parse options in shell script

options

#!/usr/bin/env bash

ACTION=
OPTION_A='undefined'
OPTION_B='undefined'
OPTIONS=("$@")
LENGTH=${#OPTIONS[@]}

main() {
  echo "  Action: $ACTION"
  echo "Option A: $OPTION_A"
  echo "Option B: $OPTION_B"
}

usage() {
  echo ''
  echo '  Usage:'
  echo "    $(basename $0) [options] <action>"
  echo ''
  echo '  Options:'
  echo "    -a --option-a  the option A"
  echo "    -b --option-b  the option B"
  echo ''
}

if [ $LENGTH = 0 ]; then
  usage
  exit 1
fi

while (("$#")); do
  OPTION=${OPTIONS[$LENGTH - $#]}

  case "$OPTION" in
    -a|--option-a)
      shift
      OPTION_A=${OPTIONS[$LENGTH - $#]}
      ;;
    --option-a=*)
      OPTION_A=${OPTION/--option-a=}
      ;;
    -b|--option-b)
      shift
      OPTION_B=${OPTIONS[$LENGTH - $#]}
      ;;
    --option-b=*)
      OPTION_B=${OPTION/--option-b=}
      ;;
    *)
      ACTION=${OPTION}
  esac

  shift
done

if [ -z "${ACTION}" ]; then
  usage
  exit 1
fi

main && exit 0 || exit 1