Environment variables

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An environment variable is a named object that contains data used by one or more applications. In simple terms, it is a variable with a name and a value. The value of an environmental variable can for example be the location of all executable files in the file system, the default editor that should be used, or the system locale settings. Users new to Linux may often find this way of managing settings a bit unmanageable. However, environment variables provide a simple way to share configuration settings between multiple applications and processes in Linux.

Utilities

The coreutils package contains the programs printenv and env. To list the current environmental variables with values:

$ printenv
Note: Some environment variables are user-specific. Check by comparing the outputs of printenv as an unprivileged user and as root.

The env utility can be used to run a command under a modified environment. The following example will launch xterm with the environment variable EDITOR set to vim. This will not affect the global environment variable EDITOR.

$ env EDITOR=vim xterm

The Bash builtin set allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables. For more information, see the set builtin documentation.

Each process stores their environment in the /proc/$PID/environ file. This file contained each key value pair delimited by a nul character (\x0). A more human readable format can be obtained with sed, e.g. sed 's:\x0:\n:g' /proc/$PID/environ.

Defining variables

Globally

Most Linux distributions tell you to change or add environment variable definitions in /etc/profile or other locations. Keep in mind that there are also package-specific configuration files containing variable settings such as /etc/locale.conf. Be sure to maintain and manage the environment variables and pay attention to the numerous files that can contain environment variables. In principle, any shell script can be used for initializing environmental variables, but following traditional UNIX conventions, these statements should be only be present in some particular files.

The following files should be used for defining global environment variables on your system: /etc/environment, /etc/profile and shell specific configuration files. Each of these files has different limitations, so you should carefully select the appropriate one for your purposes.

  • /etc/environment is used by the pam_env module and is shell agnostic so scripting or glob expansion cannot be used. The file only accepts variable=value pairs. See pam_env(8) and pam_env.conf(5) for details.
  • Global configuration files of your shell, initializes variables and runs scripts. For example Bash#Configuration files or Zsh#Startup/Shutdown files.
  • /etc/profile initializes variables for login shells only. It does, however, run scripts and can be used by all Bourne shell compatible shells.

In this example, we add ~/bin directory to the PATH for respective user. To do this, just put this in your preferred global environment variable config file (/etc/profile or /etc/bash.bashrc):

# If user ID is greater than or equal to 1000 & if ~/bin exists and is a directory & if ~/bin is not already in your $PATH
# then export ~/bin to your $PATH.
if [[ $UID -ge 1000 && -d $HOME/bin && -z $(echo $PATH | grep -o $HOME/bin) ]]
then
    export PATH="${PATH}:$HOME/bin"
fi

Per user

Note: The dbus daemon and the user instance of systemd do not inherit any of the environment variables set in places like ~/.bashrc etc. This means that, for example, dbus activated programs like Gnome Files will not use them by default. See Systemd/User#Environment variables.

You do not always want to define an environment variable globally. For instance, you might want to add /home/my_user/bin to the PATH variable but do not want all other users on your system to have that in their PATH too. Local environment variables can be defined in many different files:

To add a directory to the PATH for local usage, put following in ~/.bash_profile:

export PATH="${PATH}:/home/my_user/bin"

To update the variable, re-login or source the file: $ source ~/.bash_profile.

Graphical applications

Environment variables for GUI applications can be set in xinitrc, or in xprofile when using a display manager, for example:

~/.xinitrc
export PATH="${PATH}:~/scripts"
export GUIVAR=value

Per session

Sometimes even stricter definitions are required. One might want to temporarily run executables from a specific directory created without having to type the absolute path to each one, or editing shell configuration files for the short time needed to run them.

In this case, you can define the PATH variable in your current session, combined with the export command. As long as you do not log out, the PATH variable will be using the temporary settings. To add a session-specific directory to PATH, issue:

$ export PATH="${PATH}:/home/my_user/tmp/usr/bin"

Examples

The following section lists a number of common environment variables used by a Linux system and describes their values.

