DOC_LM_Chap 1 and 2. document -1 unix.pdf

Unix & Shell Programming (Introduction to Unix)
Basic Introduction
● In the 1965, the MIT (Massachusetts Institute of Technology), AT&T Bell Labs, and General
Electric developed an experimental time sharing operating system called Multics (Multiplexed
Information & Computing Service).
● In 1969, Ken Thompson and Dennis Ritchie, other System engineers at AT&T's Bell Labs, created
the UNICS (UNiplexed Information and Computing Service ) on PDP – 7 (Programmed Data
Processor)
● UNICS was developed in Assembly Language.
● In 1972, UNIX was rewritten in the C programming language and renamed as “UNIX” and moved
to the PDP – 11/20.
Features of UNIX
UNIX offers several features like Multiuser, Multitasking, Inter Process Communication, Security,
Portability, and Open Source.
1) Multiuser :
In Multiuser System, the same computer resources say hard disk, memory etc. are accessible to
many users. The Users are given different terminals to operate from. A terminal is a keyboard and
a monitor, which are the input and output devices for that user. All terminals are connected to
main computer whose resources are available by all users. So a user at any of the terminals can
use not only computer but also any peripherals that may be attached e.g. Printer, Scanner etc.
At the heart of UNIX installation, is host machine which is known as Server or Consol. The no of
terminals connected to the host machine depends on no of ports that are present in its controller
part.
2) The Building - Block Approach
The Unix developers thought about keeping small commands for every kind of work. So Unix has
so many commands, each of which performs one simple job only. You can use 2 commands by
using pipes (‘|’). Example : $ ls | wc. Here, | (pipe) connects 2 commands to create a pipeline.
This command counts the number of files in the directory. These types of connected commands
that can filter/manipulate data in other ways are called filters.
Nowadays, many UNIX tools are designed in a way that the output of 1 can be used as an input
for the others. We can create a large number of combinations by connecting a number of tools.
3) The UNIX Toolkit
Unix has a kernel but the kernel alone can’t do much that could help the user. So, we need to use
the host of applications that usually come along with the UNIX systems. The applications are
quite diversified. General-purpose tools, text manipulation utilities (called filters), compilers and
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interpreters, networked programs, and system administration tools are all included. With every
UNIX release, new tools are being added and the older ones are modified/ removed.
4) Pattern Matching
UNIX also provide pattern matching features. The * (metacharacter) is a special character used by
the system to indicate that it can match a number of file names. For example chapter*. It will
match find all file starting from chapter ( chapter1, chapter2, chapter3 etc.).
Some of the most advanced and useful tools also use a special expression called a regular
expression that is framed with characters from this set.
5) Multitasking:
UNIX is capable of carrying out more than one job at same time i.e. in UNIX, a single user can also
run multiple tasks concurrently. It is usual for a user to edit a file, print another on the printer,
send email to a friend and browse the World Wide Web – all without leaving any of the
applications. The kernel is designed to handle a user’s multiple needs. In such a multitasking
situation, only one job runs in the foreground while the rest run in the background. You can
switch jobs between background and foreground, suspend or even terminate them. Multitasking
is an important component of the process management system.
6) Inter process Communication (IPS):
IPC is a mechanism that allows processes to communicate with each other. This can be useful for
sharing data, coordinating activities, or synchronizing tasks. This feature allows user to pass on
data, exchange mail or program to another users in networks. This communication may be within
the Network of single main computer or between two or more such Computer Network
7) Security:
UNIX allows sharing of data but to secure them it provides three levels of protection.
a) By assigning login name and password to individual user ensuring that no other user can
have access to your work.
b) At file level, there are read, write and execute permission to each file. It decides who can
access particular file, who can modify it and who can execute it.
We can apply this permission in combination to User, Group and Other.
c) By performing encryption, the file converts in unreadable format. So that even if
someone succeeds in opening it, your secretes are safe.
