A straightforward way to have subcommands that each handle their own command lines, in a bash script.

subcommands.sh

#!/bin/bash
set -euo pipefail

a() {
    # TODO: parse options or whatever.
    echo "a called with args:" "$@"
}

b() {
    echo "b called with args:" "$@"
}


usage='usage: subcommands.sh [options] <command> <args> [then <command> <args>]...'
while getopts 'h' opt; do
    case "$opt" in
    h) echo "$usage"; exit 0;;
    esac
done
shift $((OPTIND-1))
while [[ $# -gt 0 ]]; do
    func=$1; shift
    args=()
    while [[ $# -gt 0 ]]; do
        [[ "$1" == then ]] && shift && break
        args+=($1)
        shift
    done
    "$func" "${args[@]}"
done