yes /etc/bash.bashrc yes no no yes 23.8. RHEL5 overview Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash startup scripts. Table 30.2. Red Hat User Environment script su su - ssh gdm ~./bashrc yes yes yes yes ~/.bash_profile no yes yes yes /etc/profile no yes yes yes /etc/bashrc yes yes yes yes
  • 205. 23.9. solution: user profiles 1. Make a list of all the profile files on your system. ls -a ~ ; ls -l /etc/pro* /etc/bash* 2. Read the contents of each of these, often they source extra scripts. 3. Put a unique variable, alias and function in each of those files. 4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrl- alt- F1, ...) and verify which of your custom variables, aliases and function are present in your environment. 5. Do you also know the order in which they are executed? same name aliases, functions and variables will overwrite each other 6. When an application depends on a setting in $HOME/.profile, does it matter whether $HOME/.bash_profile exists or not ? Yes it does matter. (man bash /INVOCATION)
  • 207. Chapter 24. groups Users can be listed in groups. Groups allow you to set permissions on the group level instead of having to set permissions for every individual user. Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users are advised to use this graphical tool. More experienced users can use command line tools to manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant files directly with vi or vigr.
  • 208. 24.1. groupadd Groups can be created with the groupadd command. The example below shows the creation of five (empty) groups. 24.2. group file Users can be a member of several groups. Group membership is defined by the /etc/group file. The first field is the group's name. The second field is the group's (encrypted) password (can be empty). The third field is the group identification or GID. The fourth field is the list of members, these groups have no members. 24.3. groups A user can type the groups command to see a list of groups where the user belongs to. root@laika:~# groupadd tennis root@laika:~# groupadd football root@laika:~# groupadd snooker root@laika:~# groupadd formula1 root@laika:~# groupadd salsa root@laika:~# tail -5 /etc/group tennis:x:1006: football:x:1007: snooker:x:1008: formula1:x:1009: salsa:x:1010: root@laika:~# [harry@RHEL4b ~]$ groups harry sports [harry@RHEL4b ~]$
  • 209. 24.4. usermod Group membership can be modified with the useradd or usermod command. Be careful when using usermod to add users to groups. By default, the usermod command will remove the user from every group of which he is a member if the group is not listed in the command! Using the -a (append) switch prevents this behaviour. 24.5. groupmod You can change the group name with the groupmod command. 24.6. groupdel You can permanently remove a group with the groupdel command. root@laika:~# usermod -a -G tennis inge root@laika:~# usermod -a -G tennis katrien root@laika:~# usermod -a -G salsa katrien root@laika:~# usermod -a -G snooker sandra root@laika:~# usermod -a -G formula1 annelies root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: snooker:x:1008:sandra formula1:x:1009:annelies salsa:x:1010:katrien root@laika:~# root@laika:~# groupmod -n darts snooker root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: formula1:x:1009:annelies salsa:x:1010:katrien darts:x:1008:sandra root@laika:~# groupdel tennis root@laika:~#
  • 210. 24.7. gpasswd You can delegate control of group membership to another user with the gpasswd command. In the example below we delegate permissions to add and remove group members to serena for the sports group. Then we su to serena and add harry to the sports group. Group administrators do not have to be a member of the group. They can remove themselves from a group, but this does not influence their ability to add or remove members. Information about group administrators is kept in the /etc/gshadow file. To remove all group administrators from a group, use the gpasswd command to set an empty administrators list. [root@RHEL4b ~]# gpasswd -A "" sports [root@RHEL4b ~]# gpasswd -A serena sports [root@RHEL4b ~]# su - serena [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry) [serena@RHEL4b ~]$ gpasswd -a harry sports Adding user harry to group sports [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry),522(sports) [serena@RHEL4b ~]$ tail -1 /etc/group sports:x:522:serena,venus,harry [serena@RHEL4b ~]$ [serena@RHEL4b ~]$ gpasswd -d serena sports Removing user serena from group sports [serena@RHEL4b ~]$ exit [root@RHEL4b ~]# tail -1 /etc/gshadow sports:!:serena:venus,harry [root@RHEL4b ~]#
