Process arguments and options in shell script

arg_opt_sample.sh

#!/bin/bash

usage() {
  cat << EOS >&2
sample.sh

USAGE:
  bash arg_opt_sample.sh [options] <arg1> [arg2]

ARGS:
  arg1  Required argument.
  arg2  Optional argument. The default argument is 'arg'.

OPTIONS:
  -a, --option-a        Option that has no argument.
  -b, --option-b <arg>  Option that has required argument.
  -c, --option-c [arg]  Option that has optional argument.
                        The default argument is 'ccc'.
  -h, --help            Show this help message.

EOS
}

invalid() {
  usage 1>&2
  echo "$@" 1>&2
  exit 1
}

ARGS=()
while (( $# > 0 ))
do
  case $1 in
    -h | --help)
      usage
      exit 0
      ;;
    -a | --option-a)
      if [[ -n "$OPT_A" ]]; then
        invalid "Duplicated 'option-a'."
        exit 1
      fi
      OPT_A=1
      ;;
    -b | --option-b | --option-b=*)
      if [[ -n "$OPT_B" ]]; then
        invalid "Duplicated 'option-b'."
        exit 1
      elif [[ "$1" =~ ^--option-b= ]]; then
        OPT_B=$(echo $1 | sed -e 's/^--option-b=//')
      elif [[ -z "$2" ]] || [[ "$2" =~ ^-+ ]]; then
        invalid "'option-b' requires an argument."
        exit 1
      else
        OPT_B="$2"
        shift
      fi
      ;;
    -c | --option-c | --option-c=*)
      if [[ -n "$OPT_C" ]]; then
        invalid "Duplicated 'option-c'."
        exit 1
      elif [[ "$1" =~ ^--option-c= ]]; then
        OPT_C=$(echo $1 | sed -e 's/^--option-c=//')
      elif [[ -z "$2" ]] || [[ "$2" =~ ^-+ ]]; then
        OPT_C=ccc
      else
        OPT_C="$2"
        shift
      fi
      ;;
    -*)
      invalid "Illegal option -- '$(echo $1 | sed 's/^-*//')'."
      exit 1
      ;;
    *)
      ARGS=("${ARGS[@]}" "$1")
      ;;
  esac
  shift
done

if [[ "${#ARGS[@]}" -lt 1 ]]; then
  invalid "Too few arguments."
  exit 1
elif [[ "${#ARGS[@]}" -gt 2 ]]; then
  invalid "Too many arguments."
  exit 1
fi

get_nth () {
  local n=$1
  shift
  eval echo \$${n}
}

ARG1=$(get_nth 1 "${ARGS[@]}")
ARG2=$(get_nth 2 "${ARGS[@]}")

if [[ -z "$ARG2" ]]; then
  ARG2=arg
fi

echo "ARG1 $ARG1"
echo "ARG2 $ARG2"
echo "OPT_A $OPT_A"
echo "OPT_B $OPT_B"
echo "OPT_C $OPT_C"