#!/usr/bin/env sh
set -eu ${VERBOSE:+-x}

execute() {
  local cmd="$1"
  shift

  case "$cmd" in
    start) echo a b|exec xargs -P0 -n1 "$0";;
    a) sleep 1; echo a; sleep 2; exec echo a;;
    b) sleep 2; exec echo b "$@";;
    c) execute b foo;;
    *) ! :;;
  esac
}

execute "$@"