  • 211. 24.8. newgrp You can start a child shell with a new temporary primary group using the newgrp command. 24.9. vigr Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since it will do proper locking of the file. Only experienced senior administrators should use vi or root@rhel65:~# mkdir prigroup root@rhel65:~# cd prigroup/ root@rhel65:~/prigroup# touch standard.txt root@rhel65:~/prigroup# ls -l total 0 -rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt root@rhel65:~/prigroup# echo $SHLVL 1 root@rhel65:~/prigroup# newgrp tennis root@rhel65:~/prigroup# echo $SHLVL 2 root@rhel65:~/prigroup# touch newgrp.txt root@rhel65:~/prigroup# ls -l total 0 -rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt -rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt root@rhel65:~/prigroup# exit exit root@rhel65:~/prigroup#
  • 213. 24.10. solution: groups 1. Create the groups tennis, football and sports. 2. In one command, make venus a member of tennis and sports. 3. Rename the football group to foot. 4. Use vi to add serena to the tennis group. 5. Use the id command to verify that serena is a member of tennis. 6. Make someone responsible for managing group membership of foot and sports. Test that it works. groupadd tennis ; groupadd football ; groupadd sports usermod -a -G tennis,sports venus groupmod -n foot football vi /etc/group id (and after logoff logon serena should be member) gpasswd -A (to make manager) gpasswd -a (to add member)
  • 214. Chapter 25. standard file permissions This chapter contains details about basic file security through file ownership and file permissions.
  • 215. 25.1. file ownership 25.1.1. user owner and group owner The users and groups of a system can be locally managed in /etc/passwd and /etc/group, or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files. Actually, every file has a user owner and a group owner, as can be seen in the following screenshot. paul@rhel65:~/owners$ ls -lh total 636K -rw-r--r--. 1 paul snooker 1.1K Apr 8 18:47 data.odt -rw-r--r--. 1 paul paul 626K Apr 8 18:46 file1 -rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2 -rw-rw-r--. 1 root root paul@rhel65:~/owners$ 0 Apr 8 18:47 stuff.txt User paul owns three files; file1 has paul as user owner and has the group paul as group owner, data.odt is group owned by the group snooker, file2 by the group tennis. The last file is called stuff.txt and is owned by the root user and the root group. 25.1.2. listing user accounts You can use the following command to list all local user accounts. paul@debian7~$ cut -d: -f1 /etc/passwd | column root ntp sam bert naomi daemon mysql tom rino matthias2 bin paul wouter antonio bram sys maarten robrecht simon fabrice sync kevin bilal sven chimene games yuri dimitri wouter2 messagebus man william ahmed tarik roger lp yves dylan jan frank mail kris robin ian toon news hamid matthias ivan rinus uucp vladimir ben azeddine eddy proxy abiy mike eric bram2 www-data david kevin2 kamel keith backup chahid kenzo ischa jesse list stef aaron bart frederick irc joeri lorenzo omer hans gnats glenn jens kurt dries nobody yannick ruben steve steve2 libuuid christof jelle constantin tomas Debian-exim george stefaan sam2 johan statd joost marc bjorn tom2 sshd arno thomas ronald
  • 216. 25.1.3. chgrp You can change the group owner of a file using the chgrp command. 25.1.4. chown The user owner of a file can be changed with chown command. You can also use chown to change both the user owner and the group owner. root@rhel65:/home/paul/owners# ls -l file2 -rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2 root@rhel65:/home/paul/owners# chgrp snooker file2 root@rhel65:/home/paul/owners# ls -l file2 -rw-r--r--. 1 root snooker 185 Apr 8 18:46 file2 root@rhel65:/home/paul/owners# root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown paul FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown root:project42 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul
  • 217. 25.2. list of special files When you use ls -l, for each file you can see ten characters before the user and group owner. The first character tells us the type of file. Regular files get a -, directories get a d, symbolic links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets an s. Table 32.1. Unix special files first character file type - normal file d directory l symbolic link p named pipe b block device c character device s socket Below a screenshot of a character device (the console) and a block device (the hard disk). And here you can see a directory, a regular file and a symbolic link. paul@debian6lt~$ ls -ld /dev/console /dev/sda crw------- 1 root root 5, 1 Mar 15 12:45 /dev/console brw-rw---- 1 root disk 8, 0 Mar 15 12:45 /dev/sda paul@debian6lt~$ ls -ld /etc /etc/hosts /etc/motd drwxr-xr-x 128 root root 12288 Mar 15 18:34 /etc -rw-r--r-- 1 root root 372 Dec 10 17:36 /etc/hosts lrwxrwxrwx 1 root root 13 Dec 5 10:36 /etc/motd -> /var/run/motd