8) Portability:
UNIX is written in high level language (i.e. in C) making it easier to read, understand, change and
move to other machines.
9) Open System:
UNIX has an open architecture using which user can add tools to the program by writing
program and storing the executable in a separate area in this file system.
10) Programming Facility :
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The Unix Shell is also a programming language. The Shell is able to be programmed. It is used to
write Shell Script. It has all the control structures, loops, and variables required for programming
purposes. These features are used to design the shell scripts (programs that can invoke the UNIX
commands). Many of the system’s functions can be controlled and automated by using these
shell script.
11) Documentation:
UNIX documentation is very much important for Command reference and their configuration
files. The online help facility available is the man command. Also, other resources are available
on internet as newsgroups on UNIX where you can fire your queries related to Shell
Programming or a Network Configuration.
UNIX Architecture or Organization
The functioning of UNIX is managed in three levels. On the outer level reside the application
programs and other utilities, which speak our language.
At the heart of UNIX is the kernel, which interacts with the actual hardware in machine language.
Above figure shows the three layers of Unix OS. The Shell or Command Interpreter, which interprets
the commands that we give and then conveys them to the kernel, which ultimately executes them.
The Kernel is a collection of routines (Processes or functions) written is C. It is loaded into memory
when system is booted & communicates directly with h/w. User Program that needs to access the
h/w, uses the service of Kernel which perform the job on behalf of user. This Program access the
kernel through a set of functions known as System Call.
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The kernel manages files, carries out all the data transfer between the file system and the
hardware, and also manages memory. The Scheduling of various programs running in memory or
allocation of CPU time to all running programs also lies with kernel. It also handles any interrupts
issued, as it is the entity that has direct dealing with the hardware.
The Kernel program is usually stored in a file called “Unix” whereas the shell program is in a file
called “sh”. For each user working with UNIX at any time has different shell programs. Thus, at a
particular point in time there may be several shells running in memory but only one kernel. This is
because, at any instance Unix is capable of executing only one program as the other programs wait
for their turn. And since it is the kernel which executes the program one kernel is sufficient.
However, different users at different terminals are trying to seek kernel’s attention. And since the
user interacts with the kernel through the shell, different shells are necessary.
What is Shell?
● Shell is nothing but Command Interpreter.
● Shell is most widely used Utility program on all Unix System.
● Shell is the key to interact with the Unix System.
● The Shell is itself a program that is written in ‘C’.
● It is loaded into memory when a user logs in and interacts with the user enables to
communicate with the kernel.
● The Shell is also called as Command Interpreter because it interprets the commands from
the user and passes the commands on the kernel.
● The Shell is responsible for returning the result of the commands to the terminal, a file, or
another device such as a printer.
● Following figure shows
[Relationship of the Shell to UNIX]
A unique feature of Unix OS is that all UNIX command exists as Utility Program. This Program
located as individual file in system directory such as /bin, /etc, /userbin. The Shell can be considered
as a Master Utility Program which enable a user to gain access to all other utility & resources of the
computer.
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Types of Shell
The shell runs like any other program under the UNIX System. Due to this feature a no of shells have
been developed in response to different needs of User. Some of the popular Shells are listed below.
1) Bourne Shell :
● This is one of the most widely use Shell in Unix World.
● It is developed by Steve Bourne at AT&T Bell Laboratories in late 1970.
● It is primary Unix command interpreter and comes along with every Unix System.
● It provides Dollar Prompt ($) on Unix installation is the trademark of the Bourne Shell.
● The executable file name is ‘sh’.
2) C Shell:
● Mr. Bill Joy developed it at University of California. It has two advantages over Bourne Shell.
1. It allows aliasing of commands. This is useful when lengthy commands are used and
renamed. Instead of typing the entire command you can simply use the short alias at
the command line.
2. C Shell has a command history feature means previously typed command can be
recalled. This is similar to the DOSKEY in MSDOS environment.
