Bash completion overwrites current word

Question

I am trying to create a bash completion script for my command. ls has the behaviour that I want. I need to get this same behaviour form my command.

This is what I get for ls

$ ls /home/tux/<Tab><Tab>
Downloads  Documents  Pictures
$ ls /home/tux/Do<Tab><Tab>
Downloads  Documents

I.e. bash only shows the next paths relatively, not absolutely (i.e. it does add Downloads to the completion list, but not /home/tux/Downloads)

I wanted to write a completion script that works the same way. This is what I have tried.

_testcommand ()
{
    local IFS=$'\n'
    local cur=${COMP_WORDS[COMP_CWORD]}
    COMPREPLY=( $(compgen -o bashdefault -d -- "$cur") )
    if [ "${#COMPREPLY[@]}" -ne 1 ]
    then
        # remove prefix "$cur", so the preview of paths gets shorter
        local cur_len=$(echo $cur | sed 's|/[^/]*$||' | wc -c)
        for i in ${!COMPREPLY[@]}
        do
            COMPREPLY[$i]="${COMPREPLY[i]:$cur_len}"
        done
    fi
    return 0
}

complete -o nospace -F _testcommand testcommand

However the result is like this:

$ testcommand /home/tux/<Tab><Tab>
Downloads  Documents  Pictures
$ testcommand /home/tux/Do<Tab>
Downloads  Documents
$ testcommand Do

How can I make my completion not remove /home/tux/ from the command line?

Note: I think I can not add '-f' or '-d' etc. to the complete call on the bottom. In reality, the completion must, in some cases, also complete words instead of paths.

Answer 1

You can check which completion function is used by "ls": complete -p | grep ls. And you can check the function with: type _longopt (the previous command's result). In the _longopt function you can find the _filedir function.

And finally an interesting article about completion: https://spin.atomicobject.com/2016/02/14/bash-programmable-completion/

Answer 2

Most programs in the bash completion dir (pkg-config --variable=completionsdir bash-completion) use the _filedir function provided by bash-completion itself. It seems legitimate to reuse _filedir - No need to worry about your own implementation!

MWE:

_testcommand()
{
    # init bash-completion's stuff
    _init_completion || return

    # fill COMPREPLY using bash-completion's routine
    # in this case, accept only MarkDown and C files
    _filedir '@(md|c)'
}
complete -F _testcommand testcommand

Of course, you can still use this while completing non-files:

if ...
then
    # any custom extensions, e.g. words, numbers etc
    COMPREPLY=( $(compgen ...) )
else
    # fill COMPREPLY using bash-completion's routine
    _filedir '@(md|c)'
fi

How did I find it?

Thanks to @csm: Using their answer, check type _longopt, which calls _filedir.