[TUHS] Systematic approach to command-line interfaces

[Dan Cross]

On Sun, Aug 1, 2021 at 1:44 PM Chet Ramey <chet.ramey at case.edu> wrote:

> On 7/31/21 12:19 PM, Dan Cross wrote:
> > There was someone posting here on TUHS a while back about leveraging a
> > special context-sensitive `--shell-help` or similar command line program
> > and synthesizing a protocol between the shell and a program to provide
> > TOPS-20 like command completion. It was nowhere near what you get from
> the
> > COMND JSYS, but seemed like a reasonable approximation.
>
> This is essentially how the existing shells do it (bash, zsh, tcsh, etc.),
> but in an ad-hoc fashion. There is no standard way to obtain possible
> completions or list possible arguments, so the shells push that to external
> generators.
>
> Since you have to perform the completion in the shell, there has to be some
> way to tell the shell the possible completions for each command of
> interest, whether that's options or arguments. The different shells have
> solved that in essentially the same way, with a few syntactic variations.
>
> Bash provides a framework (complete/compgen/compctl) and pushes a lot of
> the command-specific work to external completers. It provides access to the
> shell internals (lists of builtins, functions, aliases, variables, and so
> on) and built-in ways to perform common completions (filenames, directory
> names, command names, etc.), and leaves the rest to external commands or
> shell functions.
>
> The real power and flexibility comes from being able to invoke these
> external commands or shell functions to generate lists of possible
> completions, and defining an API between the shell and those generators to
> specify enough of the command line to make it easy to find the word to be
> completed, the command for which completion is being attempted, and
> clarifying context around that word. In the same way, the shell provides an
> API for those generators to return possible completions.
>
> The knowledge about each command's options and arguments is embedded in
> these generators.
>
> A standard way to handle command line options and arguments would make
> generators easier to write, but doesn't address the other issues of what,
> exactly, the user wants to complete, so the existing mechanisms would
> likely not change very much. Something like `--shell-help', as long as it
> were context-sensitive, would help more.


Thanks for the useful background information on existing solutions.

If I understood the proposal correctly, it was that the program in question
would, itself, be the generator as described above. Perhaps it was coupled
with a standard structured format for consumption by the shell, which seems
like it would be useful for this sort of expansion.

Of course, the process model in TOPS-20 was very different than in Unix,
and in that system, as soon as you typed the _name_ of a command it's image
was "run up" in your process. So the interactive help system was provided
by a running instance of the program itself. What I gathered from the
proposed model was that it involved multiple invocations of the program,
but with a special option that would trigger behavior informally described
as, "here's the context I've built so far; let me know what options are
available here." I don't know that it's terribly "Unixy", but I can see how
it would be useful for interactive use.

As an aside, I maintain some older "machines" at home (even modest hardware
can emulate a PDP-10 or Honeywell DPS8), and find that doing so provides me
with perspective that can be very useful. Looking at other systems that
were available roughly around the time of Unix (TENEX, Multics), it strikes
me that the Unix was a bit of an odd-duck with the way it handled exec in
terms of destructively overlaying the memory of the user portion of a
process with a new image; am I wrong here? I wonder why the "one program
per process and exec destroys what was running before" mechanism was
implemented? I can imagine it had a lot to do with the constraints that
early Unix machines must have imposed on design, not to mention
implementation simplicity, but I wonder what the designers thought of other
systems' process models and whether they were considered at all? Perhaps
Doug and Ken might have thoughts here?

        - Dan C.
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