Linux shell env

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

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.

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.

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

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

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

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)

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.

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
???

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

Default Shell Env. Variables
 List of Default Variables…

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.

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

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

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]}
???

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

Linux shell env

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Linux shell env