DCL provides all the information you need to determine what qualifiers and parameters were present on the command line used to invoke your program, the values of those parameters and/or qualifiers, even the verb that was used (some programs - like COPY - accept more than one verb (APPEND, COPY)).
Need some examples?
CREMBX.BAS illustrates how to get the value portion of a "/qualifier=value" expression as stated on the command line (CLI$GET_VALUE).
CREATE55.CLD shows how the VMS V5.5 CREATE.CLD was modified for the /MAILBOX qualifier and the related additional qualifiers. Look for "define verb create", then scan down to where you find "qualifier mailbox, syntax=mbx". Then go back to the top of the file and look for "define syntax MBX".
The "image" keyword indicates which image will be invoked for each variant of the command. Unless an alternate syntax modifies the "image", CREATE will be run. CREATE/MAILBOX invokes the CREMBX image.
To "install" a new (or modified) command, use the SET COMMAND statement in DCL:
$ SET COMMAND filespec
The command will become part of your process's command table. To write the resulting command table to a file, use the /OUTPUT qualifier with an appropriate filespec.:
$ SET COMMAND filespec/OUTPUT=filespec
You can even use this form of the command to modify the system-wide DCLTABLES.EXE. However, it is more advisable to prepare your own command table image, and set that in the UAF records for those users who need the modified command table. Remember that DCLTABLES.EXE, if modified, must be /REPLACEd by the System Manager using the INSTALL command.
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