  • 218. 25.3. permissions 25.3.1. rwx The nine characters following the file type denote the permissions in three triplets. A permission can be r for read access, w for write access, and x for execute. You need the r permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a directory. You need the w permission to create files in or remove files from a directory. Table 32.2. standard Unix file permissions permission on a file on a directory r (read) read file contents (cat) read directory contents (ls) w (write) change file contents (vi) create files in (touch) x (execute) execute the file enter the directory (cd) 25.3.2. three sets of rwx We already know that the output of ls -l starts with ten characters for each file. This screenshot shows a regular file (because the first character is a - ). Below is a table describing the function of all ten characters. Table 32.3. Unix file permissions position position characters function 1 - this is a regular file 2-4 rwx permissions for the user owner 5-7 r-x permissions for the group owner 8-10 r-- permissions for others When you are the user owner of a file, then the user owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you belong to the group that is the group owner of a file, then the group owner permissions apply to you. The rest of the permissions have no influence on your access to paul@RHELv4u4:~/test$ ls -l proc42.bash -rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash
  • 219. the file. When you are not the user owner of a file and you do not belong to the group owner, then the others permissions apply to you. The rest of the permissions have no influence on your access to the file.
  • 220. 25.3.3. permission examples Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions. paul@laika:~/perms$ ls total 12K -lh drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete -rwxrwxrwx 1 paul paul 0 2007-02-07 22:21 EveryoneFullControl.txt -r--r----- 1 paul paul 0 2007-02-07 22:21 OnlyOwnersRead.txt -rwxrwx--- 1 paul paul 0 2007-02-07 22:21 OwnersAll_RestNothing.txt dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter dr-x------ 2 paul paul paul@laika:~/perms$ 4.0K 2007-02-07 22:25 OnlyUserEnter To summarise, the first rwx triplet represents the permissions for the user owner. The second triplet corresponds to the group owner; it specifies permissions for all members of that group. The third triplet defines permissions for all other users that are not the user owner and are not a member of the group owner.
  • 221. 25.3.4. setting permissions (chmod) Permissions can be changed with chmod. The first example gives the user owner execute permissions. This example removes the group owners read permission. This example removes the others read permission. This example gives all of them the write permission. You don't even have to type the a. You can also set explicit permissions. Feel free to make any kind of combination. Even fishy combinations are accepted by chmod. paul@laika:~/perms$ ls -l permissions.txt -rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u+x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod g-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod o-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod a+w permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod +x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rw permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
  • 222. 25.3.5. setting octal permissions Most Unix administrators will use the old school octal system to talk about and set permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1. Table 32.4. Octal permissions binary octal permission 000 0 --- 001 1 --x 010 2 -w- 011 3 -wx 100 4 r-- 101 5 r-x 110 6 rw- 111 7 rwx This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod command will accept these numbers. paul@laika:~/perms$ chmod 777 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 664 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 750 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
