[Per_Bothner]

Completion for program P should be written and maintained by the "owner" of program P - and installed with program P. This is of course difficult when there are many different "shells" that each have their own "language" for specifying completions. A multi-shell completion library can help with this problem.

To me it make sense that completion for program P should be handled by program P itself. That way, completions are unlikely to get out of sync with the application, and the completion handler can use the same option parser as the application. A way to do this is to use a special "hidden" switch to request completion.

Specifically the DomTerm terminal emulator (https://domterm.org) handles its own completions. Bash allows you to register a command that handles completions for some other command. The following tells bash that to handle completions for the domterm command it should call domterm with the magic "#complete-for-bash" option followed by the existing line and position.

    complete -o nospace -C 'domterm "#complete-for-bash" "$COMP_LINE" "$COMP_POINT"' domterm

When domterm is run with "#complete-for-bash" as the first argument (chosen to avoids accidental use) it processes the remaining arguments, and prints out the completion suggestions to bash.

I don't know how to do something similar for other shells; suggestions welcome. Some other shells (such as Fish) include help text as part of the suggested completions. Handling that would require a more complex protocol. Perhaps one that returs a result in JSON format?

If a such a protocol were standardized (perhaps invoked by a "#completions-as-json" option?), we just need to solve a final piece of the puzzle: How does a shell know if a command understands the "#completions-as-json" option? My suggestion is that we define a new keyword-value pair in the "desktop" file specification; the shell would search for a P.desktop file,