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.