UNIT I LINUX.docx

Mrs. Bhuvaneswari R, Department of Computer Science, DKM College for Women(Autonomous),
Vellore, TamilNadu, India
UNIT I : BASICS OF LINUX OPERATING SYSTEM
Linux is one of popular version of UNIX operating System. It is open source as its source
code is freely available. It is free to use. Linux was designed considering UNIX
compatibility. Its functionality list is quite similar to that of UNIX.
Components of Linux System
Linux Operating System has primarily three components
 Kernel − Kernel is the core part of Linux. It is responsible for all major activities of
this operating system. It consists of various modules and it interacts directly with the
underlying hardware. Kernel provides the required abstraction to hide low level
hardware details to system or application programs.
 System Library − System libraries are special functions or programs using which
application programs or system utilities accesses Kernel's features. These libraries
implement most of the functionalities of the operating system and do not requires
kernel module's code access rights.
 System Utility − System Utility programs are responsible to do specialized,
individual level tasks.
Kernel Mode vs User Mode
Kernel component code executes in a special privileged mode called kernel mode with full
access to all resources of the computer. This code represents a single process, executes in
single address space and do not require any context switch and hence is very efficient and

fast. Kernel runs each processes and provides system services to processes, provides
protected access to hardware to processes.
Support code which is not required to run in kernel mode is in System Library. User
programs and other system programs works in User Mode which has no access to system
hardware and kernel code. User programs/ utilities use System libraries to access Kernel
functions to get system's low level tasks.
Architecture
The following illustration shows the architecture of a Linux system −
The architecture of a Linux System consists of the following layers −
 Hardware layer − Hardware consists of all peripheral devices (RAM/ HDD/ CPU
etc).
 Kernel − It is the core component of Operating System, interacts directly with
hardware, provides low level services to upper layer components.
 Shell − An interface to kernel, hiding complexity of kernel's functions from users.
The shell takes commands from the user and executes kernel's functions.
 Utilities − Utility programs that provide the user most of the functionalities of an
operating systems.
Linux History

Evolution of Computer
In earlier days, computers were as big as houses or parks. So you can imagine how difficult it
was to operate them. Moreover, every computer has a different operating system which made
it completely worse to operate on them. Every software was designed for a specific purpose
and was unable to operate on other computer. It was extremely costly and normal people
neither can afford it nor can understand it.
Evolution of Unix
In 1969, a team of developers of Bell Labs started a project to make a common software for
all the computers and named it as 'Unix'. It was simple and elegant, used 'C' language instead
of assembly language and its code was recyclable. As it was recyclable, a part of its code now
commonly called 'kernel' was used to develop the operating system and other functions and
could be used on different systems. Also its source code was open source.
Initially, Unix was only found in large organizations like government, university, or larger
financial corporations with mainframes and minicomputers (PC is a microcomputer).
Unix Expansion
In eighties, many organizations like IBM, HP and dozen other companies started creating
their own Unix. It result in a mess of Unix dialects. Then in 1983, Richard Stallman
developed GNU project with the goal to make it freely available Unix like operating system
and to be used by everyone. But his project failed in gaining popularity. Many other Unix like
operating system came into existence but none of them was able to gain popularity.
Evolution of Linux
In 1991, Linus Torvalds a student at the university of Helsinki, Finland, thought to have a
freely available academic version of Unix started writing its own code. Later this project
became the Linux kernel. He wrote this program specially for his own PC as he wanted to use
Unix 386 Intel computer but couldn't afford it. He did it on MINIX using GNU C compiler.
GNU C compiler is still the main choice to compile Linux code but other compilers are also
used like Intel C compiler.
He started it just for fun but ended up with such a large project. Firstly he wanted to name it
as 'Freax' but later it became 'Linux'.
He published the Linux kernel under his own license and was restricted to use as
commercially. Linux uses most of its tools from GNU software and are under GNU
copyright. In 1992, he released the kernel under GNU General Public License.
Linux Today

