This document provides an overview of Linux shells and shell environment variables. It discusses that shells provide a command line interface to allow users to enter commands and that common shells include bash, tcsh, csh, ksh, and sh. It also describes global and local environment variables, how to view, set, and remove them, and important default variables like HOME, PATH, and MAIL. Finally, it covers features like variable arrays, aliasing commands, and shell startup files.

1. Linux Shell Environment
Content Updating Program(CUP)
On
Linux Programming

2. Shell Overview
What is a Shell?
 Linux shells provide a "command line" interface which allows
the user to enter commands.
 A shell is a program which reads and executes commands for the
user.
 Shell also usually provide features such as Job Control,
Input/output Redirection and a Command Language for writing
Shell Scripts.
 The user can pick their shell (just like the applications, desktop
manger, window manager, etc. on a LINUX system).
 On the UMBC GL network, the default UNIX shell is tcsh -
Turbo C Shell.

3. Shell Overview
 Shells available include:
 tcsh - Turbo C SHell
 csh - C SHell
 ksh - Korn SHell
 bash - Bourne Again SHell
 sh - SHell
 Linux Default Shell
 Most Linux systems (especially home installations) default to
the bash shell.
 Changing Your Shell - On a Home Based System
 Usually there is a command called chsh that stands for
change shell.
 You have to enter your password and then the absolute path
to the new shell that you wish to use.

4. Shell Environment Variables
 The bash shell uses a feature called ‘environment
variables’ to store information about the shell session
and the working environment.
 USE: - Many programs and scripts uses environment
variables
- to obtain system information
- to store temporary data
- configuration information.
 There are two types of environment variables
- Local Environment Variables
- Global Environment Variables

5. Gobal Environment Variables
 Global Env. Variables(GEV) are visible from
the shell session and from any child process that
the shell spawns.
 Linux system sets several GEV on start of Shell.
 System environment variables uses CAPITAL
letter to differ from user environment variables.
 To view the GEV, use command
$ printenv

6. GEV contiuned…..
 To display a value of individual env variable
 Use command
$ echo $HOME
Example of GEV available to Child process also…

7. Local Environment Variables
 LEV can be seen in the local processes in which
they are defined.
 Linux system also define standard LEV for user
by default.
 To view LEV use the command
$ set
(additional variables not in printenv are LEV)

8. SETTING Env Variables
 Setting LEV: Users are allowed to create local
variables that are visible within user shell process
 We can assign either numerical or string value to
and environment variable using equal sign (=)
 Example:
 $ test=testing
$ echo $test
 Standard Convention is use Lower case for LEV
and user Upper Case for GEV.

9. SETTING GEV continued…
 SETTING GEV:
Create a LEV and export it to global env.
Example:
$ test=‘testing your patience’
$ echo $test
$ export test
$ bash
$ echo $test
???

10. Removing Env. Variables
 Remove the Environment variable using
command – unset
Example:
$ unset test (don’t use $before variable..)
$ echo $test
Example for GEV…

11. Default Shell Env. Variables
 List of Default Variables…

12. Important Environment Variables
 HOME - your home directory.
 USER and LOGNAME - your login ID.
 HOSTNAME - the name of the host computer.
 PWD - the current working directory.
 MAIL - where your mail is located.
 PATH - a list of directories in which to look for executable
commands.
 Certain applications and commands may communicate with the
shell and reference the environment variables that it maintains.
 For example, it seems that frm and nfrm seem not to work if $MAIL is
not defined. frm and nfrm are commands to list the contents of your
inbox without logging into pine.

13. Setting the PATH env. variable
 PATH env. Variable includes all of the directories
where your applications resides.
 We can add new search directories to existing
PATH environment variable without rebuild it
from scratch.
 Example:
$ echo $PATH
---------------
$ PATH=$PATH:/home/user
$ echo $PATH

14. Locating System Env. variables
 When we start a bash shell by logging on to the
system, it searches several files for command,
These file are called Startup files.
Following is the order in which bash processes these
files:
 /etc/profile
 $HOME/.bash_profile
 $HOME/.bash_login
 $HOME/.profile

15. Variable Arrays
 Environment variables can be used as arrays.
 So it can hold multiple values.
 To set list them in parentheses separated by
comma.
 Example:
$ mytest=( one two three four five )
$ echo $mytest
????
$ echo ${mytest[2]}
???

16. Aliasing Commands
 A helpful feature, especially for many users new to LINUX, is
the alias function.
 The alias command assigns a command, possibly with many
options and flags, to another name. Usually it is a shorter name
or one that is easier to remember.
Setting up an alias:
 The exact syntax depends on the shell that you are using. We will
cover how to do it under tcsh and bash. Most other shells use a
similar or identical syntax.
 tcsh syntax:
 alias <aliased name> <original command>
 bash syntax:
 alias <aliased name>=<original command>
Example:
$ alias dir=‘ls –l’
$dir

17. Thank you …