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 shell builtin set(1p) allows you to change the values of shell options, set the positional parameters and to display the names and values of shell variables.

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

Defining variables

To avoid needlessly polluting the environment, you should seek to restrict the scope of variables. In fact, graphical sessions and systemd services require you to set variables in certain locations for them to take effect. The scopes of environment variables are broken down into the contexts they affect:

Globally

Using shell initialization files

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 only be present in some particular files.

The following files can be used for defining global environment variables on your system, each with different limitations:

  • /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.
  • /etc/profile initializes variables for login shells only. It does, however, run scripts (e.g. those in /etc/profile.d/) and can be used by all Bourne shell compatible shells.
  • Shell specific configuration files - Global configuration files of your shell, initializes variables and runs scripts. For example Bash#Configuration files (e.g. ~/.bashrc) or Zsh#Startup/Shutdown files (e.g. ~/.zshrc).

In this example, we will create a function to add several directories (e.g. ~/bin and ~/scripts) to PATH for the respective user. To do this, just put this in your preferred global environment variable configuration file (/etc/profile or /etc/bash.bashrc): 

set_path(){

    # Check if user id is 1000 or higher
    [ "$(id -u)" -ge 1000 ] || return

    for i in "$@";
    do
        # Check if the directory exists
        [ -d "$i" ] || continue

        # Check if it is not already in your $PATH.
        echo "$PATH" | grep -Eq "(^|:)$i(:|$)" && continue

        # Then append it to $PATH and export it
        export PATH="${PATH}:$i"
    done
}

set_path ~/bin ~/scripts

One method for sharing environment variables between different shells is to create a file without any comments, blank lines, or spaces (bash) that can be read directly by envsubst (inspired by [1]): 

.env
EDITOR=vim
XDG_CACHE_HOME=$HOME/.cache
XDG_CONFIG_HOME=$HOME/.config
XDG_DATA_HOME=$HOME/.local/share
XDG_STATE_HOME=$HOME/.local/state
~/.bashrc
export $(envsubst < .env)
~/.config/fish/config.fish
export (envsubst < .env)

Using pam_env

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

Note:
  • These files are read before other files, in particular before ~/.profile, ~/.bash_profile and ~/.zshenv.
  • The deprecated ~/.pam_environment is not read anymore. See FS#68945.

/etc/environment must consist of simple VARIABLE=value pairs on separate lines, for example: 

/etc/environment
EDITOR=nano

/etc/security/pam_env.conf has the following format: 

/etc/security/pam_env.conf
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: 

/etc/security/pam_env.conf
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: 

/etc/security/pam_env.conf
GOPATH DEFAULT=${XDG_DATA_HOME}/go

VARIABLE=value pairs are also allowed, but variable expansion is not supported in those pairs. See pam_env.conf(5) for more information.

Per user

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.

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.
Tip: You can issue export -p to review the global and local environment variables declared for the user session.

Graphical environment

If an environment variable only affects graphical applications, you may want to restrict the scope of it by only setting it within the graphical session. In order of decreasing scope:

Per desktop environment session

Some graphical environments, (e.g. KDE Plasma) support executing shell scripts at login: they can be used to set environment variables. See KDE#Autostart for example.

Per Xorg session

The procedure for modifying the environment of the Xorg session depends on how it is started:

Though the end of the script depends on which file it is, and any advanced syntax depends on the shell used, the basic usage is universal: 

~/.xprofile, ~/.xinitrc, or ~/.xsession
...
export GUI_VAR=value
...
Per Wayland session

Since Wayland does not initiate any Xorg related files, GDM and KDE Plasma source systemd user environment variables instead. 

~/.config/environment.d/envvars.conf
GUI_VAR=value

No other display managers supporting Wayland sessions (e.g. SDDM) provide direct support for this yet. However, LightDM and SDDM source startup scripts for login shells on Wayland sessions too.

greetd also sources /etc/profile and ~/.profile - this behavior is controlled by its source_profile setting, enabled by default.

If your display manager sources startup scripts like ~/.bash_profile and you want to use environment.d, you can source it like so: 

~/.bash_profile
# use systemd-environment-d-generator(8) to generate environment, and export those variables
set -o allexport
source <(/usr/lib/systemd/user-environment-generators/30-systemd-environment-d-generator)
set +o allexport
Note: Other generators in /usr/lib/systemd/user-environment-generators like 60-flatpak may not quote the values of environment variables. In this case the output should be sourced with export -- "$(/usr/lib/systemd/user-environment-generators/60-flatpak)"
Per application

To set environment variables only for a specific application instead of the whole session, edit the application's .desktop file. See Desktop entries#Modify environment variables for instructions.

For Steam games, you can configure a program's environment by editing its launch options; see Steam#Launch options.

Per session or shell

The factual accuracy of this article or section is disputed.

Reason: Since PATH is exported by default (at least in Bash; see declare -p | grep PATH), both PATH=... and export PATH=... do the same; it is probably better to use another variable for at least one of these examples. (Discuss in Talk:Environment variables)

Sometimes only temporary variables 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 use the path in a short temporary shell script.

For example, you can define the PATH variable in your current shell, or use the export command to define it for all shells until you log out of the session. To add a session-specific directory to PATH, issue: 

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

To add only a shell-specific directory to PATH, issue: 

$ 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.

  • XDG_CURRENT_DESKTOP is a freedesktop.org variable containing a colon separated list of strings that the current desktop environment identifies as [4]. Standardized values for actively developed environments are GNOME, GNOME-Flashback, KDE, LXDE, LXQt, MATE, TDE, Unity, XFCE, EDE, Cinnamon, Pantheon, and DDE [5].
    • Cinnamon was registered later than the rest of the desktop environments. As a result, some software may still be expecting its pre-registration value X-CINNAMON, such as Qt[6].
    • Hyprland is informally recognized for Hyprland.
  • DE is a legacy variable indicating the desktop environment being used. There is no central documentation for what possible values are, but xdg-utils provides a reference for many desktop environments.
  • DESKTOP_SESSION is another legacy variable, similar to DE but less common. It may be a path to the session's desktop entry, in /usr/share/xsessions/ [7].
  • WINDOW_MANAGER is a variable sometimes used to choose the window manager to be used in a desktop environment, as opposed to the other variables here which are set by the already chosen display manager or desktop environment, for other programs to read.
  • PATH contains a colon-separated list of directories in which your system looks for executable files. When a regular command (e.g. ls, systemctl or pacman) 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, a relative or absolute path to the executable must be given, e.g. ./a.out or /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 malicious 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.
  • OLDPWD contains the path to the 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 unset. See manpath(5).
  • 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. In the event that it has no value, Bash will automatically set this variable to the user's login shell as defined in /etc/passwd or to /bin/sh if this cannot be determined.
  • 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:
[ -n "$DISPLAY" ] && export EDITOR=gedit || export EDITOR=nano
  • VISUAL contains command to run the full-fledged editor that is used for more demanding tasks, such as editing mail (e.g., vi, vim, emacs etc).
  • BROWSER contains the path to the web browser. Helpful to set in an interactive shell configuration file so that it may be dynamically altered depending on the availability of a graphic environment, such as X:
[ -n "$DISPLAY" ] && export BROWSER=firefox || export BROWSER=links
Tip: These default programs can also be set conditionally if a Wayland compositor is running by using the WAYLAND_DISPLAY environment variable.

See also

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