Difference between revisions of "Environment variables"
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− | [[Category: | + | [[Category:System administration]] |
− | + | [[de:Umgebungsvariablen]] | |
− | + | 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. | |
− | An environment variable is a named object that contains | ||
==Utilities== | ==Utilities== | ||
− | The {{ | + | The {{Pkg|coreutils}} package contains {{ic|printenv}} and {{ic|env}}. To list the current environmental variables, use {{ic|printenv}} to print the names and the values of each. Note that some environment variables are user-specific - check by comparing the {{ic|printenv}} output as root: |
− | |||
− | |||
$ printenv | $ printenv | ||
− | {{ | + | The {{ic|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 {{ic|EDITOR}} outside this session. | |
− | The [[Bash]] builtin {{ | + | 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== | ||
− | The following | + | The following section lists a number of common environment variables used by a Linux system and describes their values. |
+ | |||
+ | *{{ic|DE}} indicate the '''D'''esktop '''E'''nvironment 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 {{pkg|libgnome}}. For Xfce, 'exo'. Recognised values of {{ic|DE}} variable are: '''gnome''', '''kde''', '''xfce''', '''lxde''' and '''mate'''. | ||
+ | |||
+ | The {{ic|$DE}} environment variable needs to be exported before starting the window manager. For example: | ||
+ | {{hc|~/.xinitrc|<nowiki> | ||
+ | export DE="xfce" | ||
+ | exec openbox | ||
+ | </nowiki>}} | ||
− | ---- | + | 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. |
− | |||
− | + | *{{ic|DESKTOP_SESSION}}. In [[LXDE]] desktop enviroment, when DESKTOP_SESSION is set to LXDE, xdg-open will use pcmanfm file associations. | |
− | + | *{{ic|PATH}} Contains a colon-separated list of directories in which your system looks for executable files. When a regular command (i.e. {{ic|ls}}, {{ic|rc-update}} or {{ic|emerge}}) is interpreted by the shell (i.e. {{ic|bash}}, {{ic|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 {{ic|PATH}}, the absoute path to the executable must be given: {{ic|/bin/ls}}. | |
− | + | {{Note|It is advised not to include the current working directory (.) into your {{ic|PATH}} for security reasons, as it may trick the user to execute vicious commands.}} | |
− | {{ | + | *{{ic|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. |
− | + | *{{ic|PWD}} Contains the path to your working directory. | |
− | + | *{{ic|OLDPWD}} Contains the path to your previous working directory, that is, the value of PWD before last {{ic|cd}} was executed. | |
− | + | *{{ic|SHELL}} Contains the name of the running, interactive shell, i.e {{ic|bash}} | |
− | + | *{{ic|TERM}} Contains the name of the running terminal, i.e {{ic|xterm}} | |
− | |||
+ | *{{ic|PAGER}} Contains the path to the program used to list the contents of files, i.e. {{ic|/bin/less}}. | ||
− | + | *{{ic|EDITOR}} Contains the path to the lightweight program used for editing files, i.e. {{ic|/usr/bin/nano}}. | |
− | - | + | *{{ic|VISUAL}} Contains the path to full-fledged editor that is used for more demanding tasks, such as editing mail; e.g., {{ic|vi}}, [[vim]], [[emacs]], etc. |
− | |||
− | + | *{{ic|MAIL}} Contains the location of incoming email. The traditional setting is {{ic|/var/spool/mail/$LOGNAME}}. | |
− | + | *{{ic|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 | ||
− | + | *{{ic|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" | ||
− | + | *{{ic|MANPATH}} Contains a colon-separated list of directories in which {{ic|man}} searches for the man pages. Note that in {{ic|/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. {{ic|/usr/share/man:/usr/local/share/man}} | |
− | + | *{{ic|INFODIR}} Contains a colon-separated list of directories in which the info command searches for the info pages, i.e. {{ic|/usr/share/info:/usr/local/share/info}} | |
− | |||
== Defining Variables Globally == | == Defining Variables Globally == | ||
− | Most Linux distributions tell you to change or add environment variable definitions in {{ | + | Most Linux distributions tell you to change or add environment variable definitions in {{ic|/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: {{ic|/etc/profile}}, {{ic|/etc/bash.bashrc}} and {{ic|/etc/environment}}. |
== Defining Variables Locally == | == Defining Variables Locally == | ||
− | + | You do not always want to define an environment variable globally. For instance, you might want to add {{ic|/home/my_user/bin}} to the PATH variable but do not want all other users on your system to have that in their {{ic|PATH}} too. The following files should be used for local environment variables on your system: {{ic|~/.bashrc}}, {{ic|~/.profile}}, {{ic|~/.bash_login}} and {{ic|~/.bash_logout}}. | |
− | + | To add a directory to {{ic|PATH}} for local usage, put following in {{ic|~/.bashrc}}: | |
− | + | PATH="${PATH}:/home/my_user/bin" | |
− | + | To update the variable, re-login or source the file: {{ic|$ source ~/.bashrc}}. | |
− | |||
− | |||
== Session Specific Variables == | == Session Specific Variables == | ||
− | Sometimes even stricter definitions are required. | + | 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 {{ic|~/.bashrc}} for the short time needed to run them. |
− | |||
− | |||
− | + | In this case, you can define the {{ic|PATH}} variable in your current session, combined with the {{ic|export}} command. As long as you do not log out, the {{ic|PATH}} variable will be using the temporary settings. To add a session-specific directory to {{ic|PATH}}, issue: | |
$ export PATH="${PATH}:/home/my_user/tmp/usr/bin" | $ export PATH="${PATH}:/home/my_user/tmp/usr/bin" | ||
− | == | + | ==See also== |
− | + | *Gentoo Linux Documentation [http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#book_part2_chap5] | |
− | + | *[[Default Applications]] | |
− | + | *[[Xdg-open]] | |
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− | Gentoo Linux Documentation [http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#book_part2_chap5] |
Revision as of 10:36, 28 May 2013
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.
