shell - Accessing bash completions for specific commands programmatically

Question

I'm trying to write a small command launcher application, and would like to use bash's tab completions in my own completion system. I've been able to get a list of completions for general commands using compgen -abck.
However, I would also like to get completions for specific commands: for instance, the input git p should display completion for git's commands.

Is there any way I can use compgen to do this? If not, are there any other ways I can get a list of completions programmatically?

[EDIT: To clarify, I'm not trying to provide completion to bash - my app is a GUI command launcher. I'd simply like to use bash's existing completions in my own app.]

Answer 1

I don't really know how it works, but the awesome window manager uses the following Lua code for getting access to bash completion's result:

https://github.com/awesomeWM/awesome/blob/master/lib/awful/completion.lua#L119

1. Via complete -p we find complete -o bashdefault -o default -o nospace -F _git git. We remember "_git" for later.
2. The length of "git l" is 5, so we set COMP_COUNT=6. We are completing the first argument to "git", so COMP_CWORD=1.

All together we use the following script:

__print_completions() {
    printf '%s
' "${COMPREPLY[@]}"
}

# load bash-completion functions
source /etc/bash_completion

# load git's completion function
_completion_loader git

COMP_WORDS=(git l)
COMP_LINE='git l'
COMP_POINT=6
COMP_CWORD=1
_git
__print_completions

Output: "log"