environment variable


environment variable

(programming, operating system)A variable that is bound inthe current environment. When evaluating an expression insome environment, the evaluation of a variable consists oflooking up its name in the environment and substituting itsvalue.

Most programming languages have some concept of an environmentbut in Unix shell scripts it has a specific meaningslightly different from other contexts. In shell scripts,environment variables are one kind of shell variable. Theydiffer from local variables and command line arguments inthat they are inheritted by a child process. Examples arethe PATH variable that tells the shell the file systempaths to search to find command executables and the TZvariable which contains the local time zone. The variablecalled "SHELL" specifies the type of shell being used.

These variables are used by commands or shell scripts todiscover things about the environment they are operating in.Environment variables can be changed or created by the useror a program.

To see a list of environment variables type "setenv" at thecsh or tcsh prompt or "set" at the sh, bash, jshor ksh prompt.

In other programming languages, e.g. functional programminglanguages, the environment is extended with new bindings whena function's parameters are bound to its actual arguments or when new variables are declared. In ablock-structured procedural language, the environmentusually consists of a linked list of activation records.

environment variable

An item of memory-resident data that provides a mechanism for users, applications and the operating system to interact with each other. Set up in memory by the operating system each time it is booted, the variables hold the current environment configuration, such as computer and login name, CPU type and the names of the system, application, home and temporary folders.

Command Line Assistance
A common environment variable is the Path, which holds any number of folder hierarchies that the OS can search when a program is run at the command line so that the full path to the executable does not have to be explicitly entered each time. See path environment variable and path.