Today, supercomputers, smart phones, desktop, web servers, tablet, laptops and home
appliances like washing machines, DVD players, routers, modems, cars, refrigerators, etc use
Linux OS.
Linux Features
o Multiuser capability: Multiple users can access the same system resources like
memory, hard disk, etc. But they have to use different terminals to operate.
o Multitasking: More than one function can be performed simultaneously by dividing
the CPU time intelligently.
o Portability: Portability doesn't mean it is smaller in file size or can be carried in pen
drives or memory cards. It means that it support different types of hardware.
o Security: It provides security in three ways namely authenticating (by assigning
password and login ID), authorization (by assigning permission to read, write and
execute) and encryption (converts file into an unreadable format).
o Live CD/USB: Almost all Linux distros provide live CD/USB so that users can
run/try it without installing it.
o Graphical User Interface (X Window system): Linux is command line based OS
but it can be converted to GUI based by installing packages.
o Support's customized keyboard: As it is used worldwide, hence supports different
languages keyboards.
o Application support: It has its own software repository from where users can
download and install many applications.
o File System: Provides hierarchical file system in which files and directories are
arranged.
o Open Source: Linux code is freely available to all and is a community based
development project.
Why Use Linux
Linux is completely different from other operating systems in many ways.
o It is an open source OS which gives a great advantage to the programmers as they can
design their own custom operating systems.
o It gives you a lot of option of programs having some different features so you can
choose according to your need.
o A global development community look at different ways to enhance its security,
hence it is highly secured and robust so you don't need an anti virus to scan it
regularly. Companies like Google, Amazon and Facebook use linux in order to protect
their servers as it is highly reliable and stable.
o Above all you don't have to pay for software and server licensing to install Linux, its
absolutely free and you can install it on as many computers as you want.

o Its completely trouble free operating system and don't have an issue with viruses,
malware and slowing down your computer.
Linux Bash
The Linux Bash is also known as 'Bourne-again Shell.' It is a command language
interpreter for the Linux based system. It is a replacement of Bourne shell (sh). It was
developed under the GNU Project and written by Brian Fox. Nowadays, Bash is the default
user shell of most of the Linux distributions.
The Linux/Unix shell allows us to interact with the Linux system through the commands. It
let us invoke an executable file to create a running process. Moreover, it also allows us to
interact with the Linux file system. It is designed in such a way that we can perform all the
Linux operations through Bash.
The Bash is a command language interpreter as well as a programming language. It
supports variables, functions, and flow control, like other programming languages. It can
also read and execute the commands from a file, which is called a shell script.
It offers various functional improvements over Bourne Shell (sh) for both interactive and
programming use. Although many sh scripts can be run by Bash without any change. The
Bash contains the following improvements over sh:
o It provides command-line editing
o It contains unlimited size command history
o It provides Job Control
o It facilitates with Shell Functions and Aliases
o It provides the indexed arrays of unlimited size
o It contains integer arithmetic in any base from 2 to 64.
How to download Bash
It can be downloaded from the official GNU server via both HTTP
(http://ftp.gnu.org/gnu/bash/) and FTP (ftp://ftp.gnu.org/gnu/bash/ ) servers.
People often get confused between bash, shell, and shell script. Let's walk through shell and
scripting to clear a picture of bash, shell, and scripting.
What is Shell
If we are a new Linux user, and we open the terminal, it is assumed that we are well confused
as to what to do with it. Here the Shell comes in the role.

The terminal contains the shell; it allows us to execute the commands to interact with the
system. We can perform various operations such as store and retrieve data, process
information, and various other simple as well as complex tasks.
To open the terminal, press CTRL+ALT+T keys. Perform some basic operations such
as date, cal, ls, and pwd to take a tour with it.
Consider the below image:
As we can see from the above image, the shell allows us to interact with the Linux system.
When we have executed the date and cal command, the shell interacts with the system and
retrieves data.
What is Scripting
Suppose we are required to execute some basic commands every day, for example above four
commands. Linux supports a feature called scripting that allows us to execute more than one
task at once. So, it is good to define a script rather than performing repetitive tasks.
To understand how to use Linux script, let's define a script in a combination of some tasks.
To define a script, create a file with a .sh extension. We are using the VI text editor.
However, any text editor can be used to define a script. Consider below command:
1. vi tasks.sh

