Thursday, 9 May 2013

UNIX: Set Environment Variable

How do I set environment variables on UNIX systems?

UNIX and all UNIX-like operating systems such as OpenBSD, Linux, Redhat, CentOS, Debian allows you to set environment variables. When you log in on UNIX, your current shell (login shell) sets a unique working environment for you which is maintained until you log out. Following are most command examples of environment variables used under UNIX operating systems:

  • PATH - Display lists directories the shell searches, for the commands.
  • HOME - User's home directory to store files.
  • TERM - Set terminal emulator being used by UNIX.
  • PS1 - Display shell prompt in the Bourne shell and variants.
  • MAIL - Path to user's mailbox.
  • TEMP - Path to where processes can store temporary files.
  • JAVA_HOME - Sun (now Oracle) JDK path.
  • ORACLE_HOME - Oracle database installation path.
  • TZ - Timezone settings
  • PWD - Path to the current directory.
  • HISTFILE - The name of the file in which command history is saved
  • HISTFILESIZE -The maximum number of lines contained in the history file
  • HOSTNAME -The system's host name
  • LD_LIBRARY_PATH -It is a colon-separated set of directories where libraries should be searched for.
  • USER -Current logged in user's name.
  • DISPLAY -Network name of the X11 display to connect to, if available.
  • SHELL -The current shell.
  • TERMCAP - Database entry of the terminal escape divs to perform various terminal functions.
  • OSTYPE - Type of operating system.
  • MACHTYPE - The CPU architecture that the system is running on.
  • EDITOR - The user's preferred text editor.
  • PAGER - The user's preferred text pager.
  • MANPATH - Colon separated list of directories to search for manual pages.

Display Environment Variable

Open the terminal and type the following commands to display all environment variables and their values under UNIX-like operating systems:
$ set
$ printenv
$ env

Sample outputs:

Fig.01: Displaying all environment variables and their values command

To display search path, enter:
echo $PATH
To display prompt settings, enter:
echo $PS1
A few more examples:
echo $USER
echo $PWD
echo $MAIL

Change or Set Environment Variable

You can use the following command to change the environment variable for the current session as per your shell.

For Korn shell (KSH)

The syntax is as follows:
export var
To set JAVA_PATH, enter:
export JAVA_PATH

For Bourne shell (sh and bash)

The syntax is as follows:
export var=value
To set PATH, enter:
export PATH=$PATH:/opt/bin:/usr/local/bin:$HOME/bin

For C shell (csh or tcsh)

The syntax is as follows:
setenv var value
Set EDITOR to vim, enter:
setenv EDITOR vim

Example: UNIX C Shell Startup Configuration Files For Environment Variable

C shell use the following files:
  1. /etc/csh.login - It is executed if C shell is your login shell.
  2. $HOME/.cshrc and $HOME/.login - These files are executed every time C Shell starts. The ~/.login is csh login script, read by login shell, after ~/.cshrc at login.
The above set or setenv commands can be placed in the ~/.cshrc or ~/.login files. A sample $HOME/.cshrc file is as follows:
alias h  history 25alias j  jobs -l
alias la ls -a
alias lf ls -FA
alias ll ls -lA

umask 22
set path = (/sbin /bin /usr/sbin /usr/bin /usr/games /usr/local/sbin /usr/local/bin $HOME/bin)
setenv EDITOR vi
setenv PAGER more

if ($?prompt) then
 # An interactive shell -- set some stuff up
 set filec
 set history = 100
 set savehist = 100
 set mail = (/var/mail/$USER)
 if ( $?tcsh ) then
  bindkey "^W" backward-delete-word
  bindkey -k up history-search-backward
  bindkey -k down history-search-forward
# Traps CTRL-D
's to avoid accidental system log off
set ignoreeof

# Set prompt
set prompt = "[\!] %"

# Sequentially keeps a buffer of your last events.
set history=100
set savehist=100

# Stops C Shell from overwriting and destroying the information in an existing file.
set noclobber
A sample ~/.login file is as follows:
# Show fortune :)if ( -x /usr/games/fortune ) /usr/games/fortune

# Sets the system variable TERM to recognize the xterm
setenv TERM xterm

# This command sets the time zone variable
setenv TZ IST

# set PATH 
setenv PATH /opt/gnu/bin:/bin/posix:/bin:/usr/bin:/usr/local/bin:/etc:/users/vivek:.

# set mail boxset mail=/usr/mail/vivek

# alias bye is easier to remember alias bye logoutalias c clear
# read mail as soon as I get into the systems

Example: UNIX KSH Shell Startup Configuration Files For Environment Variable

KSH shell use the following files:
  1. /etc/profile - This default system file is executed by the KSH and sets up default environment variables.
  2. $HOME/.profile - Put your customization in this file.
A sample $HOME/.profile for the ksh shell:

stty sane susp ^Z

# email notification if mail -e
   echo "You have mail."fi
# promptPS1="$ "
# Check system messages
msgs -q
# Allow terminal messagesmesg y
WHERE=$(hostname -s)
export PS1
set -o vi
stty erase ^?

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