Environment variables are variables that help set the environment. Since that was a horrible definition, I’ll try to define by example. The environment is the scene of the play, and the variables are the props that help set the scene (everything from the character’s clothing to the backdrop). Basically, the environment variables are strings that the computer sees as something else. One example of this is the $AMBERHOME variable, which is used to run the Amber program. Instead of typing /usr/local/amber18, users can simply type $AMBERHOME/ with the rest of the command they want to use. The specified path of an environment variable can be checked through $ echo $VARIABLE

A list of all environment variables will be given with the env command.

Setting Bash Environment Variables

To set an environment variable in a bash environment, use:

export VARIABLE=/path/to/variable

The AMBERHOME variable would be set through

export VARIABLE=/path/to/variable

which can be checked with

$ echo $AMBERHOME

Setting C-Shell Environment Variables

To set an environment variable in a C-shell environment, use:

setenv VARIABLE /path/to/variable

$PATH

The $PATH variable makes it possible to access programs simply by running their name, which is actually the case for most commands (like ls and head). PATH makes all of these programs accessible simultaneously, by making them available anywhere. Installed programs are added to the PATH. You can also add directories or files to the PATH by defining them in the configuration file.

For a .bash_profile or .profile (you shouldn’t add user-defined path definitions to the .bashrc):

export PATH=$PATH:/home/rest/of/path

For a .cshrc or .login:

setenv PATH $PATH\:/home/rest/of/path

Remember to source the file after things have been added.

Since the PATH includes a lot of important things, and almost none of them are user-defined, you should not delete variables from the PATH.