Allow setting prefix to remove before completing directories?

[yedayak]

Hello, I'm trying to write custom completions for skopeo, and I ran into something that I don't know if it's possible to do using the builtin cobra completions.

I'm trying to make this complete directories:

skopeo copy dir:<TAB>

I tried returning nil, cobra.ShellCompDirectiveFilterDirs | cobra.ShellCompDirectiveNoFileComp, but this doesn't complete anything. It seems to be because _filedir -d looks at $cur when completing (when it eventually calls compgen here), and since cur is dir: it doesn't find any completions. I manually edited the completion file and it did manage to complete directories like I wanted:

            cur=${cur#"dir:"}
            _filedir -d

Is there some way to solve this issue with cobra?
The PR I created for skopeo: containers/skopeo#2524

I only tested this with bash
Bash version: 5.1.16(1)-release

[marckhouzam]

@yedayak thanks for diagnosing the situation, not a lot of people dive into completion logic so deep 👍

However, remember that cobra (and surely skopeo by transitivity) support multiple shells, not just bash, so we need a generic solution.

What I recommend is that you provide the list of directories yourself in your ValidArgsFunction, and prefix each one with “dir:”.

It is better to use the shell when you can, but in this case since you cannot, this is a valid alternative.