特殊:Badtitle/NS100:EnvironmentVariables

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1. title Environment Variables

<<Include(Tag/ContentCleanup)>> Environment variables provide a way to influence the behavior of software on the system. For example, the "LANG" environment variable determines the language in which software programs communicate with the user. Environment variables consist of names that have values assigned to them. For example, on a typical system in the US we would have the value "en_US.UTF-8" assigned to the "LANG" variable. The meaning of an environment variable and the format of its value are determined by the application using it. There are quite a few well-known environment variables for which the meaning and the format have been agreed upon and they are used by many applications.

Manipulating environment variables and values

While quite a few graphical system configuration applications actually manipulate environment variables in the background, the command-line allows for maximum flexibility when manipulating environment variables. Note: The shell techniques explained in the following sections apply to the Bourne Shell family of command line shells, which includes sh, ksh, and bash, which is the default shell shipped with Ubuntu. The commands may be different on other shells such as csh.

Setting values to environment variables

In order to set a value to an existing environment variable, we use an assignment expression. For instance, to set the value of the "LANG" variable to "he_IL.UTF-8", we use the following command:

LANG=he_IL.UTF-8

If we use an assignment expression for a variable that doesn't exist, the shell will create a shell variable, which is similar to an environment variable but does not influence the behavior of other applications. A shell variable can be exported to become an environment variable with the export command To create the "EDITOR" environment variable and assign the value "nano" to it. There are several ways to accomplish this. However, we will use the following formats:

EDITOR=nano
export EDITOR

The bash shell (the default command-line shell in Ubuntu) provides a shortcut for creating environment variables. The previous example could be be performed with the following single command:

export EDITOR=nano

Examining values of environment variables

The printenv command prints the names and values of all currently defined environment variables:

printenv 

To examine the value of a particular variable, we can specify its name to the printenv command:

printenv TERM

Another way to achieve that is to use the dollar sign ($), as used in the following example:

echo $TERM

There is a command for doing temporary, short-term changes to the environment. It can also be used to display the current environment. This command is env.

env

The dollar sign can actually be used to combine the values of environment variables in many shell commands. For example, the following command can be used to list the contents of the "Desktop" directory within the current user's home directory.

ls $HOME/Desktop

Erasing environment variables

While simply setting an empty value to an environment variable, as shown in the example below, may nullify its effect in most cases, there are a few variables such as "POSIXLY_CORRECT" whose mere existence, even with an empty value, influences the behavior of programs.

export LC_ALL=

The unset command can be used in order to completely erase the existence of an environment variable:

unset LC_ALL

It is also possible to use the "-n" switch to the export command in order un-export an environment variable and therefore demote it to become a shell variable while preserving its value.

export -n LC_ALL

Working principles of environment variables

A few simple principles govern how environment variables work and achieve their effect.

Process locality

The values of environment variables are local, which means they are specific to the running process in or for which they were set. This means that if we open two terminal windows (which means we have two separate bash processes running), and change a value of an environment variable in one of the windows, that change will not be seen by the shell in the other window or any other program currently on the desktop.

Inheritance

When a parent process creates a child process, for example when we run the "gedit" command from the terminal and "bash" (the parent process) creates "gedit" (the child process), the child process inherits all the environment variables and values the parent process had. This means that if we set a new value to the "LANG" environment variable in the terminal, and then run "gedit" from that same terminal, "gedit" will inherit the new value of "LANG", and therefore may display in a different language than the rest of the processes on the desktop. Note that because of the Process Locality principle explained above, once we run Gedit, changes to environment variables of the parent process will not be seen by the child process and vice-versa. Note: in the Gnome graphical desktop environment, gnome-session is the parent process of all the processes running on the desktop. This fact (along with the Inheritance principle) is the key to our ability to powerfully influence the operation of our desktop with environment variables. The equivalent process in KDE is kde-session.

Case sensitivity

The names of environment variables are case sensitive. This means that lang is not the same variable as LANG, Lang, or laNg. It is a common practice to name all environment variables with only English capital letters and underscore (_) signs.

