The Environment variable reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Environment variable

Environment variables are a set of dynamic values that can affect the way running processes will behave.

In most Unix shells, each process has its own private set of environment variables that most likely were copies of the environment variables of the parent process. All Unix operating system flavors and MS-DOS have environment variables, however they do not all use the same variable names. Examples of environment variables include

1. the PATH, which tells the shell where to search for commands the user may type.
2. HOME (Unix-like) and userprofile (MS Windows) indicate where a user's home directory is located in the file system.

Under Unix-like systems, they are also used to pass configuration information to running programs, e.g.

1. CVS_RSH is used to tell CVS which rsh-like program to use.
2. MAIL is used to indicate where a user's mail is to be found.

Shell scripts and batch files use environment variables to store temporary values for reference later in the script, and also to communicate data and preferences to child processes.

Usually an environment variable that is changed in a script or compiled program will only affect that process and possibly child processes. The parent process and any unrelated processes will not be affected.

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