  • 223. 25.3.6. umask When creating a file or directory, a set of default permissions are applied. These default permissions are determined by the umask. The umask specifies permissions that you do not want set on by default. You can display the umask with the umask command. As you can also see, the file is also not executable by default. This is a general security feature among Unixes; newly created files are never executable by default. You have to explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the umask has no meaning--a umask of 0022 is the same as 0033. 25.3.7. mkdir -m When creating directories with mkdir you can use the -m option to set the mode. This screenshot explains. 25.3.8. cp -p To preserve permissions and time stamps from source files, use cp -p. paul@laika:~/perms$ cp file* cp paul@laika:~/perms$ cp -p file* cpp paul@laika:~/perms$ ll * -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 cp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:34 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42 cpp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 [Harry@RHEL4b ~]$ umask 0002 [Harry@RHEL4b ~]$ touch test [Harry@RHEL4b ~]$ ls -l test -rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test [Harry@RHEL4b ~]$ paul@debian5~$ mkdir -m 700 MyDir paul@debian5~$ mkdir -m 777 Public paul@debian5~$ ls -dl MyDir/ Public/ drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/ drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/
  • 224. 25.4. solution: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. mkdir ~/permissions ; touch ~/permissions/myfile.txt 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? cp /etc/hosts ~/permissions/ The copy is owned by you. 3. As root, create a file in the users ~/permissions directory. (become root)# touch /home/username/permissions/rootfile 4. As normal user, look at who owns this file created by root. ls -l ~/permissions The file created by root is owned by root. 5. Change the ownership of all files in ~/permissions to yourself. chown user ~/permissions/* You cannot become owner of the file that belongs to root. 6. Make sure you have all rights to these files, and others can only read. chmod 644 (on files) chmod 755 (on directories) 7. With chmod, is 770 the same as rwxrwx--- ? yes 8. With chmod, is 664 the same as r-xr-xr-- ? No 9. With chmod, is 400 the same as r-------- ? yes 10. With chmod, is 734 the same as rwxr-xr-- ? no 11a. Display the umask in octal and in symbolic form. umask ; umask -S 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. umask -S u=rwx,go=
  • 225. 12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. Yes, others can read this file 13b. Can root read this file ? Can root write to this file with vi ? Yes, root can read and write to this file. Permissions do not apply to root. 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. You can not yet do the last part of this exercise... (become root) # echo hello > /home/username/root.txt # chmod 744 /home/username/root.txt (become user) vi ~/root.txt echo hello > file ; chmod 744 file mkdir /home/project42 ; groupadd project42 chgrp project42 /home/project42 ; chmod 775 /home/project42
  • 226. 319 Chapter 26. advanced file permissions
  • 227. 26.1. sticky bit on directory You can set the sticky bit on a directory to prevent users from removing files that they do not own as a user owner. The sticky bit is displayed at the same location as the x permission for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there is no x for others). The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets. You will typically find the sticky bit on the /tmp directory. 26.2. setgid bit on directory setgid can be used on directories to make sure that all files inside the directory are owned by the group owner of the directory. The setgid bit is displayed at the same location as the x permission for group owner. The setgid bit is represented by an s (meaning x is also there) or a S (when there is no x for the group owner). As this example shows, even though root does not belong to the group proj55, the files created by root in /project55 will belong to proj55 since the setgid is set. You can use the find command to find all setgid directories. root@RHELv4u4:~# mkdir /project55 root@RHELv4u4:~# ls -ld /project55 drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# chmod +t /project55/ root@RHELv4u4:~# ls -ld /project55 drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# root@RHELv4u4:~# chmod 1775 /project55/ root@RHELv4u4:~# ls -ld /project55 drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# root@barry:~# ls -ld /tmp drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp root@RHELv4u4:~# groupadd proj55 root@RHELv4u4:~# chown root:proj55 /project55/ root@RHELv4u4:~# chmod 2775 /project55/ root@RHELv4u4:~# touch /project55/fromroot.txt root@RHELv4u4:~# ls -ld /project55/ drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/ root@RHELv4u4:~# ls -l /project55/ total 4 -rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt root@RHELv4u4:~# paul@laika:~$ find / -type d -perm -2000 2> /dev/null /var/log/mysql /var/log/news /var/local ...
