The DISPLAY environment variable is used when you're logged on to a Unix/Linux system graphically. It identifies where X applications should "send" their graphical screens to. When you log on to a system remotely, this variable is set to your local IP address and the screen number you're using on this system. Most of the time, when you're logged on locally, it's content is ":0.0" (the first screen on the system).

Note that "screen" here isn't the hardware device, but a name given to a running X instance.

The EDITOR variable identifies the default text editor you want to use. Applications that spawn a text editor (for instance, visudo) to edit one or more files, use this variable to know which text editor to launch.

Locales are discussed later in this chapter. Its environment variables (LANG and the various LC_* variables) identify the users' language, timezone, currency, number formatting and more.

The PATH variable identifies the directories where the system should look for executable files (being binaries or shell scripts). If unset or set incorrectly, you cannot execute a command without providing the entire path to this command (except built-in shell commands as those are no executable files).

Below is a small example of a PATH variable:

~$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/opt/bin:/usr/i686-pc-linux-gnu/gcc-bin/4.1.2:
/opt/blackdown-jdk-1.4.2.03/bin:/opt/blackdown-jdk-1.4.2.03/jre/bin:
/usr/kde/3.5/bin:/usr/qt/3/bin:/usr/games/bin:/home/swift/bin/

An example of what happens when PATH is not set:

~$ ls
(... listing of current directory ...)
~$ unset PATH
~$ ls
-bash: ls: No such file or directory
~$ /bin/ls
(... listing of current directory ...)

The TERM variable allows command-line programs with special characters to identify which terminal you use to run them. Although nowadays the xterm TERM is most used, sometimes you will find yourself logged on to a different system which doesn't know xterm or where the application looks really awkward. In such cases a solution could be to set the TERM variable to, for instance, vt100.

When you want to set an environment variable for a specific session, you can use the shell set or export command:

~$ ls -z
ls: invalid option -- z
Try `ls --help` for more information.
~$ export LANG="fr"
~$ ls -z
ls: option invalide -- z
Pour en savoir davantage, faites: `ls --help`

Which one to use depends on what you actually want to achieve:

User specific environment settings are best placed inside the .bashrc file. This file is automatically read when a user is logged on (at least when he is using the bash shell). A more shell-agnostic file is .profile. Inside the file, define the variables as you would for a specific session:

export LANG="fr"

To make an environment variable system wide, you must make sure that your environment variable is stored in a file or location that every session reads out when it is launched. By convention, /etc/profile is a script in which system wide environment variables can be placed. Gentoo offers a nice interface for this: inside /etc/env.d you can manage environment variables in a more structured approach, and the env-update.sh script will then make sure that the environment variables are stored elsewhere so that /etc/profile reads them out.

Inside /etc/env.d, you will find environment files which use a simple key=value syntax. For instance, the /etc/env.d/20java file defines, amongst other environment variables, the PATH and MANPATH variables:

# cat /etc/env.d/20java
...
MANPATH=/opt/blackdown-jdk-1.4.2.03/man
PATH=/opt/blackdown-jdk-1.4.2.03/bin:/opt/blackdown-jdk-1.4.2.03/jre/bin

With these settings, the value of MANPATH (location where man will search for its manual pages) and PATH (location where the system will look for executable binaries every time you enter a command) is extended with the given values (note that the variables are not rewritten: their value is appended to the value previously assigned to the variable).

The order in which variable values are appended is based on the filename inside /etc/env.d. This is why most files start with a number (as most people find it easier to deal with order based on numbers, plus that the filenames themselves are still explanatory to what purpose they serve).

If you want to change a system variable globally, you can either add another file to /etc/env.d or manipulate an existing one. In the latter case, you should be aware that application upgrades automatically update their entries inside /etc/env.d without warning (this location is not protected, unlike many other configuration locations).

As such, it is adviseable to always add your own files rather than manipulate existing ones.

When you have altered an environment file or added a new one, you need to call env-update to have Gentoo process the changes for you:

# env-update

This command will read in all environment files and write the final result in /etc/profile.env (which is sourced by /etc/profile, which is always sourced when a user logs on).