8.1 UNIX Variables
Variables are a way of passing information from the shell to programs when you run them. Programs look "in the environment" for particular variables and if they are found will use the values stored. Some are set by the system, others by you, yet others by the shell, or any program that loads another program. Standard UNIX variables are split into two categories, environment variables and shell variables. In broad terms, shell variables apply only to the current instance of the shell and are used to set short-term working conditions; environment variables are exported and have a farther reaching significance; those set at login are valid for the duration of the session. By convention, environment variables are written in UPPERCASE while shell variables usually have lowercase names.
8.2 Environment Variables
An example of an environment variable is the $SHELL variable. The value of this is the current shell you are using. Type
% echo $SHELL
More examples of environment variables are
- $USER (your login name)
- $HOME (the path name of your home directory)
- $PWD (current working directory)
- $DISPLAY (the name of the computer screen to display X windows; only set if X is enabled)
- $PATH (the directories the shell should search to find a command)
Finding Out the Current Values of These Variables
Environment variables are set using the export command (bash or ksh) or the setenv command (tcsh or csh), displayed using the printenv (bash, tcsh) or env (bash, ksh) commands, and unset using the unsetenv command. To show all values of these variables, type
% printenv | less
To set a value of an environment variable, type (for bash)
% export VAR=value
or (for tcsh)
% setenv VAR value
8.3 Using and Setting Variables
Each time you login to a UNIX host, the system looks in your home directory for initialisation files. Information in these files is used to set up your working environment. The bash shell uses files called .profile, .bash_profile, and .bashrc, whereas ksh uses .profile and .kshrc, and the the C and TC shells use files called .login and .cshrc or .tcshrc (note that all these file names begin with periods or dots; hence they are called dotfiles). At login the bash shell first sources .bash_profile or .profile (if .bash_profile exists .profile will be ignored). Child shells source .bashrc. If you wish for your login shell to source the .bashrc also, add the lines
if [ -f ~/.bashrc ];
then . ~/.bashrc
to the .bash_profile script. The ksh reads .profile and child shells source .kshrc. The C shell reads .login for login shells and .tcshrc for all other shells. The .bash_profile, .profile, or .login is to set conditions which will apply to the whole session and/or to perform actions that are relevant only at login. The .bashrc, .kshrc, or .cshrc file is used to set conditions and perform actions specific to the shell and to each invocation of it. The rc stands for resource; many UNIX dotfiles use this convention to set resources.
Warning: NEVER put commands that run graphical displays (e.g. web browsers) in your dotfiles. If you change your .bashrc you can force the shell to reread it by using the shell source command.
% source .bashrc
8.4 Setting the Path
When you type a command, your path (or $PATH) variable defines in which directories the shell will look to find the command you typed. If the system returns a message saying "command: Command not found", this indicates that either the command doesn't exist at all on the system or it is simply not in your path. For example, to run units, you either need to directly specify the units path (~/units174/bin/units), or you need to have the directory ~/units174/bin in your path. You can add it to the end of your existing path (the $PATH represents this) by issuing the command:
% export PATH=$PATH:$HOME/units174/bin
If you have units installed you can test that this worked by trying to run units in any directory other than where units is actually located.
% cd; units
Hint: You can run multiple commands on one line by separating them with a semicolon. To add this path permanently, add the line to your .bashrc file after the list of other commands. Make sure that you include the $PATH when you reset it, or you will lose access to basic system commands!
M.Stonebank@surrey.ac.uk, © October 2001. Modified for ARCS February 2012.