Contents
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 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.
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 ofDE
variable are: gnome, kde, xfce, lxde and mate.
The $DE
environment variable needs to be exported before starting the window manager. For example:
~/.xinitrc
export DE="xfce" exec openbox
This will make xdg-open use the more user-friendly exo-open, because it assumes it is inside Xfce. Use exo-preferred-applications for configuring.
DESKTOP_SESSION
. In LXDE desktop enviroment, when DESKTOP_SESSION is set to LXDE, xdg-open will use pcmanfm file associations.
PATH
Contains a colon-separated list of directories in which your system looks for executable files. When a regular command (i.e.ls
,rc-update
oremerge
) is interpreted by the shell (i.e.bash
,zsh
), the shell looks for an executable file with same name as your command in the listed directories, and executes it. To run executables that are not listed inPATH
, the absoute path to the executable must be given:/bin/ls
.
PATH
for security reasons, as it may trick the user to execute vicious commands.HOME
Contains the path to the home directory of the current user. This variable can be used by applications to associate configuration files and such like with the user running it.
PWD
Contains the path to your working directory.
OLDPWD
Contains the path to your previous working directory, that is, the value of PWD before lastcd
was executed.
SHELL
Contains the name of the running, interactive shell, i.ebash
TERM
Contains the name of the running terminal, i.exterm
PAGER
Contains the path to the program used to list the contents of files, i.e./bin/less
.
EDITOR
Contains the path to the lightweight program used for editing files, i.e./usr/bin/nano
.
VISUAL
Contains the path to full-fledged editor that is used for more demanding tasks, such as editing mail; e.g.,vi
, vim, emacs, etc.
MAIL
Contains the location of incoming email. The traditional setting is/var/spool/mail/$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 "$DISPLAY" ]; then BROWSER=firefox else BROWSER=links fi
ftp_proxy and http_proxy
Contains FTP and HTTP proxy server, respectively:
ftp_proxy="ftp://192.168.0.1:21" http_proxy="http://192.168.0.1:80"
MANPATH
Contains a colon-separated list of directories in whichman
searches for the man pages. Note that in/etc/profile
, there is a comment that states "Man is much better than us at figuring this out", so this variable should generally be left as default, i.e./usr/share/man:/usr/local/share/man
INFODIR
Contains a colon-separated list of directories in which the info command searches for the info pages, i.e./usr/share/info:/usr/local/share/info
Defining Variables Globally
Most Linux distributions tell you to change or add environment variable definitions in /etc/profile
or other locations. Be sure to maintain and manage the environment variables and pay attention to the numerous files that can contain environment variables. In principle, any shell script can be used for initializing environmental variables, but following traditional UNIX conventions, these statements should be only be present in some particular files. The following files should be used for defining global environment variables on your system: /etc/profile
, /etc/bash.bashrc
and /etc/environment
.
Defining Variables Locally
You do not always want to define an environment variable globally. For instance, you might want to add /home/my_user/bin
to the PATH variable but do not want all other users on your system to have that in their PATH
too. The following files should be used for local environment variables on your system: ~/.bashrc
, ~/.profile
, ~/.bash_login
and ~/.bash_logout
.
To add a directory to PATH
for local usage, put following in ~/.bashrc
:
PATH="${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"
See also
- Gentoo Linux Documentation [1]
- Default Applications
- Xdg-open