fish tab completion based on file extension

I have a command foo that can only run *.bar files. I would like to implement tab completion for foo such that only *.bar files are listed as options. In bash, this would look like:

complete -f -X '!*.bar' foo

Is it possible to do this in fish?

Completing based on file extension is an attractive nuisance. You think you want it, but most of the time it is wrong. Most programs that typically use files with a specific extension will also happily work with others, and if they don't that's a weird restriction that could just as well be removed.

And if you restrict your completions to a specific extension and then need files without that's awkward to use.

Fish ships a helper function called __fish_complete_suffix that instead sorts the wanted extensions first. You would use it like this:

complete -c patch -s i -l input -x -k -a "(__fish_complete_suffix .patch .diff)"

This tells fish to complete the command patch with a -i or --input option which requires an argument but not via fish's builtin file completion (that's the -x flag), and keeps whatever order the completions were generated in (-k/--keep-order). To generate the completions, it runs __fish_complete_suffix .patch .diff, which sorts .patch and .diff files first.

You could adapt this to your command with:

complete -c foo -x -k -a "( __fish_complete_suffix .bar)"

If you really are sure that your command needs the file to be called ".bar", you can also generate this yourself with something like

complete -c foo -f -a '(complete -C"\'\' $(commandline -ct)" | string match -re "(?:/|.bar)$")'

This tells fish to disable the builtin file completions (-f), and then matches files (by running the file completions by hand by asking it to complete the command '') ending in ".bar" or "/", so you get directories that might contain ".bar" files. You could also try to match .bar files everywhere, but that's prohibitively expensive typically - imagine if you try to run this in the filesystem root ($PWD of "/").