Persistent environment variables（环境变量的设置）

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. The way to achieve that is to place a command setting on an environment variable value in one of the script files that get executed automatically in one of the stages of the system and user's session startup processes. Along with the inheritance principle, this ensures the value is seen by applications loaded at later stages.

Session-wide environment variables

In order to set environment variables in a way that effects a user's entire desktop session, one may place commands to set their values in one of the "hidden" script files in the user's home directory. The more common such files are outlined below.

• ~/.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 effect the system as a whole (rather then just a particular user's desktop session) can be placed in any of the many system-level scripts that get executed when the system or the desktop session are loaded. Ubuntu defines several locations dedicated to placing such settings:

• /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.
• /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.

Note: When dealing with end-user/home desktop systems it is probably best to place settings in the session-wide files discussed above rather then the system-wide ones since those files do not require one to utilize root privileges in order to edit.

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 behaviour 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 in ld and in /etc/ld.so.conf
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 systems. 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 behaviour 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 my 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.