  • DE indicates the Desktop Environment being used. xdg-open will use it to choose more user-friendly file-opener application that desktop environment provides. Some packages need to be installed to use this feature. For GNOME, that would be libgnomeAUR; for Xfce this is exo. Recognised values of DE variable are: gnome, kde, xfce, lxde and mate.
The DE environment variable needs to be exported before starting the window manager. For example:
~/.xinitrc
export DE="xfce"
exec openbox
This will make xdg-open use the more user-friendly exo-open, because it assumes it is running inside Xfce. Use exo-preferred-applications for configuring.
  • DESKTOP_SESSION is similar to DE, but used in LXDE desktop environment: when DESKTOP_SESSION is set to LXDE, xdg-open will use pcmanfm file associations.
  • PATH contains a colon-separated list of directories in which your system looks for executable files. When a regular command (e.g., ls, rc-update or ic|emerge) is interpreted by the shell (e.g., bash or zsh), the shell looks for an executable file with the same name as your command in the listed directories, and executes it. To run executables that are not listed in PATH, the absolute path to the executable must be given: /bin/ls.
Note: It is advised not to include the current working directory (.) into your PATH for security reasons, as it may trick the user to execute vicious commands.
  • HOME contains the path to the home directory of the current user. This variable can be used by applications to associate configuration files and such like with the user running it.
  • PWD contains the path to your working directory.
  • OLDPWD contains the path to your previous working directory, that is, the value of PWD before last cd was executed.
  • TERM contains the type of the running terminal, e.g. xterm-256color. It is used by programs running in the terminal that wish to use terminal-specific capabilities.
  • MAIL contains the location of incoming email. The traditional setting is /var/spool/mail/$LOGNAME.
  • ftp_proxy and http_proxy contains FTP and HTTP proxy server, respectively:
ftp_proxy="ftp://192.168.0.1:21"
http_proxy="http://192.168.0.1:80"
  • MANPATH contains a colon-separated list of directories in which man searches for the man pages.
Note: In /etc/profile, there is a comment that states "Man is much better than us at figuring this out", so this variable should generally be left as default, i.e. /usr/share/man:/usr/local/share/man
  • INFODIR contains a colon-separated list of directories in which the info command searches for the info pages, e.g., /usr/share/info:/usr/local/share/info
  • TZ can be used to to set a time zone different to the system zone for a user. The zones listed in /usr/share/zoneinfo/ can be used as reference, for example TZ=":/usr/share/zoneinfo/Pacific/Fiji". When pointing the TZ variable to a zoneinfo file, it should start with a colon per the GNU manual.

Default programs

  • SHELL contains the path to the user's preferred shell. Note that this is not necessarily the shell that is currently running, although Bash sets this variable on startup.
  • PAGER contains command to run the program used to list the contents of files, e.g., /bin/less.
  • EDITOR contains the command to run the lightweight program used for editing files, e.g., /usr/bin/nano. For example, you can write an interactive switch between gedit under X or nano, in this example:
export EDITOR="$(if [[ -n $DISPLAY ]]; then echo 'gedit'; else echo 'nano'; fi)"
  • VISUAL contains command to run the full-fledged editor that is used for more demanding tasks, such as editing mail (e.g., vi, vim, emacs etc).
  • BROWSER contains the path to the web browser. Helpful to set in an interactive shell configuration file so that it may be dynamically altered depending on the availability of a graphic environment, such as X:
if [ -n "$DISPLAY" ]; then
    export BROWSER=firefox
else 
    export BROWSER=links
fi

Using pam_env

The PAM module pam_env(8) loads the variables to be set in the environment from the following files: /etc/security/pam_env.conf, /etc/environment and ~/.pam_environment.

  • /etc/environment must consist of simple VARIABLE=value pairs on separate lines, for example:
    EDITOR=NANO
  • /etc/security/pam_env.conf and ~/.pam_environment share the same following format:
    VARIABLE [DEFAULT=value] [OVERRIDE=value]
    @{HOME} and @{SHELL} are special variables that expand to what is defined in /etc/passwd. The following example illustrates how to expand the HOME environment variable into another variable:
    XDG_CONFIG_HOME   DEFAULT=@{HOME}/.config
    Note: The variables ${HOME} and ${SHELL} are not linked to the HOME and SHELL environment variables, they are not set by default.
    The format also allows to expand already defined variables in the values of other variables using ${VARIABLE} , like this:
    GNUPGHOME        DEFAULT=${XDG_CONFIG_HOME}/gnupg
    VARIABLE=value pairs are also allowed, but variable expansion is not supported in those pairs. See pam_env.conf(5) for more information.
Note: These files are read before other files, in particular before ~/.profile, ~/.bash_profile and ~/.zshenv.

See also