6.3 Predefined Environment Variables

We've said that environment variables are used to store information that you'd rather not worry about, and that there are a number of standard environment variables that many UNIX programs use. These are often called "predefined" environment variables - not because their values are predefined, but because their names and uses are predefined. Here are the most important ones:

• PATH (6.4) contains your command search path (8.7). This is a list of directories in which the shell looks to find commands. It's usually set in one of your shell setup files (2.2).

• EDITOR can be loaded with the name of your favorite editor. It's usually set in one of your shell setup files. Some programs distinguish between EDITOR (usually set to a line editor (33.1) such as ed) and VISUAL (set to a full-screen editor like vi). Many people don't follow that convention; they set both to the same editor. (The Korn shell checks VISUAL and EDITOR, in that order, to determine your command editing mode (11.13).)

• PRINTER (43.4) can be loaded with the name of your default printer. It's quite useful at a site with many printers - you don't need to tell lpr (43.2) which printer to use. This variable is usually set in one of your shell setup files.

• PWD contains the absolute pathname of your current directory. It's set automatically by the cd command in some UNIX shells. PWD may be fooled (14.13) by cding through symbolic links.

• HOME (14.11) (called LOGDIR on some systems) contains the absolute pathname of your home directory. It's set automatically when you log in.

• SHELL contains the absolute pathname of your login shell. It's set automatically whenever you log in.

• USER or LOGNAME contains your username. It's set automatically when you log in, and doesn't change.

• TERM (5.10) contains the name of your terminal type in the termcap or terminfo database. It's usually set in a shell setup file.

• TERMCAP (5.4) can be loaded with the complete termcap database entry for the terminal you are using. This may make some programs start up more quickly, but it's not necessary. It's set (under some conditions) by the tset command, which is usually run in your shell setup file.

• ENV contains the name of an initialization file to be executed whenever a new Korn shell is started. (See article 2.2.) Korn shell only.

• PAGER can be set to the name of your favorite page-by-page screen display program like more (25.3) or less (25.4). (Programs like man (50.1) use PAGER to determine which paging program to use if their output is longer than a single screen.)

• EXINIT (30.35, 6.10) stores setup options for the vi editor (and the ex editor, where EXINIT got its name).

• PS1 contains the primary prompt (i.e., interactive command prompt) for Bourne shells. (The C shell doesn't store the prompt in an environment variable. It uses a shell variable called prompt because the .cshrc file (2.2) is read to set up each instance of the shell. See article 7.2.)

• PS2 (9.13) contains the secondary prompt (used within compound commands like while and for) for Bourne shells.

• MANPATH (50.10), if your man (50.1) command supports it, is a colon-separated list of directories to search for manual pages.

• TZ (6.6) contains the time zone. This is a name of a file in /usr/lib/zoneinfo that provides time zone information for your locality. It is read by commands like date (51.10, 6.7).

• DISPLAY is used by the X Window System (1.31) to identify the display server (keyboard and screen handling program) that will be used for input and output by X applications.

Because Bourne-type shells don't make as strict a distinction between environment variables and shell variables as the C shell does, we've included a few things here that might not be on other people's lists.

We may have implied that environment variables are relatively constant (like your favorite editor). That's not true. For example, in a windowing environment, the current length of your window might be kept in an environment variable. That can change as often as you resize your window. What is true (fortunately) is exactly what we've said: environment variables store information that you'd rather not have to worry about.