Difference between revisions of "Environment variables"

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will set the default editor to vim in the new xterm session. This will not affect the {{ic|EDITOR}} outside this session.
 
will set the default editor to vim in the new xterm session. This will not affect the {{ic|EDITOR}} outside this session.
  
The [[Bash]] builtin {{ic|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: [http://www.gnu.org/software/bash/manual/bash.html#The-Set-Builtin gnu.org].
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The [[Bash]] builtin {{ic|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 [http://www.gnu.org/software/bash/manual/bash.html#The-Set-Builtin documentation] on  {{ic|set}} built-in command.
  
 
==Examples==
 
==Examples==

Revision as of 20:54, 13 October 2011

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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 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 Template:Package Official package contains printenv and env. To list the current environmental variables, use printenv to print the names and the values of each. Note that 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.

Examples

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

  • PATH Contains a colon-separated list of directories in which your system looks for executable files. When a regular command (i.e. ls, rc-update or emerge) is interpreted by the shell (i.e. 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 Template:Codeline 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 suchlike 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, i.e bash
  • TERM Contains the name of the running terminal, i.e xterm
  • PAGER Contains the path to the program used to list the contents of files, i.e. /bin/less.
  • EDITOR Contains the path to the lightweight program used for editing files, i.e. /usr/bin/nano.
  • VISUAL Contains the path to full-fledged editor that is used for more demanding tasks, such as editing mail; e.g., Template:Codeline, vim, emacs, etc.
  • MAIL Contains the location of incoming email. The traditional setting is Template:Filename.
  • 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
	BROWSER=firefox
else
	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 that in Template:Filename, 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, i.e. /usr/share/info:/usr/local/share/info

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.

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 ~/.bashrc:

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

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

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 ~/.bashrc 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"

References

Gentoo Linux Documentation [1]