[TUHS] Systematic approach to command-line interfaces

Paul Winalski paul.winalski at gmail.com
Sun Aug 1 01:56:51 AEST 2021

On 7/31/21, Michael Siegel <msi at malbolge.net> wrote:
> While doing that, I learned that there is a better way to approach
> this problem – beyond using getopt(s) (which never really made sense to
> me) and having to write case statements in loops every time: Define a
> grammar, let a pre-built parser do the work, and have the parser
> provide the results to the program.

This method for handling command lines dates back at least to the
1970s.  The COMND JSYS (system call) in TOPS-20 operated this way, as
does the DCL command line interface in OpenVMS.  As you pointed out it
can greatly simplify the code in the application.  It also permits
command completion.  If the command has a long-winded option, such as
-supercalifragilisticexpialidocious, I can type -super then hit the
TAB key and as long as there is only one option that starts with
-super the parser will fill in the rest of the long keyword.  It also
means that you can provide interactive help.  At any point the user
can type a question mark and the command interpreter will say what
syntactic element is expected next.  The TOPS-20 COMND JSYS
implemented both of these features, and I think that command
completion was eventually added to the VMS command interpreter, too.

This method of command line parsing also enforces a degree of
uniformity of syntax between the command lines of the various
utilities and applications.

-Paul W.

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