Handbook:Parts/Working/EnvVar/en

Introduction
An environment variable is a named object that contains information used by one or more applications. Many users (and especially those new to Linux) find this a bit weird or unmanageable. However, this is a mistake: by using environment variables one can easily change a configuration setting for one or more applications.

Important examples
The following table lists a number of variables used by a Linux system and describes their use. Example values are presented after the table.

Below is an example definition of all these variables:

The env.d directory
To centralize the definitions of these variables, Gentoo introduced the directory. Inside this directory a number of files are available, such as, , etc. which contain the variables needed by the application mentioned in their name.

For instance, when is installed, a file called  was created by the ebuild which contains the definitions of the following variables:

Other distributions might tell their users to change or add such environment variable definitions in or other locations. Gentoo on the other hand makes it easy for the user (and for Portage) to maintain and manage the environment variables without having to pay attention to the numerous files that can contain environment variables.

For instance, when is updated, the  file is updated too without requesting any user-interaction.

This not only benefits Portage, but also the user. Occasionally users might be asked to set a certain environment variable system-wide. As an example we take the http_proxy variable. Instead of messing about with, users can now just create a file (say ) and enter the definition(s) in it:

By using the same file for all self-managed variables, users have a quick overview on the variables they have defined themselves.

env-update
Several files in define the PATH variable. This is not a mistake: when is executed, it will append the several definitions before it updates the environment variables, thereby making it easy for packages (or users) to add their own environment variable settings without interfering with the already existing values.

The env-update script will append the values in the alphabetical order of the files. The file names must begin with two decimal digits.

The concatenation of variables does not always happen, only with the following variables: ADA_INCLUDE_PATH, ADA_OBJECTS_PATH , CLASSPATH , KDEDIRS , PATH , LDPATH , MANPATH , INFODIR , INFOPATH , ROOTPATH , CONFIG_PROTECT , CONFIG_PROTECT_MASK , PRELINK_PATH , PRELINK_PATH_MASK , PKG_CONFIG_PATH , and PYTHONPATH. For all other variables the latest defined value (in alphabetical order of the files in ) is used.

It is possible to add more variables into this list of concatenate-variables by adding the variable name to either COLON_SEPARATED or SPACE_SEPARATED variables (also inside an file).

When executing, the script will create all environment variables and place them in (which is used by ). It will also extract the information from the LDPATH variable and use that to create. After this, it will run to recreate the  file used by the dynamical linker.

To notice the effect of immediately after running it, execute the following command to update the environment. Users who have installed Gentoo themselves will probably remember this from the installation instructions:

User specific
It might not be necessary to define an environment variable globally. For instance, one might want to add and the current working directory (the directory the user is in) to the PATH variable but do not want all other users on the system to have that in their PATH too. To define an environment variable locally, use or :

After logout/login, the PATH variable will be updated.

Session specific
Sometimes even stricter definitions are requested. For instance, a user might want to be able to use binaries from a temporary directory created without using the path to the binaries themselves or editing for the short time necessary.

In this case, just define the PATH variable in the current session by using the command. As long as the user does not log out, the PATH variable will be using the temporary settings.