Bash's quick assignment and inheritance trick

The bash shell has a trick to allow us to set one or more environment variables and run a child process with single command. For example, in order to set the "LANG" and "FOO" environment variables and then run "gedit", we would use the following command:

LANG=he_IL.UTF-8 FOO=bar gedit

Note: When using this command, the new values are only assigned to the environment variables of the child process (in this case gedit). The variables of the shell retain their original values. For instance, in the example above, the value of "LANG" will not change from its original value, as far as subsequent commands to the shell are concerned. A similar behavior can be achieved with other shells by using the env command.

Persistent environment variables

So far we've only discussed ways set an environment variable value temporarily until the shell session in which it was set is closed. One may wonder if there is a way to somehow permanently set an environment variable to a certain value. Note: The shell config files mentioned below are only meant to be read by particular shells and desktop environments. GDM for example used to source /etc/profile, and but stopped this misbehaviour, which is good. See "man pam_env" for a proper solution. (Using /etc/security/pam_env.conf, /etc/environment and ~/environment.)

Session-wide environment variables

In order to set environment variables in a way that affects a particular user's environment, one should not place commands to set their values in particular shell script files in the user's home directory, but use:

• ~/.pam_environment - This file is specifically meant for setting a user's environment. It is not a script file, but rather consists of assignment expressions, one per line.

Not recommended:

• ~/.profile' - This is probably the best file for placing environment variable assignments in, since it gets executed automatically by the Display'Manager during the startup process desktop session as well as by the login shell when one logs-in from the textual console.
• ~/.bash_profile or ~./bash_login - If one of these file exist, bash executes it rather then "~/.profile" when it is started as a login shell. (Bash will prefer "~/.bash_profile" to "~/.bash_login"). However, these files won't influence a graphical session by default.
• ~/.bashrc - Because of the way Ubuntu currently sets up the various script files by default, this may be the easiest place to set variables in. The default configuration nearly guarantees that this file will be executed in each and every invocation of bash as well as while logging in to the graphical environment. However, performance-wise this may not be the best thing to do since it will cause values to be unnecessarily set many times.

System-wide environment variables

Environment variable settings that affect the system as a whole (rather then just a particular user) should not be placed in any of the many system-level scripts that get executed when the system or the desktop session are loaded, but into

• /etc/environment - This file is specifically meant for system-wide environment variable settings. It is not a script file, but rather consists of assignment expressions, one per line. Specifically, this file stores the system-wide locale and path settings.

Not recomended:

• /etc/profile' - This file gets executed whenever a bash login shell is entered (e.g. when logging in from the console or over ssh), as well well as by the Display'Manager when the desktop session loads. This is probably the file you will get referred to when asking veteran UNIX system administrators about environment variables. In Ubuntu, however, this file does little more then invoke the /etc/bash.bashrc file.
• /etc/bash.bashrc - This is is the system-wide version of the ~/.bashrc file. Ubuntu is configured by default to execute this file whenever a user enters a shell or the desktop environment.

Note: When dealing with end-user/home desktop systems may be appropriate to place settings in the user's ~/.pam_environment files discussed above rather then the system-wide ones, since those files do not require one to utilize root privileges in order to edit and are easily moved between systems.

List of common environment variables

Each application is free to define and use its own environment variables. Many manual pages include long lists of environment variables that can effect the behavior of the application they describe. However, the most useful variables are common to many applications.

File-location related variables

The following variables determine how the system locates various files in order to operate.

Variable	Value Examples	What it's for
PATH	/usr/sbin:/usr/bin:/sbin:/bin	When you type a command to run, the system looks for it in the directories specified by PATH in the order specified
MANPATH	/usr/share/man:/usr/local/man	List of directories for the system to search manual pages in
LD_LIBRARY_PATH	/opt/app/oracle/lib	List of directories where the system searches for runtime libraries in addition to those hard-defined by ld and in /etc/ld.so.conf.d/*.conf files. Note: You can only set this environment variable inside an interactive shell. Since Ubuntu 9.04 Jaunty Jackalope, LD_LIBRARY_PATH cannot be set in $HOME/.profile, /etc/profile, nor /etc/environment files. You must use /etc/ld.so.conf.d/*.conf configuration files. See Launchpad bug #366728 for more information.
TMPDIR	/var/tmp	The directory used for temporary file creation by several programs

Locale setting variables

The following environment variables determine the locale-related behavior of the systems such as the language of messages sent to the user and the way times and dates are presented. The values that can assigned to the locale environment variables are names of locale setting packages installed on the system. To see which such packages are installed on the system, one can use the locale -a command. Locale setting packages can be generated with the locale-gen command. However, Ubuntu includes pre-generated locale setting packages in the language-pack packages available within the package management system.

