The syntax for creating an executable extension is:

        set .ext=command [options]

This tells CMD.EXE to run the specified command whenever you name a file with the extension .ext at the prompt.

.EXT is the executable file extension; command is the name of the internal command, external program, alias, or batch file to run; and [options] are any command-line startup options you want to specify for the program, batch file, or alias.

Normally, when you type a filename (as opposed to an alias or internal command name) as the first word on the command line, CMD.EXE looks for a file with that name to execute. The file's extension may be .EXE or .COM to indicate that it contains a program, it may have a batch file extension like .BTM, or the file's contents may indicate that it is executable. (To change the default list of extensions see the PATHEXT environment variable and the PathExt setting in CMD.INI.)

You can add to the default list of extensions, and have CMD.EXE take the action you want with files that are not executable programs or batch files. The action taken is always based on the file's extension. For example, you could start your text editor whenever you type the name of a .DOC file, or start your database manager whenever you type the name of a .DAT file.

You can use environment variables to define the internal command, external program, batch file, or alias to run for each defined file extension. To create an executable extension, use the SET command to create a new environment variable. An environment variable is recognized as an executable extension if its name begins with a period.

For example, if you want to run a word processor called EDITOR whenever you type the name of a file that has an extension of .EDT, you could use this command:

        [c:\] set .edt=c:\edit\editor.exe

If the command specified in an executable extension is a batch file or external program, CMD.EXE will search the PATH for it if necessary. However, you can make sure that the correct program or batch file is used, and speed up the executable extension, by specifying the full name including drive, path, filename, and extension.

Once an executable extension is defined, any time you name a file with that extension the corresponding program, batch file, or alias is started, with the name of your file passed to it as a parameter.

The following example defines B.EXE (the Boxer text editor) as the processor for .MAK files:

        [c:\] set .mak=c:\boxer\b.exe -s

With this definition, if you have a file named INIT.MAK in the current directory and enter the command:

        [c:\source] init

CMD.EXE will execute the command:

        c:\boxer\b.exe -s c:\source\init.mak

Notice that the full pathname of INIT.MAK is automatically included. If you enter parameters on the command line, they are appended to the end of the command. For example, if you changed the above entry to:

        [c:\source] init -w

the command processor would execute the command:

        c:\boxer\b.exe -s c:\source\init.mak -w

In order for executable extensions to work, the command, program, batch file, or alias must be able to interpret the command line properly. For example, if a program you want to run doesn't accept a file name on its command line as shown in these examples, then executable extensions won't work with that program.

Executable extensions may include wildcards, so you could, for example, run your text editor for any file with an extension beginning with T by defining an executable extension called .T*. Extended wildcards (e.g., DO[CT] for .DOC and .DOT files) may also be used.

The search for executable files starts in the current directory, then proceeds to each subdirectory specified by the PATH environment variable (if a “.” is used in the PATH the current directory is not searched first; see the PATH command for details).

You may need to take this search order into account when using executable extensions. Using the .MAK example above, if you had a file named FORMAT.MAK in the current directory and entered the command FORMAT A:, your command would run the Boxer editor specified by the executable extension, instead of finding the standard OS/2 FORMAT command as you perhaps intended. You can get around this by remembering that the OS/2 FORMAT command is in the file FORMAT.COM. If you entered the command FORMAT.COM A: then the .MAK executable extension would not be checked, and the search would continue until it found the FORMAT.COM file.

To remove an executable extension, use the UNSET command to remove the corresponding variable.