The above command will open the vi editor in normal mode. Switch it to insert mode by
pressing ESC, and after that enter 'i' keys, enter your desired tasks. Every task should be
defined in a new line. Consider the below tasks:
1. date
2. cal
3. pwd
4. ls
After entering the tasks, press ESC and :wq! Keys to save and exit from the editor.
Now, make the created file as executable, use the chmod command with +x option as
follows:
1. chmod +x tasks.sh
We have created our first script. To execute the shell script, execute the file name by
predefining './ ' with the file name. Consider the below command:
1. ./task.sh
Consider the below output:
As we can see from the above output, by using the shell, we can automate the tasks. We can
use this whenever we need it. It can be used repeatedly any number of times.
Features of Bash
All the built-in command of the sh shell is available in Bash; moreover, it facilitates us with
many other features. Some key features of Bash are as follows:

o Shell Syntax: The shell syntax contains shell operations, quoting, and comments.
The shell operations are the basic operation of the shell. Quoting allows how to
remove the special meaning from characters, and comments are meant to specify the
comments.
o Shell commands: Shell commands are the types of commands that you can execute.
These commands can be simple commands, pipelines, lists, compound commands,
and more.
o Shell Functions: Shell functions are used to group commands by name. They are
executed as traditional commands. When we use the name of a shell function, the list
of commands associated with that is executed.
o Shell parameters: Basically, a parameter is an entity that stores value; it can be a
name, number, or special character. The shell parameters specify how the shell store
value. They can be a positional parameter or a special parameter. Positional
parameters are the shell's command-line arguments, and the special parameters are
denoted by a special character.
o Shell Expansions: Shell expansion is a technique that is used by Bash to expand the
parameters. Expansion is performed on the command line after the input has been
splitted into tokens.
o Redirections: It is a way to manage and control the input and output.
o Command execution: It decides how the system will react when we execute a
command.
o Shell Scripts: It is a text file that has shell commands and executes them when it is
used. Bash reads and executes the commands then exits.
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
Initially it was command
based OS, but later
Common Desktop

are mostly used GUI. 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 vs. Windows
Linux vs. Windows has always been one of the most frequently asked questions related to the
operating system. Users often get confused about which one is better for them. There is
diversity between the users as most users prefer the graphical user interface (GUI) and most
command-line interface (CLI). There are many disagreements and acrimonious behavior
among users, and it seems that it will be forever.

In this section, we will discuss the differences between Linux and Windows by considering
several parameters such as performance, usability, security, ease of use, and more to clear
a picture of using both operating systems. Further, we will see the advantages of Linux
over other operating systems such as Windows and Mac OS. This will help you to decide
which one is better for you.
Difference between Linux and Windows operating system
To understand the difference between Linux and Windows, let's see a brief introduction to
both operating systems. Later we will talk on their features and security options.
What is the Windows operating system?
Windows is a graphical operating system developed and marketed by Microsoft. It is also
referred to as Microsoft Windows. Several versions of Windows have been introduced in the
market; the current version is Windows 10. The first version of Windows was introduced
on November 20, 1985, as a graphical operating system for MS-DOS.
Microsoft Windows is a family of various operating systems. It comes with two versions, i.e.,
64 bit and 32 bit. It facilitates both client and server versions. The latest client version is
Windows 10, and the server version is Windows server 2019.
Windows is a straight forward and simple to use. Generally, it is designed for users having no
programming knowledge. So, mostly it is used for business and alternative industrial
purposes.

