Do Complete func

do_complete.py

https://github.com/mitsuhiko/click/blob/master/click/_bashcomplete.py#L42
def do_complete(cli, prog_name):
    cwords = split_arg_string(os.environ['COMP_WORDS'])
    cword = int(os.environ['COMP_CWORD'])
    args = cwords[1:cword]
    try:
        incomplete = cwords[cword]
    except IndexError:
        incomplete = ''

    ctx = resolve_ctx(cli, prog_name, args)
    if ctx is None:
        return True

    choices = []
    if incomplete and not incomplete[:1].isalnum():
        for param in ctx.command.params:
            if not isinstance(param, Option):
                continue
            choices.extend(param.opts)
            choices.extend(param.secondary_opts)
    elif isinstance(ctx.command, MultiCommand):
        choices.extend(ctx.command.list_commands(ctx))

    for item in choices:
        if item.startswith(incomplete):
            echo(item)

    return True