● It provides Percentage Prompt (%) on Unix installation is the trademark of C Shell.
● The executable file name is ‘csh’.
3) Korn Shell:
● It is very powerful, and is a superset of Bourne Shell.
● The Korn Shell includes all the enhancements which are in the C Shell, like Command History &
aliasing, and offers a few more features itself.
● It is developed by David Korn at AT&T Bell Laboratories.
● It provides Dollar Prompt ($).
● The executable file name is ‘ksh’.
Features of Shell
1) Interactive Environment:
● The Shell allows user to create communication between User and Host UNIX System. This
Communication terminates when user ends the Session.
2) Shell Script:
● The Shell contains internal command which can be utilized by the User for programming.
● The Shell Script is group of UNIX commands and executes as individual programming file.
3) I/O Redirection:
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● Output Redirection is function of the Shell that redirects the Output of the program as an input
to other file or command.
● Similarly, the Shell can make a program that accepts input from other than keyboard by
redirecting its input from another Source like file.
4) Piping Mechanism:
● Piping Facility allows the output of one command to be used as input of another command.
E.g. who | wc
o Here, the output of ‘who’ command will the input of ‘wc’ command.
5) Meta character Facility (Filename Substitution) :
Shell recognizes *, . ,?, [ .. ] as Special characters when reading the argument to a command
line. Shell then perform filename expansion on this list before executing the requested
command.
E.g. ls s*
o Here this command displays the list of all files begin with ‘s’.
6) Background Process:
● Multitasking facilities allows the user to run commands in background.
● This allows the commands to be processed while the user can proceed with other task.
● When a background task is completed, the user is notified.
7) Customize Environment:
● The Shell is working environment and gives facilities by which users can change according to
their needs.
8) Shell Variable:
● The User can declare the variables to store the data which can be access anywhere in program.
What is kernel?
It is the heart of Operation System. The kernel assigns memory to each of the programs that are
running, carries out all the data transfer between the file system and the hardware, partitions time
fairly so that each program can get its job done, handles all l/O operations, all other low-level
services, scheduling of various programs running in memory and so on.
Internal Command and External Command
● The command built into the Shell is known as Internal Command. The Shell does not create a
separate process to run internal command. They are directly run by Shell.
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E.g. cd, echo, pwd
● A command stored in /bin directory is known as External Command. Each command requires
the Shell to create a new Process.
E.g. ls, cat, date
Kernel Architecture
The Unix Operating System has three levels.
The above figure gives a top view of the Operating System. The User Level includes all the user
programs, libraries and system call interface using which the users interact with the kernel.
Following figure shows block diagram of kernel as follows:
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The Kernel has the file subsystem and process control subsystem. Also the kernel has three
components within the process control subsystem 1) inter-process communication, 2) scheduler 3)
memory management.
The File Subsystem of the kernel, it uses Device drivers to actually read or write disk files. 1) It uses
the Buffer Cache (high speed memory), which regulates data flow between the kernel and the
secondary storage device. The Buffer Cache interacts with Block Device drivers to initiate the data
transfer to and from the kernel, for block devices like disks and tapes, Character Device drivers are
used to interact with character devices like terminals and any device which is not a Block Device,
without buffer cache interaction. 2) Device drivers are the set of kernel modules that control the
operations on peripheral devices by making them appear as random storage devices to the rest of
the system.
The Process Control Subsystem and the File Subsystem interact with each other whenever a process
needs an executable file to be loaded into memory for execution. The process control subsystem is
responsible for inter-process communication, CPU Scheduling to processes, and memory
management. 1) The Memory management module of the process control subsystem controls the
allocation of memory to processes. Thus if the kernel finds that the main memory space is not
sufficient for the processes, then the kernel moves the processes between the main memory and
secondary memory. 2) The Scheduler module of the process control subsystem allocates the CPU to
processes. This Scheduling of the CPU is done in turns, where each process is given a time quantum.
