There are a number of settings that the shell requires to control the way your system behaves. Settings such as the location of your mailbox and the type of terminal you have are all set using environment variables. The Linux operating system has several default environment variables such as PS1, HOME, LOGNAME, SHELL, and PATH.

When a predefined environment variable is placed in one of the initialization files, it is recognized globally in any shell or subshell.

You can also temporarily change your environment variables at the command line. This affects only the current shell. When you exit the shell in which the environment variable has been assigned, it is released or set back to its default value. However, permanent changes can be made by modifying the initialization files.

Note: The Bourne and Bash shells use the same format for setting environment variables.