Variable	What it's for
LANG	The basic language setting used by applications on the system, unless overridden by one of the other locale environment variables
LC_CTYPE	The character set used to display and input text
LC_NUMERIC	How non-monetary numeric values are formatted on screen
LC_TIME	How date and time values are formatted
LC_COLLATE	How to sort various information items (e.g. defines the order of the alphabet so items can be ordered alphabetically by the sort command)
LC_MONETARY	How monetary numeric values are formatted
LC_MESSAGES	Which language is to display messages to the end user
LC_PAPER	Definitions of paper formats and standards
LC_NAME	How names are formatted
LC_ADDRESS	How to display address information
LC_TELEPHONE	How telephone numbers are structured
LC_MEASUREMENT	What units of measurement are used
LC_IDENTIFICATION
LC_ALL	This variable serves as a powerful override over all the other locale environment variables. When its value is set, applications use that value to determine which locale settings to use regardless of the values of the other variables

By utilizing various combinations of settings for the locale variables, you can make interesting tweaks to the behavior of your system. For example, you can make your system display message in US-English while using number, date, and measurement formats that are more common to European countries. The locale variables can effectively override each other's value in some combinations. Therefore examining the values of the variables themselves may not always provide clear indication of how the system will behave. The locale command can be used to examine what the effective values are to the applications.

Preferred application variables

These environment variables indicate to various programs what the user's preferred applications are for performing certain tasks. These variables are typically not respected by GUI applications that tend to include their own built-in text display windows and editors. Most desktop environments also contain their own preferred application selection system.

Variable	Value Examples	What it's for
PAGER	/usr/bin/less	The name of the utility used to display long text by commands such as man.
EDITOR	/usr/bin/nano	The name of the user's preferred text editor. Used by programs such as the mutt mail client and sudoedit.
VISUAL	/usr/bin/gedit	Similar to the "EDITOR" environment variable, applications typically try the value in this variable first before falling back to "EDITOR" if it isn't set.
BROWSER	/usr/bin/lynx	The name of the user's preferred web browser. This variable is arguably less common than the rest

Graphical desktop-related variables

Variable	Value Examples	What it's for
DISPLAY	:0.0>localhost:10.0>terminal01:0.0	This variable is used to indicate to graphical applications where to display the actual graphical user interface, the value consists of 3 parts: A host-name followed by a colon (:), a display number followed by a dot (.) and a screen number. The host-name part can be used to have the graphical output sent to a remote machine over the network. It can be omitted when the output is meant for an X server running on the local machine. The display number allows selecting among multiple X servers running on the same machine (Ubuntu uses multiple X servers to enable multiple graphical desktop sessions). Although the screen number is used to select among multiple physical screen that are managed by the same X server, it is rarely set to anything other then "0" nowadays. Manually setting the "DISPLAY" environment variable's value is rarely needed nowadays since it can be automatically and intelligently adjusted by many applications such as "GDM" and "SSH" when needed.
XDG_DATA_HOME	~/.local/share	Indicates to applications that conform to the freedesktop.org specifications, where to place the user's private data. This variable is typically unset since a sensible default fall-back value was defined by the specifications.
XDG_CONFIG_HOME	~/.local/share	Indicates to applications that conform to the freedesktop.org specifications, where to place the user's configuration information. This variable is typically unset since a sensible default fall-back value was defined by the specifications.
XDG_DATA_DIRS	/usr/local/share:/usr/share	A colon-separated list (similar to "PATH") of directories where data is searched for by applications that conform to the freedesktop.org specifications. This variable is typically unset since a sensible default fall-back value was defined by the specifications.
XDG_CONFIG_DIRS	/etc/xdg	A colon-separated list (similar to "PATH") of directories where configuration information is searched for by applications that conform to the freedesktop.org specifications. This variable is typically unset since a sensible default fall-back value was defined by the specifications.
XDG_CACHE_HOME	~/.cache	A location used by applications that conform to the freedesktop.org specifications to cache temporary data. This variable is typically unset since a sensible default fall-back value was defined by the specifications.