The moment the time quantum expires for any process, the next high priority process will be run.
The previous process will again get CPU time as and when it becomes next high priority process. 3)
Inter-process Communication is implemented using advanced features of piping between
processes.
The Hardware control consists of modules for handling interrupts caused by devices like disk, tapes
or terminals.
File Types in UNIX
In UNIX all information is treated as a file. So, beside a User’s program files and data files, there are
also special files such as files that contains information about directory contents or files that store
information about various input output devices connected to the system. In Unix, there are three
categories of files.
1) Ordinary Files (Regular Files) :
All files created by User come under this category of file. These include all data files, files
containing programs, compiled and executable program files. A user can make changes to such
files. There are two subcategory of this category.
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a) Text File :
These text files normally contain only ASCII codes and UNIX has a command called cat to
display the content of the text file. These files contain all the source prorgrams and the
documents.
b) Binary File :
Binary file contain compile programs and nontext informations. Any byte of the binary file
contain any possible 256 combinations of 8 bits. UNIX provides command called od (octal
dump) to display the content of compiled object file.
2) Directory Files :
For each directory there is a file, by the same name as the directory which contains information
about files under that directory. A directory file is automatically created by Unix where a
directory is created. A directory file cannot be modified by a user but is instead modified
automatically by the system when a new file or a sub directory is added or deleted from the
directory.
3) Special Files (Device Files) :
Most of the System files in UNIX are special files are typically associated with input output
devices. Such files are found in the Standard UNIX directories such as /dev and /etc. Special files
cannot be altered by Users. The kernel accesses the inode for the required special file, picks up
this address and then executes the actual device driver.
4) FIFO Files :
FIFO files are used by UNIX to implement pipes. It is a file which is also treated as a stream of
bytes, but in a FIFO manner, i.e, the byte which is written to this file first, is the one which is put
out first by the FIFO file. This is the reason it is called “First In First Out (FIFO)”.
UNIX File System
All the Utilities, application, data in Unix is stored as Files. Even a directory is treated as a file which
contains several other files. The UNIX file system resembles an upside down tree. Thus, the file
system begins with a directory called root. The root directory is denoted as slash(/). Branching from
the root, there are several other directories called bin, lib, usr, etc, tmp, dev. The root directory also
contains a file UNIX, which is UNIX kernel itself. These directories are called sub-directories. Each
sub-directory contains several files and directories called sub-sub-directories. The figure shows
basic structure of Unix File system.
The main reason behind creation of directories is to keep related files together and separate them
from other group. E.g. to keep all user related files in the usr directory, all device related files in the
dev directory, all temporary files in tmp directory and so on.
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1) The ‘/ ’ directory (Root directory): The ‘/ ’ directory is called as the root directory. It is also
known as the super user’s directory. The user who logs in as root will be placed in ‘ / ‘
initially.
2) The /bin directory: The /bin directory contains essential UNIX utilities. The word ‘bin’ stands
for binary, hence this directory contains binary files as well as shell script. Binary files are
executables files.
3) The /dev directory: The /dev or device directory is where all device files are kept. UNIX
treats devices such as printer, disk storage devices, terminals and even areas of the
computer’s memory as a file. The files in /dev are termed as special files, since the file types
are neither directory nor ordinary file. The /dev directory will contain a file for each device
on the network. One important thing to note about the /dev directory is that there are no
file sizes. Instead, there are major and minor device numbers.
The major number represents the type of device being referred to, while minor number
distinguishes between possible instances of the same device or it indicates the special
characteristic of the device.