  • 228. 26.3. setgid and setuid on regular files These two permissions cause an executable file to be executed with the permissions of the file owner instead of the executing owner. This means that if any user executes aprogram that belongs to the root user, and the setuid bit is set on that program, then the program runs as root. This can be dangerous, but sometimes this is good for security. Take the example of passwords; they are stored in /etc/shadow which is only readable by root. (The root user never needs permissions anyway.) Changing your password requires an update of this file, so how can normal non-root users do this? Let's take a look at the permissions on the /usr/bin/passwd. When running the passwd program, you are executing it with root credentials. You can use the find command to find all setuid programs. In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will result in these programs to run with the credentials of their group owner. 26.4. setuid on sudo The sudo binary has the setuid bit set, so any user can run it with the effective userid of root. paul@laika:~$ find /usr/bin -type f -perm -04000 /usr/bin/arping /usr/bin/kgrantpty /usr/bin/newgrp /usr/bin/chfn /usr/bin/sudo /usr/bin/fping6 /usr/bin/passwd /usr/bin/gpasswd ... root@RHELv4u4:~# ls -l /etc/shadow -r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow root@RHELv4u4:~# ls -l /usr/bin/passwd -r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd paul@rhel65:~$ ls -l $(which sudo) ---s--x--x. 1 root root 123832 Oct 7 2013 /usr/bin/sudo paul@rhel65:~$
  • 229. 26.5. solution: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. groupadd sports mkdir /home/sports chown root:sports /home/sports 1b. Members of the sports group should be able to create files in this directory. chmod 770 /home/sports 1c. All files created in this directory should be group-owned by the sports group. chmod 2770 /home/sports 1d. Users should be able to delete only their own user-owned files. chmod +t /home/sports 1e. Test that this works! Log in with different users (group members and others and root), create files and watch the permissions. Try changing and deleting files... 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. A normal user cannot change password now. 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 4755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd paul@laika:~$ sudo su - [sudo] password for paul: root@laika:~# mkdir attr root@laika:~# cd attr/ root@laika:~/attr# touch file42 root@laika:~/attr# lsattr ./file42 root@laika:~/attr# chattr +i file42
  • 230. root@laika:~/attr# lsattr ----i------------- ./file42 root@laika:~/attr# rm -rf file42 rm: cannot remove `file42': Operation not permitted root@laika:~/attr# chattr -i file42 root@laika:~/attr# rm -rf file42 root@laika:~/attr#
  • 232. Chapter 27. access control lists Standard Unix permissions might not be enough for some organisations. This chapter introduces access control lists or acl's to further protect files and directories.
  • 233. 27.1. acl in /etc/fstab File systems that support access control lists, or acls, have to be mounted with the acl option listed in /etc/fstab. In the example below, you can see that the root file system has acl support, whereas /home/data does not. root@laika:~# tail -4 /etc/fstab /dev/sda1 / ext3 acl,relatime 0 1 /dev/sdb2 /home/data auto noacl,defaults 0 0 pasha:/home/r /home/pasha nfs defaults 0 0 wolf:/srv/data /home/wolf nfs defaults 0 0 27.2. getfacl Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl of file33 with getfacl. 27.3. setfacl Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how to change the acl of file33 with setfacl. First we add user sandra with octal permission 7 to the acl. paul@laika:~/test$ setfacl -m u:sandra:7 file33 Then we add the group tennis with octal permission 6 to the acl of the same file. paul@laika:~/test$ setfacl -m g:tennis:6 file33 The result is visible with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- mask::rwx other::r-- paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- user:sandra:rwx group::r-- group:tennis:rw- mask::rwx other::r--
  • 234. 27.4. remove an acl entry The -x option of the setfacl command will remove an acl entry from the targeted file. Note that omitting the u or g when defining the acl for an account will default it to a user account. 27.5. remove the complete acl The -b option of the setfacl command will remove the acl from the targeted file. 27.6. the acl mask The acl mask defines the maximum effective permissions for any entry in the acl. This mask is calculated every time you execute the setfacl or chmod commands. You can prevent the calculation by using the --no-mask switch. paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- user:sandra:rwx #effective:rw- group::r-- mask::rw- other::r-- paul@laika:~/test$ setfacl -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 | grep sandra user:sandra:rwx paul@laika:~/test$ setfacl -x sandra file33 paul@laika:~/test$ getfacl file33 | grep sandra paul@laika:~/test$ setfacl -b file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- other::r--
  • 235. 27.7. eiciel Desktop users might want to use eiciel to manage acls with a graphical tool. You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to manage acls. paul@laika:~$ sudo aptitude install eiciel nautilus-actions
  • 236. Chapter 28. file links An average computer using Linux has a file system with many hard links and symbolic links. To understand links in a file system, you first have to understand what an inode is.
