Environment Variable - Synopsis

Synopsis

In all Unix and Unix-like systems, each process has its own separate set of environment variables. By default, when a process is created, it inherits a duplicate environment of its parent process, except for explicit changes made by the parent when it creates the child. At API level, these changes must be done between running fork and exec. Alternatively, from command shells such as bash, a user can change environment variables for a particular command invocation by indirectly invoking it via env or using the ENVIRONMENT_VARIABLE=VALUE notation. All Unix operating system flavors, MS-DOS, and Microsoft Windows have environment variables; however, they do not all use the same variable names. A running program can access the values of environment variables for configuration purposes.

Examples of environment variables include:

• PATH - A list of directory paths. When the user types a command without providing the full path, this list is checked if it contains a path that leads to the command.
• HOME (Unix-like) and USERPROFILE (Microsoft Windows) - indicate where a user's home directory is located in the file system.
• HOME/{.AppName} (Unix-like) and APPDATA\{DeveloperName\AppName} (Microsoft Windows) - for storing application settings. Many open source programs incorrectly use USERPROFILE for application settings in Windows - USERPROFILE should only be used in dialogs that allow user to choose between paths like Documents/Pictures/Downloads/Music, for programmatic purposes APPDATA (roaming), LOCALAPPDATA or PROGRAMDATA (shared between users) is used.
• TERM (Unix-like) - specifies the type of computer terminal or terminal emulator being used (e.g., vt100 or dumb).
• PS1 (Unix-like) - specifies how the prompt is displayed in the Bourne shell and variants.
• MAIL (Unix-like) - used to indicate where a user's mail is to be found.
• TEMP - location where processes can store temporary files

Shell scripts and batch files use environment variables to communicate data and preferences to child processes. They can also be used to store temporary values for reference later in a shell script. However, in Unix, other variables are usually used for this.

In Unix, 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. In MS-DOS, changing or removing a variable's value inside a BATCH file will change the variable for the duration of command.com's existence.

In Unix, the environment variables are normally initialized during system startup by the system init scripts, and hence inherited by all other processes in the system. Users can, and often do, augment them in the profile script for the command shell they are using. In Microsoft Windows, each environment variable's default value is stored in the Windows registry or set in the autoexec.bat file.