What is a Linux operating system
Linux is an open-source operating system. As it is open-source, it is special and different
from other operating systems, which means that you can customize it by editing source
code. It provides programming as well as a graphical user interface. Linux is built by Linux
Torvalds because he wanted to create a free operating system kernel that anyone can use.
Linux is a collection of operating systems that are based on Linux kernel. The first version of
Linux was released in the year 1991. The Linux system is most commonly used for servers;
however, it is available in desktop versions as well.
Ubuntu, Devian, and Fedora are some popular Linux distributions. Also, we have SUSE
Linux Enterprise Server (SLES) and RedHat Enterprise Linux for the commercial
distribution of Linux. As it is open-source, we can modify the source code and make
variations in the operating system.
Let's discuss some features and parameters to understand the difference between both
operating systems:
o File System
Windows uses different drives such as C, D, E, and more, having some folders to store
files.
But Linux uses a tree structure to store and organize files. Linux file structure starts from
the root directory, and it is considered as a start point of the file system. It is represented by a
forward slash (/). In Linux, everything (Directories, devices, and files) is considered a file.
There are three types of files available in a Linux system.

o General files
o Directory files
o Device files
The general file system of Unix is as follows:
General files: General files or ordinary files are files that contain images, text, or a program.
These files are in ASCII text or Binary format. The General files are the most common in any
Linux system.
Directory files: Directory files are the depository for other files. A directory can have a
subdirectory file within it. For Windows, we can understand them as folders.
Device files: Windows represents the external devices (Pendrives, hard drives, and CD-
ROM) as letters such as E: F: But, Linux represents devices as files, such as the hard drive's
partitions are represented as dev/sda1, dev/sda2, and more (depends upon the number of
partitions). All the device files take place within the directory /dev.
o Naming Conventions for file
Linux files are case sensitive; therefore, we can have two files having the same name; one in
upper case and other in lower case. Comparatively, the Windows files are not case sensitive;
we cannot have two files with the same name.
o Users
Windows supports four types of users:
o Administrator
o Standard

o Child
o Guest
Comparatively, Linux supports three types of users:
o Regular
o Administrative(root)
o Service
Regular User
In Linux, when we install ubuntu on our system, a regular account user is created. By default,
all our files are saved in the home directory (/home/). A regular user cannot access the other
user's directories.
Root User
Apart from the regular user account, a root user account is also created during installation.
The root account is also called as superuser as it can access the restricted files, install the
software and other utility, and has administrative rights. For installing the software or editing
the system files or any other administrative task, we need the root access. For general tasks
such as creating files, playing games, browsing the internet, we don't need the root access.
Service user
The Linux system is widely used as the server operating system. The leading service
providers such as Apache, Squid, email, and more have their service accounts to increase
security. Linux service user can allow or deny access to various resources depending on the
service type.
Note: The Ubuntu Desktop version does not have a service account, and the regular
accounts are called standard accounts.
o Home Directory
Linux OS has a separate home directory for a user. The files and directories created by a user
are stored under a particular home directory. A user cannot store the files under any other
user's directory, as it is not allowed to access the other user's directory. For example, the
home directory of a user 'Alex' is automatically created as "/home/alex/" at the time of
installation.
The Windows OS also has different home directories for different users. It is as "C:
documents or settings."
o Other Directories

Linux systems have a tree structure to save the directories; comparatively, Windows uses
different drives to save the directories. In Windows, system and program files usually take
place in C drive. But in Linux, the system and program files take place in their specific
directory such as software files are stored in /bin, programs and device files are
in /dev, and boot files are stored in /boot directory.
o Kernel
The basic part of any operating system is its kernel. It interacts with hardware devices and
performs other functions such as process management, file handling, and more. Different
operating systems have different kernels.
Linux and Windows have a different kernel. The Linux kernel is monolithic, and it consumes
more running space. Comparatively, Windows uses the microkernel, which consumes less
running spaces. But, Windows running efficiency is less than Linux. The key difference
between Linux kernel and Windows kernel is that Windows is a commercial software while
Linux is open-source software.
o Pricing
Linux is an open-source operating system, so almost all the utilities and libraries are entirely
free. The GNU/Linux distribution can be simply downloaded for free. However, some
companies are providing paid support for their Linux distributions, but the underlying
software is still free.
Microsoft Windows licensed copy usually cost between $99.00 and $199.00. Microsoft
currently stops providing support for the earlier versions. The latest version, Windows 10, is
available at $139.
Let's focus some head to head comparison between Linux and Windows.
Parameter Linux Windows
Access Users can access the
source code of
kernel in Linux and
can alter the kernel
according to need.
Usually, users cannot access the
source code. However, members
of some groups can have access
to it.
Variety Linux has several
distributions that are
highly
Windows have fewer options to
customize.