  • 237. 28.1. inodes 28.1.1. inode contents An inode is a data structure that contains metadata about a file. When the file system stores a new file on the hard disk, it stores not only the contents (data) of the file, but also extra properties like the name of the file, the creation date, its permissions, the owner of the file, and more. All this information (except the name of the file and the contents of the file) is stored in the inode of the file. The ls -l command will display some of the inode contents, as seen in this screenshot. 28.1.2. inode table The inode table contains all of the inodes and is created when you create the file system (with mkfs). You can use the df -i command to see how many inodes are used and free on mounted file systems. root@rhel53 ~# df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/mapper/VolGroup00-LogVol00 4947968 115326 4832642 3% / /dev/hda1 26104 45 26059 1% /boot tmpfs 64417 1 64416 1% /dev/shm /dev/sda1 262144 2207 259937 1% /home/project42 /dev/sdb1 74400 5519 68881 8% /home/project33 /dev/sdb5 0 0 0 - /home/sales /dev/sdb6 100744 11 100733 1% /home/research In the df -i screenshot above you can see the inode usage for several mounted file systems. You don't see numbers for /dev/sdb5 because it is a fat file system. 28.1.3. inode number Each inode has a unique number (the inode number). You can see the inode numbers with the ls -li command. These three files were created one after the other and got three different inodes (the first column). All the information you see with this ls command resides in the inode, except for the filename (which is contained in the directory). root@rhel53 ~# ls -ld /home/project42/ drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/ paul@RHELv4u4:~/test$ touch file1 paul@RHELv4u4:~/test$ touch file2 paul@RHELv4u4:~/test$ touch file3 paul@RHELv4u4:~/test$ ls -li total 12 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817267 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$
  • 238. 28.1.4. inode and file contents Let's put some data in one of the files. The data that is displayed by the cat command is not in the inode, but somewhere else on the disk. The inode contains a pointer to that data. 28.2. about directories 28.2.1. a directory is a table A directory is a special kind of file that contains a table which maps filenames to inodes. Listing our current directory with ls -ali will display the contents of the directory file. paul@RHELv4u4:~/test$ ls -ali total 32 817262 drwxrwxr-x 2 paul paul 4096 Feb 5 15:42 . 800768 drwx------ 16 paul paul 4096 Feb 5 15:42 .. 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul@RHELv4u4:~/test$ paul paul 0 Feb 5 15:38 file3 28.2.2. . and .. You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself, and the dotdot .. is a mapping to the parent directory. The three other names are mappings to different inodes. paul@RHELv4u4:~/test$ ls -li total 16 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$ cat file2 It is winter now and it is very cold. We do not like the cold, we prefer hot summer nights. paul@RHELv4u4:~/test$
  • 239. 28.3. hard links 28.3.1. creating hard links When we create a hard link to a file with ln, an extra entry is added in the directory. A new file name is mapped to an existing inode. Both files have the same inode, so they will always have the same permissions and the same owner. Both files will have the same content. Actually, both files are equal now, meaning you can safely remove the original file, the hardlinked file will remain. The inode contains a counter, counting the number of hard links to itself. When the counter drops to zero, then the inode is emptied. 28.3.2. finding hard links You can use the find command to look for files with a certain inode. The screenshot below shows how to search for all filenames that point to inode 817270. Remember that aninode number is unique to its partition. paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 24 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2 paul@RHELv4u4:~/test$ paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null /home/paul/test/file2 /home/paul/test/hardlink_to_file2
  • 240. 28.4. symbolic links Symbolic links (sometimes called soft links) do not link to inodes, but create a name to name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic link gets an inode of its own. paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 32 817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2 817267 lrwxrwxrwx 1 paul paul@RHELv4u4:~/test$ paul 5 Feb 5 16:55 symlink_to_file2 -> file2 Permissions on a symbolic link have no meaning, since the permissions of the target apply. Hard links are limited to their own partition (because they point to an inode), symbolic links can link anywhere (other file systems, even networked). 28.5. removing links Links can be removed with rm. 28.6. solution : links 1. Create two files named winter.txt and summer.txt, put some text in them. 2. Create a hard link to winter.txt named hlwinter.txt. 3. Display the inode numbers of these three files, the hard links should have the same inode. 4. Use the find command to list the two hardlinked files 5. Everything about a file is in the inode, except two things : name them! The name of the file is in a directory, and the contents is somewhere on the disk. 6. Create a symbolic link to summer.txt called slsummer.txt. paul@laika:~$ touch data.txt paul@laika:~$ ln -s data.txt sl_data.txt paul@laika:~$ ln data.txt hl_data.txt paul@laika:~$ rm sl_data.txt paul@laika:~$ rm hl_data.txt echo cold > winter.txt ; echo hot > summer.txt ln winter.txt hlwinter.txt ls -li winter.txt summer.txt hlwinter.txt find . -inum xyz #replace xyz with the inode number ln -s summer.txt slsummer.txt
