Variables

Customising your Unix environment ($filesize Bytes

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Standard Unix variables are split into two categories, environment and shell variables. In broad terms, shell variables apply only to the current instance of the shell and are used to set short-term working conditions; environment variables have a farther reaching significance, and those set at login are valid for the duration of the session. By convention, environment variables have UPPER CASE and shell variable have lower case names.

Users may create their own variables in order to customise their environment but it is necessary to avoid clashes with the names of the standard variables.

Variables may be set in the .login or .cshrc initialisation files and may be subsequently referred to by prefixing their name with $, e.g. $EDITOR.

ENVIRONMENT variables are set using the setenv command, displayed using the printenv or env commands, and unset using the unsetenv command. Some commonly used environment variables are:

Name	Description
EDITOR	the editor that will be used by other programs such as the mail program.
EXINIT	customisation options for the ex and vi text editors.
HOME	the path name of your home directory.
LOGNAME	your login name
LPDEST	default destination for print jobs submitted via lp.
MAIL	the location of your mailbox for incoming email.
MANPATH	the list of directories to search for manual pages
PAGER	Sets the pager that will be used by programs such as man to display man pages.
PATH	Specifies the directories that the shell is to look through to find a command. These directories are searched in the order in which they appear.
PRINTER	default destination for print jobs submitted via lpr.
SHELL	your login shell.
SYS_TYPE	the type of system you are using, e.g. Sun, SGI
TERM	the terminal type for programs such as the editor and pager.
TZ	the time zone.
USER	your login name

Shell variables are both set and displayed using the set command. They can be unset by using the unset command. Shell variables set automatically on ISS Unix systems are:

argv	of significance to shell programmers
cwd	your current working directory
home	the path name of your home directory
path	the list of directories to search for programs
prompt	the text string used to prompt for interactive commands
shell	your login shell
status	a code indicating the degree of success of the previous command
term	the type of 'terminal' you are using

In general, environment and shell variables that have the same name (apart from the case) are distinct and independent, except for possibly having the same initial values. There are, however, exceptions.

Each time the shell variables home, user and term are changed, the corresponding environment variables HOME, USER and TERM receive the same values. However, altering the environment variables has no effect on the corresponding shell variables.
path and PATH specify directories to search for commands and programs. Both variables always represent the same directory list, and altering either automatically causes the other to be changed.