$ ls –l /dev
total 52
brw-rw-rw- 1 root sys 51, 0 Aug 28 07:28 cd0 CD-ROM
brw-rw-rw- 2 bin bin 2, 64 Mar 30 1997 fd0 Default floppy drive
brw------ 1 sysinfo sysinfo 1, 0 Mar 8 1996 fd0 First hard disk
crw------ 2 bin bin 2, 1 Dec 5 1997 fd0 Default floppy drive
File types in /dev directory are not ordinary or directory instead the type of files is either
Character Special or Block Special. A character device is simply a device from which
characters are read. These include terminals, printers and even raw memory. A block device
is a device that is used only for the file system and can only accessed by users via standard
input/output request such as for using the various UNIX utilities.
Major and Minor Numbers
Char devices are accessed through names in the filesystem. Those names are called special
files or device files or simply nodes of the filesystem tree; they are conventionally located in
the /dev directory. Special files for char drivers are identified by a “c” in the first column of
the output of ls -l. Block devices appear in /dev as well, but they are identified by a “b.”
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If you issue the ls -l command, you’ll see two numbers (separated by a comma) in the device
file entries before the date of last modification, where the file length normally appears.
These numbers are the major device number and minor device number for the particular
device. The following listing shows a few devices as they appear on a typical system. Their
major numbers are 1, 4, 7, and 10, while the minors are 1, 3, 5, 64, 65, and 129.
crw-rw-rw- 1 root root 1, 3 Feb 23 1999 null
crw------- 1 root root 10, 1 Feb 23 1999 psaux
crw------- 1 rubini tty 4, 1 Aug 16 22:22 tty1
crw-rw-rw- 1 root dialout 4, 64 Jun 30 11:19 ttyS0
crw-rw-rw- 1 root dialout 4, 65 Aug 16 00:00 ttyS1
crw------- 1 root sys 7, 1 Feb 23 1999 vcs1
crw------- 1 root sys 7, 129 Feb 23 1999 vcsa1
crw-rw-rw- 1 root root 1, 5 Feb 23 1999 zero
The major number identifies the driver associated with the device. For
example, /dev/null and /dev/zero are both managed by driver 1, whereas virtual consoles
and serial terminals are managed by driver 4; similarly, both vcs1 and vcsa1 devices are
managed by driver 7. The kernel uses the major number at open time to dispatch execution
to the appropriate driver.
The minor number is used only by the driver specified by the major number; other parts of
the kernel don’t use it, and merely pass it along to the driver.
4) The /etc directory: The /etc directory is a directory that contains various administrative
utilities together with other special system files that allow the host Unix System to start up
properly at bootstrap time.
Utilities for the handling of the system’s terminal devices are stored in /etc, as are the lists of
all the registered users of the system, including you.
5) The /tmp directory: The /tmp directory contains the temporary files created by UNIX or by
the Users. These files get automatically deleted when system is shutdown and restarted.
6) The /usr directory: The /usr directory contains several directories, each associated with a
particular user. The System administrator creates these directories when he creates accounts
for different users. Each user is allowed to work with his directory called home directory.
a) The /usr/bin directory: Within usr directory there is another /bin directory which
contains additional Unix command files that are more important to the end user.
b) The /usr/adm directory: This directory is a part of /usr directory. The ‘adm’ stands for
administrative in this context, hence the /usr/adm directory contains administrative
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utilities for user by the systems administrator, although most user will not be able to
access them.
c) The /usr/man directory: Unix has a few online help facilities of which man is one.
Man is the Unix manual feature, a small Unix utility program, that enables access to
information on all Unix utilities.
d) The /usr/scr directory: This is a directory that contains the source-code for many of
the binary utilities.
7) The /lib directory: The /lib directory contains all library functions provided by Unix for
programmers.
Features of UNIX File System
Following are the features of UNIX file system.
1) It is hierarchical file structure:
The Unix system organizes its files using upside-down hierarchical tree structure. All files will
have a ‘parent’ file, apart from a directory called ‘root’ directory, which is the parent of all
files on the system. This hierarchical component also adds to the dynamic flexibility of the
file system.
