# 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 documentation: [1].

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/profile`, `/etc/bash.bashrc` and `/etc/environment`. Each of these files has different limitations, so you should carefully select the appropriate one for your purposes.

- `/etc/profile` initializes variables for login shells only. It does, however, run scripts and can be used by all Bourne shell compatible shells.
- `/etc/bash.bashrc` initializes variables for interactive shells only. It also runs scripts but (as its name implies) is Bash specific.
- `/etc/environment` is used by the PAM-env module and is agnostic to login/non-login, interactive/non-interactive and also Bash/non-Bash, so scripting or glob expansion cannot be used. The file only accepts `variable=value` pairs.

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 $HOME/bin) ]]
    then
        export PATH=$HOME/bin:${PATH}
    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:

1. Configuration files of your shell, for example Bash#Configuration files or Zsh#Startup/Shutdown files.
2. `~/.profile` is used by many shells as fallback, see wikipedia:Unix shell#Configuration files.
3. `~/.pam_environment` is the user specific equivalent of `/etc/environment`, used by PAM-env module. See `pam_env(8)` and `pam_env.conf(5)` for details.

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

To set environment variables for GUI applications, you can put your variables in xinitrc (or 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 `~/.bash_profile` 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 libgnome; 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 enviroment: 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 absoute 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.

- `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.

- `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.

- `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).

- `MAIL` contains the location of incoming email. The traditional setting is `/var/spool/mail/$LOGNAME`.

- `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

- `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"`

### Using pam_env

Using `/etc/environment` and `~/.pam_environment` can be a little tricky, and the man pages (`pam_env(8)` and `pam_env.conf(5)`) are not particularly clear. So, here's an example:

    ~/.pam_environment

    LANG             DEFAULT=en_US.UTF-8
    LC_ALL           DEFAULT=${LANG}

    XDG_CONFIG_HOME  DEFAULT=@{HOME}/.config
    #XDG_CONFIG_HOME=@{HOME}/.config                    # is **not** valid see below
    XDG_DATA_HOME    DEFAULT=@{HOME}/.local/share

    # you can even use recently defined variables
    RCRC             DEFAULT=${XDG_CONFIG_HOME}/rcrc
    BROWSER=firefox
    #BROWSER         DEFAULT=firefox # same as above
    EDITOR=vim

In `~/.pam_environment` there are two ways to set environmental variables:

    VARIABLE=VALUE

and

    VARIABLE [DEFAULT=[value]] [OVERRIDE=[value]]

The first one doesn't allow the use of `${VARIABLES}` , while the second does. `@{HOME}` is a special variable that expands what is defined in `/etc/passwd` (same goes with `@{SHELL}` ). After defining a `VARIABLE`, you can recall it with `${VARIABLE}` . Note that curly braces and the dollar sign are needed (`${}` ) when invoking the previously defined variable.

Note: This file is read before everything, even `~/.{,bash_,z}profile` and `~/.zshenv` .

## See also

- Gentoo Linux Documentation
- Ubuntu Community Wiki - Environment Variables

Category:

- System administration