customizable.
Command-
line
The command line
usually referred
to as Terminal,
which is the most
useful tool of the
Linux system. It
used for
administration and
daily tasks. For the
end-users, it does
not look so
effective.
Windows also have a command
line, but it is not such effective as
a comparison to the Linux
terminal. Most users prefer the
GUI options for daily tasks.
Installation The Linux
installation process
is a bit complicated
to set up as it
requires many user
inputs. It takes less
time than Windows
to install.
Windows OS is easy to install
and set up on a machine; it
requires fewer user input options
during installation. However, it
takes more time to install as
compared to Linux.
Ease of use The Linux OS is
meant to be for the
technical user
because you must
have some exposure
to various Linux
commands. Users
may take more time
to be a handy user
of Linux. The
troubleshooting
process is also
complicated as
compared to
Windows.
Windows comes with simple and
rich GUI options, so it is easy to
use it. It can be simply used by
technical as well as non-technical
users. The troubleshooting
process is also much easy than
Linux.

Written in Linux is written
in assembly
language and C.
Windows is written
in C++ and assembly language.
Reliability Linux is highly
reliable and secure.
It has well-
established system
security, process
management, and
uptime.
Windows is not as much reliable
as Linux. However, now
Windows has improved reliability
but still has some security
weaknesses and system
instabilities.
Support Linux has a
good support as it
has a huge
community of user
forums and online
search.
Windows also provide good
support to its user. It provides
free as well as paid support. It has
an easily accessible online forum.
Update Linux provides full
control to its users
on updates. A user
can install the
update whenever
needed. Also, it
takes less time to
install an update.
Windows updates are annoying.
The updates will come at any
time and take too much time to
install. Sometimes, you power on
your machine, and updates are
automatically getting started.
Unfortunately, the user does not
have much control over updates.
Security Linux OS is more
secure than
Windows. It is hard
for the hackers and
attackers to find a
loophole in it. So,
Linux is hard to
breakthrough.
Windows is less secure than
Linux. Attackers primarily target
the Windows for malware and
virus. Windows is most
vulnerable without anti-virus.
License Linux is distributed
under
the GPL(GNU
Windows is distributed under
a Proprietary commercial
software license.

General Public
License) license.
Linux Distributions (Distros)
Other operating systems like Microsoft combine each bit of codes internally and release it as
a single package. You have to choose from one of the version they offer.
But Linux is different from them. Different parts of Linux are developed by different
organizations.
Different parts include kernel, shell utilities, X server, system environment, graphical
programs, etc. If you want you can access the codes of all these parts and assemble them
yourself. But its not an easy task seeking a lot of time and all the parts has to be assembled
correctly in order to work properly.
From here on distribution (also called as distros) comes into the picture. They assemble all
these parts for us and give us a compiled operating system of Linux to install and use.
Linux Distributions List
There are on an average six hundred Linux distributors providing different features. Here,
we'll discuss about some of the popular Linux distros today.
1) Ubuntu
It came into existence in 2004 by Canonical and quickly became popular. Canonical wants
Ubuntu to be used as easy graphical Linux desktop without the use of command line. It is the
most well known Linux distribution. Ubuntu is a next version of Debian and easy to use for
newbies. It comes with a lots of pre-installed apps and easy to use repositories libraries.
Earlier, Ubuntu uses GNOME2 desktop environment but now it has developed its own unity
desktop environment. It releases every six months and currently working to expand to run on
tablets and smartphones.
2) Linux Mint
Mint is based on Ubuntu and uses its repository software so some packages are common in
both.

