environment variable

programming, operating system A variable that is bound in

the current environment. When evaluating an expression in

some environment, the evaluation of a variable consists of

looking up its name in the environment and substituting its

value.

Most programming languages have some concept of an environment

but in Unix shell scripts it has a specific meaning

slightly different from other contexts. In shell scripts,

environment variables are one kind of shell variable. They

differ from local variables and command line arguments in

that they are inheritted by a child process. Examples are

the PATH variable that tells the shell the file system

paths to search to find command executables and the TZ

variable which contains the local time zone. The variable

called "SHELL" specifies the type of shell being used.

These variables are used by commands or shell scripts to

discover things about the environment they are operating in.

Environment variables can be changed or created by the user

or a program.

To see a list of environment variables type "setenv" at the

csh or tcsh prompt or "set" at the sh, bash, jsh

or ksh prompt.

In other programming languages, e.g. functional programming

languages, the environment is extended with new bindings when

a function's parameters are bound to its actualarguments or when new variables are declared. In a

block-structured procedural language, the environment

usually consists of a linked list of activation records.

(1999-01-26)