2) Files can grow dynamically ( or dynamic file expansion ):
The file system structure is dynamic, that is to say, its size is not determined by any rules
other than the amount of disk storage that it is available on the System. A file’s size can be
changed by user at any time.
3) Files have access permission:
Files are protected using file ownership mechanisms. These allow only a specific class of user
to access certain files.
4) Structureless files :
It contains sequence of bytes in a file
5) All devices are implemented as files :
E.g. hardware devices such as printer, terminal, disk devices all are treated as files.
Physical file structure of Unix
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A file system is a group of files and relevant information regarding them. Your whole hard disk may
comprise a single file system of it may be partitioned into several file system.
The disk space allotted to a Unix file system is made up of ‘blocks’ each of which are typically of 512
bytes. Some file systems have blocks of 1024 or 2048 bytes as well.
All the blocks belonging to the file system are logically divided into four parts . 1) Boot Block, 2)
Super Block, 3) I-node Block, 4) Data Block.
Boot Block Super Block I-node Block Data Block
● Boot Block:
Boot Block or Block 0 is the first block of the file system. This block is normally reserved for
booting procedures. It is also called Master Boot Record (MBR). The boot block contains the
partition table and a small ‘bootstrapping’ program. When the system is booted, the system
BIOS checks for the existence of the first block of hard disk and loads the entire segment
(code) of the boot block into memory. It then hands over control to the bootstrapping
program. This in turn loads the kernel (the file unix) into memory. However, the
bootstrapping program is read in form the boot block of the root(main or track zero) file
system. For other file system, this block is simply kept blank
● Super Block:
The Super Block or Block 1 is the ‘balance sheet’ of every Unix file System. It contains global
file information about disk usage and availability of the data block and I-nodes. It contains
o The Size of the file system.
o The length of the file system’s Logical block
o Last time of Updating
o The number of free data blocks available and a partial list of immediately allocable free
data blocks. (This term indicates that these free data blocks are immediately accessible and
ready for allocation without any additional preparation or delay. It implies that the file system
has free space that can be used without needing to perform lengthy operations like
defragmentation or restructuring.)
o Number of free I-nodes available and a partial list of immediately usable I-nodes.
o The state of the file System (whenever ‘clear’ - All changes are written to disk; no pending
updates or ‘dirty’ - There are pending changes that have not been written to disk.)
As with I-nodes, the kernel also maintains a copy of the Super Block in memory.
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● I-node Block:
We know that all entities in UNIX are treated as files. I-nodes are used to describe the file
characteristic and the locations of the data blocks which store the contents of the system.
The information related to all these files (not the contents) is stored in an I-node table on
the disk. For each file, there is an I-node entry in the table. Each entry is made up of 64 bytes
and contains the relevant details for that file. These details are:
o File Type (regular, directory, device etc)
o Number of links (the number of aliases the file has)
o The numeric UID of the owner.
o The numeric GUID of the owner.
o File mode( file access permission)
o Size of the file
o Date and Time of last modification of file data
o Date and Time of last access of file data
o Date and Time of last change of the I-node
o Addresses of blocks where the file is physically present
The I-node is accessed by number called I-node Number. This number is unique for every file in a
single file system.
● Data Block:
The remaining space of the file system is taken up by Data block or Storage block. For a directory
each data block contains 16 bytes entries. Each such entry would have a file or subdirectory
name up to 14 bytes and 2 bytes would be taken for the I-node. Thus, for directories, the table
of filenames and their I-nodes is available.
e.g. (directory called /usr)
14 bytes 2 bytes
File name 1 I-node 1
The operation system’s files, all data and programs created by users reside in this area.
Booting Sequence / Stages of System Startup
1) ROM diagnostics are run:
Hardware and memory tests are performed
2) Boot loader is loaded:
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The boot loader is loaded into RAM by the ROM
3) Kernel is loaded:
The kernel program is called by the user. Recall that the user needs to enter the name of the
program. (E.g UNIX, HICIX etc at prompt during boot process which is optional. By default
UNIX kernel is loaded).
