# 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 filesystem, 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 provides a simple way to share configuration settings between multiple applications and processes in Linux.

## Utilities

The coreutils package contains `printenv` and `env`. To list the current environmental variables, use `printenv` to print the names and the values of each.
Note: Some environment variables are user-specific - check by comparing the `printenv` output as root.

    $ printenv

The `env` utility can be used to run a command under a modified environment. In the simplest case:

    $ env EDITOR=vim xterm

will set the default editor to vim in the new xterm session. This will not affect the `EDITOR` outside this session.

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 documentation on `set` built-in command.

To see what environment variables are available to a specific process, look in `/proc/${PID}/environ`. This file contains special `\x0` characters. You can parse it with this function.

    # envof() { sudo sed 's/\x0/\n/g' /proc/${1}/environ; }
    # envof 1
    TERM=linux

## Examples

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

- `DE` indicate the Desktop Environment being used. xdg-open will use it to chose 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, 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 inside Xfce. Use exo-preferred-applications for configuring.

- `DESKTOP_SESSION`. 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 `emerge`) is interpreted by the shell (e.g., `bash`, `zsh`), the shell looks for an executable file with 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 name of the running, interactive shell, e.g., `bash`

- `TERM` Contains the name of the running terminal, e.g., `xterm`

- `PAGER` Contains the path to the program used to list the contents of files, e.g., `/bin/less`.

- `EDITOR` Contains the path to the lightweight program used for editing files, e.g., `/usr/bin/nano`, or 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 the path to 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"`

## Defining variables globally

Most Linux distributions tell you to change or add environment variable definitions in `/etc/profile` or other locations. 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`.

An example that sets all users ~/bin in their respective path, put this in your preferred global environment variable config file:

    # 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

## Defining variables locally

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. The following files should be used for local environment variables on your system: `~/.bashrc`, `~/.profile`, `~/.bash_login` and `~/.bash_logout`.

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

    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, for example:

    ~/.xinitrc

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

## Session specific variables

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"

## See also

- Gentoo Linux Documentation
- Default applications
- Xdg-open

Category:

- System administration