[SOLVED] Bash completion scripting - getting a "transparent proxy"-like behaviour

Issue

I am trying to write a simple Bash completion script for a program that runs its arguments as a command. A good example of this is kind of program is the prime-run script provided by the nvidia-prime package:

#!/bin/bash
__NV_PRIME_RENDER_OFFLOAD=1 __VK_LAYER_NV_optimus=NVIDIA_only __GLX_VENDOR_LIBRARY_NAME=nvidia "$@"

This script sets a few environment variables, which instructs the prime driver to use the Nvidia dGPU on a hybrid system. The first argument is treated as the command, and all trailing arguments are passed through. So for example you can run prime-run code . and VSCode will start in the current directory using the dGPU.

Therefore from a completion-script POV, what we want is to basically try to complete as if the prime-run token isn't there (hence "transparent proxy"-like behaviour). To give a rather contrived example:

> prime-run journalc<TAB>
(completes journalctl)
> prime-run journalctl --us<TAB>
(completes --user)

However I am finding this surprisingly difficult in Bash (not that I know how in other shells). So the question is simple: is it possible and if so how?

Ideas I've (hopelessly) had

• The simple complete -A command prime-run: the first argument gets completed as a command as expected (let's call it foo), but the following arguments are also completed as commands rather than as arguments to foo
• Use some combination of compgen and complete -p to invoke the completion function of foo, but AFAIK the completion function for all foo is locally defined and thus uncallable

Solution

TL;DR

bash-completion provides a function named _command_offset (permalink), which is exactly what I need.

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition.

Keep reading if you are interested in how I got here.

---

So I was daydreaming the other day, when it hit me - doesn't sudo basically have the exact same behaviour I want? So the task became simple - reverse engineer the completion script for sudo. Source available here: permalink.

Turns out, most of the code has to do with completing the various options, so it's safe to simply throw most of it out:

• L 8-11, 50-52: Related to sudo's edit mode. Safe to ditch.
• L 19-24, 27-39, 43-49: These complete sudo's options. Safe to ditch.

So we're left with this:

_sudo()
{
    local cur prev words cword split
    _init_completion -s || return

    for ((i = 1; i <= cword; i++)); do
        if [[ ${words[i]} != -* ]]; then
            local PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
            local root_command=${words[i]}
            _command_offset $i
            return
        fi
    done

    $split && return
} &&
    complete -F _sudo sudo sudoedit

• The for and if block are there to deal with sudo's options that precede the "guest command". Safe to ditch (after replacing all $i with 1).
• The variable $split is only referenced in _init_completion (permalink), and it seems to be used for handling different argument styles (--foo=bar v.s. --foo bar). Same with the -s flag. Irrelevant.
• Appending to $PATH and setting $root_command have to do with privilege escalation. Only relevant to sudo.

So after the dust has cleared, by process of elimination, I ended up with this simple chunk of code:

_my-script()
{
    local cur prev words cword
    _init_completion || return

    _command_offset 1

} && complete -F _my-script my-script

Declaring these four local variables and calling _init_completion is standard for all completion scipts, so really it's as simple as one command. Of course someone had to write the massively-complex _command_offset function so lucky me I guess?

Anyways, thank you for reading the story of me messing around and hopefully this will be helpful to some other person in the future.

Answered By - cyqsimon

Answer Checked By - Cary Denson (WPSolving Admin)