Great, yes doing it at the OS level is a creative approach and I can definitely see advantages to that. I would be worried about having to parse zsh and fish as well as bash, although I suppose coming up with something that works most of the time is feasible.

As mentioned elsewhere I wanted to have some invariants for correctness, but maybe not all of them were necessary. I felt it was useful to separate the problem into completing the shell language vs. completing the argv.

Feel free to join the Oil's Zulip channel (link on home page https://www.oilshell.org/)! There are the past discussions on #shell-autocompletion, going back a couple years. It's dormant now, but as mentioned, there were multiple people who wrote code towards this. And there were debates around the issues above.

Another interesting channel that just started is #shell-gui. Short summary which I have yet to blog about: I just implemented "headless mode" for Oil (analogy to headless Chrome). So I can punt the UI for the shell to multiple other projects :) As I mentioned a few times, I realized the scope of the project is too big. I have a collaborator Subhav who has a prototype of a GUI in Go (and shell).

Basically the shell a language- and text-oriented interface, but it does NOT have to be terminal-oriented interface. It took me awhile to realize that we shouldn't conflate those things! And it was pretty easy to tease them apart in Oil. The headless mode provides a simple interface and allows integration that can't be done with bash (or any other shell AFAIK).

It needs feedback from people who want to build GUIs. An easy analogy is to imagine is a browser-like GUI for a shell with the URL bar as the prompt. The URL bar provides autocompletion, history, and shows state, etc. just like shell does.