Environment variables
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.
Contents
Utilities
The coreutils package contains the programs printenv and env. To list the current environmental variables with values:
$ printenv
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/$PID/environ file. This file contains each key value pair delimited by a nul character (\x0). A more human readable format can be obtained with sed, e.g. sed 's:\x0:\n:g' /proc/$PID/environ.
Defining variables
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 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/environmentis used by the pam_env module and is shell agnostic so scripting or glob expansion cannot be used. The file only acceptsvariable=valuepairs. 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/profileinitializes 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}:$HOME/bin"
fi
Per user
~/.bashrc etc. This means that, for example, dbus activated programs like Gnome Files will not use them by default. See Systemd/User#Environment variables.You do not always want to define an environment variable globally. For instance, you might want to add /home/my_user/bin to the PATH variable but do not want all other users on your system to have that in their PATH too. Local environment variables can be defined in many different files:
-
~/.pam_environmentis the user specific equivalent of/etc/security/pam_env.conf[1], used by pam_env module. See pam_env(8) and pam_env.conf(5) for details. - User configuration files of your shell, for example Bash#Configuration files or Zsh#Startup/Shutdown files.
-
~/.profileis used by many shells as fallback, see wikipedia:Unix shell#Configuration files. -
systemd will load environment variables from
~/.config/environment.d/*.confsee environment.d(5) and https://wiki.gnome.org/Initiatives/Wayland/SessionStart.
To add a directory to the PATH for local usage, put following in ~/.bash_profile:
export PATH="${PATH}:/home/my_user/bin"
To update the variable, re-login or source the file: $ source ~/.bash_profile.
Graphical environment
Environment variables for Xorg applications can be set in xinitrc, or in xprofile when using a display manager, for example:
~/.xinitrc
export PATH="${PATH}:~/scripts"
export GUIVAR=value
Applications running on Wayland may use systemd user environment variables instead, as Wayland does not initiate any Xorg related files:
~/.config/environment.d/envvars.conf
PATH=$PATH:~/scripts GUIVAR=value
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.
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.
-
DEindicates 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 ofDEvariable are:gnome,kde,xfce,lxdeandmate.
- The
DEenvironment 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_SESSIONis similar toDE, but used in LXDE desktop environment: whenDESKTOP_SESSIONis set toLXDE, xdg-open will use pcmanfm file associations.
-
PATHcontains 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 inPATH, a relative or absolute path to the executable must be given, e.g../a.outor/bin/ls.
.) into your PATH for security reasons, as it may trick the user to execute vicious commands.-
HOMEcontains 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.
-
PWDcontains the path to your working directory.
-
OLDPWDcontains the path to your previous working directory, that is, the value ofPWDbefore last cd was executed.
-
TERMcontains 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.
-
MAILcontains the location of incoming email. The traditional setting is/var/spool/mail/$LOGNAME.
-
ftp_proxyandhttp_proxycontains FTP and HTTP proxy server, respectively:
ftp_proxy="ftp://192.168.0.1:21" http_proxy="http://192.168.0.1:80"
-
MANPATHcontains a colon-separated list of directories in which man searches for the man pages.
/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).
-
INFODIRcontains 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
-
TZcan 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 exampleTZ=":/usr/share/zoneinfo/Pacific/Fiji". When pointing theTZvariable to a zoneinfo file, it should start with a colon per the GNU manual.
Default programs
-
SHELLcontains 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.
-
PAGERcontains command to run the program used to list the contents of files, e.g.,/bin/less.
-
EDITORcontains 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 $DISPLAY ]]; then echo 'gedit'; else echo 'nano'; fi)"
-
VISUALcontains command to run the full-fledged editor that is used for more demanding tasks, such as editing mail (e.g.,vi, vim, emacs etc).
-
BROWSERcontains the path to the web browser. Helpful to set in an interactive shell configuration file so that it may be dynamically altered depending on the availability of a graphic environment, such as X:
if [ -n "$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/environmentmust consist of simpleVARIABLE=valuepairs on separate lines, for example:EDITOR=NANO
-
/etc/security/pam_env.confand~/.pam_environmentshare 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 theHOMEenvironment variable into another variable:XDG_CONFIG_HOME DEFAULT=@{HOME}/.configNote: The variablesThe format also allows to expand already defined variables in the values of other variables using${HOME}and${SHELL}are not linked to theHOMEandSHELLenvironment variables, they are not set by default.${VARIABLE}, like this:GNUPGHOME DEFAULT=${XDG_CONFIG_HOME}/gnupgVARIABLE=valuepairs are also allowed, but variable expansion is not supported in those pairs. See pam_env.conf(5) for more information.
~/.profile, ~/.bash_profile and ~/.zshenv.