Earlier it was an alternative of Ubuntu because media codecs and proprietary software are
included in mint but was absent in Ubuntu. But now it has its own popularity and it uses
cinnamon and mate desktop instead of Ubuntu's unity desktop environment.
3) Debian
Debian has its existence since 1993 and releases its versions much slowly then Ubuntu and
mint.
This makes it one of the most stable Linux distributor.
Ubuntu is based on Debian and was founded to improve the core bits of Debian more quickly
and make it more user friendly. Every release name of Debian is based on the name of the
movie Toy Story.
4) Red Hat Enterprise / CentOS
Red hat is a commercial Linux distributor. There products are red hat enterprise Linux
(RHEL) and Fedora which are freely available. RHEL is well tested before release and
supported till seven years after the release, whereas, fedora provides faster update and
without any support.
Red hat uses trademark law to prevent their software from being redistributed. CentOS is a
community project that uses red hat enterprise Linux code but removes all its trademark and
make it freely available. In other words, it is a free version of RHEL and provide a stable
platform for a long time.
5) Fedora
It is a project that mainly focuses on free software and provides latest version of software. It
doesn't make its own desktop environment but used 'upstream' software. By default it has
GNOME3 desktop environment. It is less stable but provides the latest stuff.
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.
Linux Licensing
Linus Torvalds has given linux kernel license to GNU General Public License (GPL) version
2. GNU make sure that any software source code licensed under it have to make originating
source code open and freely availble to all its users. Here, freely doesn't mean by cost but it
means that it is freely available to users to distribute and modify the code.
There is the third version of GNU, GNU Lesser General Public License (LGPL) version 3.
But it imposes some more permissions on the license. Torvalds doesn't like some provisions
in version 3 and have announced that linux kernel will not come under version 3.
Linux Set Environment Variable
The environment variables are dynamic values that are stored within a system and used by
applications launched in shells or sub-shells. These variables have a name and their respected
value. The environment variable customizes the system performance and the behavior of an
application.
The environment is the track for a computer application to interact with the system. The
environment variable can have information about the default applications of the system, the
system locale, the path of the executable file and the keyboard layout setting, and more. The
environment variable makes an app available as per the system.
Common Environment Variables
Some standard environment variables are as follows:
o PATH

This variable contains a list of directories in which our system looks for files. It separates
directories by a (:) colon.
o USER
This variable holds the username.
o HOME
This variable holds the default path to the user's home directory.
o EDITOR
This variable contains the path to the specified editor.
o UID
This variable contains the path to the user's unique id.
o TERM
This variable contains the path to the default terminal emulator.
o SHELL
This variable contains the path to the default shell that is being used by the user.
o ENV
This variable displays all the environment variable.
How to set Environment Variable in Linux?
There are multiple commands in Linux that allow us to set and create the environment
variable.
Use the export command to set a new environment variable.
To create a new variable, use the export command followed by a variable name and its value.
Syntax:
1. export NAME=VALUE
To create a new variable say new_variable, execute the command as follows:

1. export new_variable=10
The echo command is used to display the variable:
1. echo new_variable
To display the value of the variable, use the $ symbol before the variable name:
1. echo $new_variable
To set Java Environment Variable, execute the command as follows:
1. export PATH=$PATH:/home/jdk1.8/bin/
We can also create a user to define a variable by directly declaring it on the terminal.
If we want to create a variable new_variable2, we can create it as follows:
1. new_variable2=100
Accessing the value of Environment Variable
To access the value of a variable, execute the echo command as follows:
1. echo $variable name
Note: The variables are case sensitive; we cannot use any variable name 'new_variable' as
'New_variable.'
The env command
The env command is used to display all the available variables in the system.
1. env

