Saturday, March 18, 2017

Followup on Bash long options

A followup on Bash long options.

The top-level option parsing while-loop I discussed works fine for regular options. Sometimes you need special parsing for subcommand options. A hypothetical example might be:

$ my-script --toplevel-thing my-subcommand --something-wonderful option-arg

Here the --toplevel-thing option is for my-script, and --something-wonderful option and its option-arg is for my-subcommand. Regular getopts parsing will try to handle all options for the top level, failing to distinguish subcommand options as separate. Further, getopts in a function does not behave quite as expected.

One solution is simple and hearkens back to the pre-getopts days. For the top level:

while (( 0 < $# ))
do
    case $1 in
        --toplevel-thing ) _toplevel_thing=true ; shift ;;
        -* ) usage >&2 ; exit 2 ;;
        * ) break ;;
    esac
done

Using a while-loop with explicit breaks avoids looking too far along the command line, and wrongly consuming options meant for subcommands. Rechecking $# each time through the loop breaks gracefully. Similarly, for subcommands expressed in a function:

function my-subcommand {
    while (( 0 < $# ))
    do
        case $1 in
            --something-special ) local option_arg="$2" ; shift 2 ;;
            * ) usage >&2 ; exit 2 ;;
        esac
    done
    # Rest of my-subcommand, using `option_arg` if provided

This uses the same pattern as the top level so you avoid needing to remember to handle top level one way, and subcommand another.

An example script using this pattern.

Post a Comment