The environment variables of the command interpreter process are inherited by the processes of any (external) commands that it executes. A few environment variables are used by the command interpreter itself. Changing them changes its operation.

Environment variables are affected by the SET, PATH, and PROMPT commands.

To unset a variable, set it to empty string, such as set myvar=.

The command interpreter inherits its initial set of environment variables from the process that created it. In the case of command interpreters invoked from desktop shortcuts this will be Windows Explorer, for example.

Command interpreters generally have textual user interfaces, not graphical ones, and so do not recognize the Windows message that informs applications that the environment variable template in the Registry has been changed. Changing the environment variables in Control Panel will cause Windows Explorer to update its own environment variables from the template in the Registry, and thus change the environment variables that any subsequently invoked command interpreters will inherit. However, it will not cause command interpreters that are already running to update their environment variables from the template in the Registry.

COMSPEC

The COMSPEC environment variable contains the full path name of the command interpreter program file. This is just inherited from the parent process, and is thus indirectly derived from the setting of COMSPEC in the environment variable template in the Registry.

PATH

The value of the PATH environment variable comprises a list of directory names, separated by semi-colon characters. This is the list of directories that are searched, in order, when locating the program file of an external command to execute.

PATHEXT

The value of the PATHEXT environment variable comprises a list of filename extensions, separated by semi-colon characters. This is the list of filename extensions that are applied, in order, when locating the program file of an external command to execute.

An example content of PATHEXT printed by echo %PATHEXT%:

• .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC

By adding ".PL" to the variable, you can ensure Perl programs get run from the command line even when typed without the ".pl" extension. Thus, instead of typing "mydiff.pl a.txt b.txt", you can type "mydiff a.txt b.txt".

Adding ".PL" to the variable in Windows Vista and later:

• setx PATHEXT %PATHEXT%;.PL
  • If you use "set" available in Windows XP, the effect will be temporary and impacting only the current console or process.

Links:

• Windows Environment Variables at ss64
• Making Python scripts run on Windows without specifying “.py” extension at stackoverflow

PROMPT