  • 241. 7. Find all files with inode number 2. What does this information tell you ? It tells you there is more than one inode table (one for every formatted partition + virtual file systems) 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? 9. Look in /lib with ls -l... 10. Use find to look in your home directory for regular files that do not(!) have one hard link. ls -l /etc/init.d ls -l /etc/rc2.d ls -l /etc/rc3.d ls -l /lib find ~ ! -links 1 -type f
  • 242. Appendix A. keyboard settings A.1. about keyboard layout Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when you are not from the USA and want a local keyboard layout on your system, then the best practice is to select this keyboard at installation time. Then the keyboard layout will always be correct. Also, whenever you use ssh to remotely manage a Linux system, your local keyboard layout will be used, independent of the server keyboard configuration. So you will not find much information on changing keyboard layout on the fly on linux, because not many people need it. Below are some tips to help you. A.2. X Keyboard Layout This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US- qwerty. When in Gnome or KDE or any other graphical environment, look in the graphical menu in preferences, there will be a keyboard section to choose your layout. Use the graphical menu instead of editing xorg.conf. A.3. shell keyboard layout When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US-qwerty configuration, followed by a Belgian azerty configuration. The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps. [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "be" [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="be-latin1" [paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/ total 52 drwxr-xr-x 2 root root 4096 Apr 1 00:14 amiga
  • 244. 339 keyboard settings drwxr-xr-x 2 root root 4096 Apr 1 00:14 atari drwxr-xr-x 8 root root 4096 Apr 1 00:14 i386 drwxr-xr-x 2 root root 4096 Apr 1 00:14 include drwxr-xr-x 4 root root 4096 Apr 1 00:14 mac lrwxrwxrwx 1 root root 3 Apr 1 00:14 ppc -> mac drwxr-xr-x 2 root root 4096 Apr 1 00:14 sun
  • 245. Appendix B. hardware B.1. buses B.1.1. about buses Hardware components communicate with the Central Processing Unit or cpu over a bus. The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card. These are all Plag and Play buses. Older x86 computers often had isa buses, which can be configured using jumpers or dip switches. B.1.2. /proc/bus To list the buses recognised by the Linux kernel on your computer, look at the contents of the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below). Can you guess which of these two screenshots was taken on a laptop ? B.1.3. /usr/sbin/lsusb To list all the usb devices connected to your system, you could read the contents of /proc/ bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is executed here on a SPARC system with Ubuntu. B.1.4. /var/lib/usbutils/usb.ids The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices. root@laika:~# ls /proc/bus/ input pccard pci usb [root@RHEL4b ~]# ls /proc/bus/ input pci usb root@shaka:~# lsusb Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage) root@shaka:~# paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head ------> /var/lib/usbutils/usb.ids <------ # # List of USB ID's # # Maintained by Vojtech Pavlik <vojtech@suse.cz>
  • 246. hardware 341 B.1.5. /usr/sbin/lspci To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci (partial output below). B.2. interrupts B.2.1. about interrupts An interrupt request or IRQ is a request from a device to the CPU. A device raises an interrupt when it requires the attention of the CPU (could be because the device has data ready to be read by the CPU). Since the introduction of pci, irq's can be shared among devices. Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a channel for IRQ's 8 to 15, and thus is the same as IRQ 9. B.2.2. /proc/interrupts You can see a listing of interrupts on your system in /proc/interrupts. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 1320048 555 IO-APIC-edge timer 1: 10224 7 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 2 1 IO-APIC-edge rtc 10: 3062 21 IO-APIC-fasteoi acpi 12: 131 2 IO-APIC-edge i8042 15: 47073 0 IO-APIC-edge ide1 18: 0 1 IO-APIC-fasteoi yenta 19: 31056 1 IO-APIC-fasteoi libata, ohci1394 20: 19042 1 IO-APIC-fasteoi eth0 21: 44052 1 IO-APIC-fasteoi uhci_hcd:usb1, uhci_hcd:usb2,... 22: 188352 1 IO-APIC-fasteoi ra0 # If you have any new entries, send them to the maintainer. # The latest version can be obtained from # http://www.linux-usb.org/usb.ids # # $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $ paul@laika:~$ lspci ... 00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139... 00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816... 00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713... 00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI 00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ... 00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A... 00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... 00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... ...