Output:
SHELL=/bin/bash
SESSION_MANAGER=local/javatpoint-GB-BXBT-2807:@/tmp/.ICE-
unix/1458,unix/javatpoint-GB-BXBT-2807:/tmp/.ICE-unix/1458
COLORTERM=truecolor
XDG_CONFIG_DIRS=/etc/xdg/xdg-ubuntu:/etc/xdg
XDG_MENU_PREFIX=gnome-
GNOME_DESKTOP_SESSION_ID=this-is-deprecated
GTK_IM_MODULE=ibus
LANGUAGE=en_IN:en
QT4_IM_MODULE=ibus
MANDATORY_PATH=/usr/share/gconf/ubuntu.mandatory.path
GNOME_SHELL_SESSION_MODE=ubuntu
SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
XMODIFIERS=@im=ibus
DESKTOP_SESSION=ubuntu
SSH_AGENT_PID=1362
GTK_MODULES=gail:atk-bridge
PWD=/home/javatpoint
LOGNAME=javatpoint
XDG_SESSION_DESKTOP=ubuntu
XDG_SESSION_TYPE=x11
GPG_AGENT_INFO=/run/user/1000/gnupg/S.gpg-agent:0:1
XAUTHORITY=/run/user/1000/gdm/Xauthority
GJS_DEBUG_TOPICS=JS ERROR;JS LOG
WINDOWPATH=2
HOME=/home/javatpoint
USERNAME=javatpoint
IM_CONFIG_PHASE=1
LANG=en_IN
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd
=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:e
x=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;
31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:
*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;3
1:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.
rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace
=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=
01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.
gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:
*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:
*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm

=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.n
uv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fl
i=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=0
1;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=0
0;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=
00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:
XDG_CURRENT_DESKTOP=ubuntu:GNOME
VTE_VERSION=5802
GNOME_TERMINAL_SCREEN=/org/gnome/Terminal/screen/dc6e48ed_7868_43d5_a086
_fd6d6a90a74a
INVOCATION_ID=fadf556d1cf5422ea55b247bf57c3c99
MANAGERPID=1252
CLUTTER_IM_MODULE=ibus
GJS_DEBUG_OUTPUT=stderr
LESSCLOSE=/usr/bin/lesspipe %s %s
XDG_SESSION_CLASS=user
TERM=xterm-256color
DEFAULTS_PATH=/usr/share/gconf/ubuntu.default.path
LESSOPEN=| /usr/bin/lesspipe %s
USER=javatpoint
GNOME_TERMINAL_SERVICE=:1.216
DISPLAY=:0
SHLVL=1
QT_IM_MODULE=ibus
XDG_RUNTIME_DIR=/run/user/1000
JOURNAL_STREAM=9:35179
XDG_DATA_DIRS=/usr/share/ubuntu:/usr/local/share/:/usr/share/:/var/lib/snapd/desktop
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/
snap/bin
GDMSESSION=ubuntu
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
_=/usr/bin/env
javatpoint@javatpoint-GB-BXBT-2807:~$
Removing an Environment Variable
By removing an environment variable we can remove all existing component of particular
variable.
To remove an environment variable, execute the unset command followed by variable name:
1. unset variable name
The above command will delete the specified variable and its components from the system.

To remove a variable new_variable from the system, execute the command as follows:
1. unset new_variable
Linux Set Command
Linux set command is used to set and unset certain flags or settings within the shell
environment. These flags and settings determine the behavior of a defined script and help in
executing the tasks without facing any issue. The values of shell attributes and parameters
can be changed or displayed by using the set command.
Syntax:
1. set [options]
To know about how to set the environment variable in Linux, visit Here.
Options: The supported options by the set command are as following:
o -a: It is used to mark variables that are modified or created for export.
o -b: It is used to notify of job termination immediately.
o -e: It is used to exit immediately if a command exits with a non-zero status.
o -f: It is used to disable the file name generation (globbing).
o -h: It is used to save the location of commands where they looked up.
o -k: It is used to place all assignment arguments in the environment variable of a
command, except those that precede the command name.
o -m: It is used to enable Job control.
o -n: It is used to read commands.
o -o: It is used for option-name.
o -p: It is used to disable the processing of the '$ENV' file and import shell functions. It
is turned on whenever the real and effective user ids do not match. Turning off this
option may cause the working uid and gid to be set as the authorized uid and gid.
o -t: It is used to exit from the command after executing one command.
o -u: It is used to treat unset variables as an error when substituting.
o -v: It is used to print shell input lines.

