Environment 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.

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)

Using and Setting Variables

Environment variables are set using the export command (bash or zsh) or the setenv command (tcsh or csh), displayed using the printenv (bash, tcsh) or env (bash, zsh) commands, and unset using the unsetenv command. To show all values of these variables, type

% printenv | more

To set a value of an environment variable, type (for bash)

% export VAR=value

Sourcing

A group of shell commands can be placed into a file and then sourced. When a file is sourced, the commands are executed as if they had been typed at the command line in the current shell. For example, if several environment variables needed to be set over and over again, they could be collected into a file such as this simple script called envs.sh:

export NUM_CPUS=16
export MEM=64

ncpus=${NUM_CPUS}

Exercise 6A

Download the envs.sh file to the Unix system you are using. Run the command

source envs.sh

Print the values of the environment variables in the file. To print the value of the shell variable ncpus, type

echo $ncpus

Dotfiles

Each time you log in to a Unix host, the system looks in your home directory for initialization files. Information in these files is used to set up your working environment. The first initialization file sourced is the login initialization. It is sourced only in the login shell. Note: modern “desktop” user interfaces tend to “swallow” the login setup file, and it may be difficult to determine what is happening in these cases if there is an error.

At login the bash shell first sources .bash_profile or .profile (if .bash_profile exists .profile will be ignored). Child shells source .bashrc. The zsh sources .zprofile and child shells source .zshrc. Two older shells, csh (C shell) and tcsh, read .login for login shells and .cshrc or .tcshrc for all other shells.

Note that all these file names begin with periods or “dots”; hence they are called dotfiles. As we have learned, dotfiles are hidden and will only be visible with ls -a.

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, .zshrc, or .tcshrc 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.

If you wish for your login shell to source the .bashrc also, add the lines

if [ -f ~/.bashrc ];
  then . ~/.bashrc
fi

to the .bash_profile script.

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

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, suppose you have installed a program called “units” into your home directory in a folder called units174. Units is a simple utility that can convert Imperial to metric and vice versa, from SI to cgi, and so forth. This folder contains a bin subdirectory in which the executable is located. To run units, you must either 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 export 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!