  • 247. 23: 632444 1 IO-APIC-fasteoi nvidia 24: 1585 1 IO-APIC-fasteoi VIA82XX-MODEM, VIA8237 B.2.3. dmesg You can also use dmesg to find irq's allocated at boot time. B.3. io ports B.3.1. about io ports Communication in the other direction, from CPU to device, happens through IO ports. The CPU writes data or control codes to the IO port of the device. But this is not only a one way communication, the CPU can also use a device's IO port to read status information about the device. Unlike interrupts, ports cannot be shared! B.3.2. /proc/ioports You can see a listing of your system's IO ports via /proc/ioports. B.4. dma B.4.1. about dma A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With dma or Direct Memory Access a device can gain (temporary) access to a specific range of the ram memory. B.4.2. /proc/dma Looking at /proc/dma might not give you the information that you want, since it only contains currently assigned dma channels for isa devices. paul@laika:~$ dmesg | grep "irq 1[45]" [ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14 [ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15 [root@RHEL4b ~]# cat /proc/ioports 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 0060-006f : keyboard 0070-0077 : rtc 0080-008f : dma page reg 00a0-00a1 : pic2 00c0-00df : dma2 00f0-00ff : fpu 0170-0177 : ide1 02f8-02ff : serial ...
  • 248. hardware 343 pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful. The screenshot below shows that during boot the parallel port received dma channel 1, and the Infrared port received dma channel 3. root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3' [ 20.576000] parport: PnPBIOS parport detected. [ 20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1... [ 20.764000] irda_init() -- [ 21.204000] pnp: Device 00:0b activated. [ 21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8... [ 21.204000] nsc-ircc, chip->init root@laika:~# cat /proc/dma 1: parport0 4: cascade
  • 249. Appendix C. License GNU Free Documentation License Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The "Invariant Sections" are certain Secondary Sections whose titles
  • 250. License 345 are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque". Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only. The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text. The "publisher" means any person or entity that distributes copies of the Document to the public. A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. 2. VERBATIM COPYING You may copy and distribute the Document in any medium, either
  • 251. commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3. You may also lend copies, under the same conditions stated above, and you may publicly display copies. 3. COPYING IN QUANTITY If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects. If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages. If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public. It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document. 4. MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version: * A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
  • 252. License 347 * B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. * C. State on the Title page the name of the publisher of the Modified Version, as the publisher. * D. Preserve all the copyright notices of the Document. * E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. * F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. * G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. * H. Include an unaltered copy of this License. * I. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. * J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. * K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. * L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. * M. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version. * N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section. * O. Preserve any Warranty Disclaimers. If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles. You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties— for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of,
  • 253. you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. 5. COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements". 6. COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. 7. AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
  • 254. License 349 8. TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. 9. TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. 10. FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/. Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies
  • 255. that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document. 11. RELICENSING "Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site. "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. "Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.