o -x: It is used to print commands and their arguments in a sequential way (as they are
executed).
o -B: It is used to perform brace expansion by the Shell.
o -C: It is used to disallow existing regular files to be overwritten by redirection of
output.
o -E: It is used if the ERR trap is inherited by the shell functions.
o -H: It is used to enable style history substitution. By default, it is on when the shell is
interactive.
o -P: It is used if we do not want to follow symbolic links when executing commands.
o -T: If this flag is set, the DEBUG trap is inherited by the shell functions.
Note: To turn off the above flags, use the '+' rather than '-' symbol.
To better understand the set command, let's see a brief introduction of the shell variable:
Shell Variable
A variable is a character string that can hold a value. The assigned value could be anything
such as number, filename, text, or any other data type. It is just like a pointer to the actual
data. The shell allows us to create, delete, and assign variables.
The variable name can have any letters, numbers, or underscore (_) character. We cannot
have the characters such as "! *, or -", because these special characters have other meanings
for the shell. According to Unix naming convention, the Unix shell variable must have their
names in UPPERCASE.
Examples of the set command
Turn debugging information on or off.
The '-x' option is used with the set command to show command and their arguments. It is
useful for debugging the shell script.
Execute the following command:
To turn on the debugging information:
1. set -x
To turn off the debugging information:
1. set +x
Disable Bash's default behavior.

To disable the default behavior of Bash, execute the command as follows:
1. set -C
Stop a script immediately.
To stop a script immediately, execute the following command:
1. set -e
Getting Help
If you get stuck during the use of the set command, you can access the help documentation
from your terminal. To access the help manual, execute the below command:
1. set --help
Linux export Command
The export command is a built-in utility of Linux Bash shell. It is used to ensure the
environment variables and functions to be passed to child processes. It does not affect the
existing environment variable.
Environment variables are set when we open a new shell session. At any time, if we change
any variable value, the shell has no way to select that change. The export command allows us
to update the current session about the changes that have been made to the exported variable.
We do not need to wait to start a new shell session.
Syntax:
1. export [-f] [-n] [name[=value] ...] or export -p
Let's have a look at various examples of the export command:
Example1: The export command without any argument
The basic export command will display all the exported environment variables of your
system. It is executed as follows:
1. export
Consider the below snap of output:

Example2: Display all exported variable on current shell
To display all the exported environment variable of the current shell, execute the command
with -p option as follows:
1. export -p
Consider the below snap of output:

Example3: Using export with functions
To use a function with the export command, use the -f option. If we do not use this option, it
will be considered as a variable, not function.
Syntax:
1. export -f function_name
We are exporting a function 'name' as follows:
1. name() { echo "Javatpoint";}
To export the above function, execute the command as follows:
1. export -f name
Now, invoke the bash shell to execute the function:
1. bash

To call the function, enter the function name:
1. name
let's create another function 'hello,' execute the command as follows:
1. function hello
2. > {
3. > echo hello, welcome to javatpoint
4. > }
To export the above function, execute the command as follows:
1. export -f hello
Example4: Assign a value before exporting a function or variable:
The export command allows us to assign a value before exporting a function. Consider the
below command:
1. export name[=value]
For example, assign a value to a variable as follows:
1. a=5
now export it as:
1. export a
we can verify the assignment by using the printenv command as follows:
1. printenv a

Example5: Set vim as default editor:
The vim editor is the most widely used text editor for the Linux systems. We can set the vim
as default text editor by using the export command.
To set the vim as a default text editor, execute the following command:
1. export EDITOR=/usr/bin/vim
2. export | grep EDITOR
The above commands will not show any confirmation. Consider the below output:
Example6: Set an environment variable
To create a new variable, use the export command followed by a variable name and its value.
Syntax:
1. export NAME=VALUE
To create a new variable, 'sys,' execute the command as follows:
1. export sys=50
The echo command is used to display the variable:
1. echo sys
To display the value of the variable, use the $ symbol before the variable name
1. echo $sys

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