Gnome-specific variables

Variable	Value Examples	What it's for
NAUTILUS_SCRIPT_SELECTED_FILE_PATHS	/home/ifireball/about.html	This environment variable is set by Nautilus, the Gnome file manager, to a newline-delimited list of the currently selected files, when a script is invoked from the right-click menu. This variable is only set if the files are local, e.g. not from a network share or an SSH connection
NAUTILUS_SCRIPT_SELECTED_URIS	file:///home/ifireball/about.html	This environment variable is set by Nautilus to a newline-delimited list of the URI addresses of the currently selected files, when a script is invoked from the right-click menu.
NAUTILUS_SCRIPT_CURRENT_URI	file:///home/ifireball	This environment variable is set to the URI address of the location currently displayed by the Nautilus window, when a script is invoked from the right-click menu.
NAUTILUS_SCRIPT_WINDOW_GEOMETRY	828x511+251+342	This environment variable is set to the on-screen position of the Nautilus window, when a script is invoked from the right-click menu.

Program execution variables

Arguably the most powerful (but dangerous) environment variables, the following allow tweaking the basic way software actually runs.

Variable	Value Examples	What it's for
LD_PRELOAD	/usr/lib/valgrind.so	This variable can be used to inject a custom dynamic library into an application's memory when it loads. It can be used to do things like replacing the application's built-in memory allocation library with a debugging version in order to detect memory leaks. It can also be used to override the way it does various things like play sounds.

Compilation and software development related variables

Variable	Value Examples	What it's for
CC	gcc	The name of the C compiler to use
CFLAGS	-o out.o	A list of debugging/optimization flags to pass to the C compiler
CXXFLAGS	-Wall	A list of debugging/optimization flags to pass to the C++ compiler
CPPFLAGS	-DDEBUG	A list of flags to pass to the C/C++ pre-processor/compiler
LIBRARY_PATH	/usr/lib/firefox	A list of directories (separated by colons) in which library files should be searched for
INCLUDE	/opt/app/src/include	A colon-separated list of directories in which header files should be searched for
CPATH	..:$HOME/include:/usr/local/include	A colon-separated list of directories in which header files should be searched for

Other environment variables

Variable	Value Examples	What it's for
USER	myuser1	The name of the currently logged-in user. This variable is set by the system. You probably shouldn't change its value manually.
LOGNAME	myuser1	Similar to "USER", some programs prefer to read this variable rather than USER.
HOME	/home/myuser1	The location of the currently logged-in user's home directory.
PWD	/home/myuser1/Desktop	The current working directory of the shell.
SHELL	/bin/bash	The user's preferred command-line shell as it is set in the /etc/passwd file.
POSIXLY_CORRECT	N/A	If this environment variable exists, regardless of its value, it causes the behavior of quite a few utilities to more closely match the behavior defined by the POSIX standard. This typically makes various GNU extensions that make life easier not work, but it may just be what's needed to make an old UNIX script execute successfully.
HOSTALIASES	/etc/host.aliases	The name of a file containing host-name aliases for use with various network programs.
TZDIR	/usr/share/zoneinfo	The path of the directory containing time-zone information files. This typically does not need to be set manually, as the system searches for such files in /usr/share/zoneinfo by default.
TZ	IST-2>:Japan	This variable was used by older UNIX systems to specify the system's time-zone. However, Ubuntu and most other modern systems use the /etc/localtime file for that purpose. This variable can, however, be used to make one user's particular session display times in a different timezone than the rest of the system.>The value of this variable can either be the name and offset of a time-zone (as seen in the first example) or the name of a zone-info file in /usr/share/zoneinfo (as seen in the second example).
TERM	xterm	The name of a terminal information file from /lib/terminfo, this file instructs terminal programs how to achieve things such as displaying color. It may help to fiddle with this variable if you're trying use an odd terminal emulator program or trying to connect a hardware serial-port terminal emulator and getting undesired results.
TERMCAP		This variable can be used instead of "TERM" to manually specify terminal information rather than point to a file.
COLUMNS	80	The number of text columns in the terminal window. Try adjusting this variable if lines don't seem to wrap properly in the console.
LINES	25	The number of text lines on the console window. Try adjusting this variable if you're getting strange results when scrolling text.