4) Kernel initialization takes place:
It involves performing memory tests, initializing the devices through device driver, swapper
scheduler process and the init process.
5) /etc/init or the init program is invoked by the kernel. This program takes over the System
Control.
6) The init program: This process has the PID1 (Process IDentification) which is the second process
of the system. The init program reads the instruction of /etc/inittab (configuration file used by init
program) file to carry out processes like identifying the run level i.e the mode in which system
should run single user/multi-user, maintaining files to track activities etc..
7) The getty process : The init program also invokes the getty program which establishes a
communication with the terminals of the UNIX system. The getty program uses the file called
/etc/gettydefs for instructions to provide the login: prompt at each terminals connected to system
and goes into suspended (sleep) mode and activated when user attempt to login.
8) The login program: Once the user types login name and password, getty transfer control to a
login program, to verify the login-name and password entered by user. Thus if the login is successful
the $ prompt appears on the screen. It is essential to know that it is the /etc/profile that enables
the system administration to provide information like time and date, system name, number of users
etc. This file can be modified by the system administrator to accommodate relevant messages for
the users.
init -> getty -> login -> shell
(Fig: process of leading to shell)
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Now init, is the only living ancestor of the shell which is goes off to sleep mode and waiting for the
death of its children. When the user logs out, shell is killed and the death is intimated to init, init
then wakes up and generate another getty for that time to monitor the next login.
Structure of /etc/passwd
All user information except the encrypted password is stored in /etc/passwd. The encrypted
password is stored in /etc/shadow. The /etc/passwd shows the entry of all registered users. Each
entries consisting of seven field. Each field is separated by colon (:).
Username: password : UID : GID : Comment : home directory : login shell
(Fig: The structure of /etc/passwd file)
The following are the significance of all the seven fields:
1) Username : It is login name of the user.
2) Password: Encrypted password of the user, but it contains an x. The encrypted password is
stored in /etc/shadow file.
3) UID : UID stand for User IDentification number. It is unique number, No two users have the
same UID.
4) GID : GID stands for Group IDentification number. It is also unique number.
5) Comment : This field contains detailed information about the user such as full name of the
user, address etc.
6) Home directory : It is the home directory of a user. When a user login to the unix system,
initially he is placed in this directory.
7) Login shell : When the user logs in then default shell is allocated. Using this shell user
communicate with the kernel.
The typical entries of /etc/passwd is as follows:
Kamal : x : 210 : 100 : kamal patel, surat : /home/kamal : /bin/ksh
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Nirmal : x : 211 : 100 : nirmal patel, surat : /home/nirmal : /bin/ksh
Karan : x : 212 : 100 : karan patel, surat :/home/karan : /bin/ksh
Structure of /etc/shadow
The /etc/shadow stores actual password of the user in encrypted format. The general structure of
/etc/shadow file is as follows.
username : pwd : Last pwd change : minimum : maximum : warn : Inactive : Expire : reserve
(Fig: The structure of /etc/shadow file)
This file contain 9 fields, each field is separated by colon. This file contain the encrypted password
information for user’s accounts and optional password aging information.
1. Username : It is login name of the user.
2. Pwd : It contains encrypted password. The password should be minimum 6-8 characters
long including special characters / digits.
3. Last password change : Days since Jan 1, 1970 that password was last changed.
4. Minimum : The number of days left before the user is allowed to change password or
minimum days before password may be changed.
5. Maximum : The maximum number of days the password is valid or days after which
password must be changed.
6. Warn : The number of days before password is to be expire the user is warned that his /
her password must be changed or days before password is to expire that user is warned.
7. Inactive : The number of days after password expires that account is disabled.
8. Expire : days since Jan 1, 1970 that account is disabled. i.e. an absolute date specifying
when the login may no longer be used.
