alias-completion-bash

#!/bin/bash

# run in sub-shell, to avoid altering environment
(

usage() {
    cmd=$(basename "${BASH_SOURCE[0]}")
    cat << _EOF_
usage: source $cmd <alias> <command>

outputs bash completion code for alias of command.

for example:

    $ source $cmd k kubectl

to load the generated completion:

    $ eval "\$(source $cmd k kubectl)"

you could use it in ~/.bashrc, or use it in completion files.
_EOF_
}

get_completion() {
    # use the completion of the following command
    cmd_upstream=$1

    # get the name of this command
    cmd_current=$2

    # try to get upstream completion specification
    completion_spec=$(complete -p "$cmd_upstream" 2>/dev/null)

    # if that failed, load upstream completion
    if [ "$completion_spec" = '' ]; then
        _completion_loader "$cmd_upstream"

        # try if it works now
        completion_spec=$(complete -p "$cmd_upstream" 2>/dev/null)

        # if failed again, bail out
        if [ "$completion_spec" = '' ]; then
            # (will never happen in current version of bash-completion,
            #  because it will always give `complete -F _minimal <command>` instead of failing)
            return 1
        fi
    fi

    # use upstream specification for our own purpose
    #
    # for example, if this is upstream:
    #
    #     complete -o default -F _pacman pacman
    #
    # we want to turn that into this:
    #
    #     complete -o default -F _pacman pacmatic
    #
    completion_spec=${completion_spec/ $cmd_upstream/ $cmd_current}

    # output
    echo "$completion_spec"
}

get_output() {
    alias=$1
    command=$2

    # build output
    output=''

    # if upstream completion is not loaded, add loading to output
    if [ "$(complete -p "$command" 2>/dev/null)" = '' ]; then
        output+="_completion_loader $(printf '%q' "$command");"$'\n'
    fi

    # get completion, and add to output
    completion=$(get_completion "$command" "$alias")
    output+="$completion;"$'\n'

    # output
    echo "$output"
}

main() {
    # handle options
    case "$1" in
        -h|--help|--usage) usage; return;;
    esac

    # this program must be sourced
    if ! [ "${BASH_SOURCE[0]}" != "$0" ]; then
        usage 1>&2
        return 1
    fi

    # get arguments
    alias=$1
    command=$2

    # validate arguments
    if ! (command -v "$alias" && command -v "$command") >/dev/null 2>&1; then
        usage 1>&2
        return 1
    fi

    # get output
    output=$(get_output "$alias" "$command")

    # output
    echo "$output"
}

main "$@"

)