9. Reserve : it is a reserved field.
17 | Page

A typical entry of /etc/shadow is as follows:
Kamal : $2$Cd3eesdf3$dfdfeefefefeffe23dfdf.:14433::88888::::
Nirmal : $3$Ef3eeshj2$dfdjhhferefeggh23dfdf.:14422::88888::::
Structrue of Inode
The inode is created, whenever a file is created. The inode is deleted whenever the related file is removed.
The inode contains the following information:
1) Owner’s user id (uid) : user id of owner
2) Owner’s group id (gid) : the group id the user belongs to
3) Protection bits: UNIX divides all the users into three categories: owner, Group and the others.
Each user can have read (r), write (w) and execute (x) rights. For each of these right, one bit is
necessary. So 9 bits (3 bits for rights X 3 bits for 3 types of users) are required for each file in its
inode.
4) File type: The file type specifies that a file is an ordinary file, a directory file, a FIFO file or a
special file.
5) File access times: The file access times specify the time at which the file was created, last used
and last modified.
6) Number of links : Number of links specify the number of symbolic name the file is known by. The
kernel physically delete a file (free all the data blocks and the inode allocated to that file) only if
this number is 0. when all the users delete this file from their directories than number of links
becomes 0.
7) File Size: The size of the file.
8) Addresses of all the blocks allocated to this file, direct blocks (data blocks) or indirect blocks
(index blocks that points to data blocks).
Each inode of particular file contain the above information. The inode reserves the memory to maintain
block numbers of 13 blocks. Assume that a block is of 1024 bytes. 4 bytes are required to represent 1 block
18 | Page

number. So each inode require 4 x 13 =52 bytes. Out of these 13 blocks, 10 block are pointed to actual data
blocks allocated to the file. This 10 blocks are numbered from 0 to 9. This 10 blocks are referred to as “Direct
Blocks”.
As a new file is created, initially all the 10 direct entries in inode are emply. When something is to be written
in a file, a block is required, the kernel goes to the list of free data blocks, choose a free data block, remove
the selected free block number from the list of free data blocks and insert this block number in inode. This
process continue upto first 10 block in a file.
Single Indirect Block
When all the first 10 blocks are filled, and new free block is required , the kernel allocates a data block from
the available free blocks, and used create an index block in which it insert and entry for newly assigned block
number. The 11th
entry in inode points to the index block. This index block also has 1024 bytes. So it contain
1024 / 4=256 entries of block number which points to actual data blocks. This index block is called a “Single
indirect” block.
Double Indirect Block
In double indirect the 12th
entry in the inode points to a double indirect index block. This index block has 256
entries again, but instead of pointing to 256 data blocks directly they point to 256 additional index blocks
which in turn points to actual data blocks.
Total number of capabilities of addressing at double indirect index is
10+256+(256x 256) = 65802 blocks ir 65.8 KB.
Total number of capabilities of addressing at Triple indirect index is
10+2561
+2562
+2563
= 1,68,43,018 blocks ir 16.84 MB.
19 | Page

The file size play an important role in address translation. Depending upon the file size, the kernel knows
whether to locate the block number in the inode itself (direct), or in an index at a specific level of indirection.
Exercise
20 | Page

Short Question
1) What is major and minor number?
2) Where the number of free i-nodes and data block stored?
3) List out functions of kernel.
4) Which process is created by Getty process?
5) Explain the purpose of super block.
6) What is the purpose of /bin directory of Unix file system?
7) List out functions of kernel.
8) What is Shell?
9) Explain fork() system call.
10) What does /etc directory contain?
11) Which process creates getty process? What is the responsibility of getty process?
Long Question
1) Explain booting sequence with init process.
2) Explain features of Shell.
3) Explain content of I-node block.
4) Explain command line interpretation of the shell.
5) Explain the directory structure of UNIX.
6) Explain in brief I-node block.
7) Define a shell. Explain meta character that shell use to match file names.
21 | Page

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