#!/bin/sh

init_variables() {
  COMMAND=${0##*/}

  OPTION=
  OPERAND=
}

parse_command_line() {
  while getopts o: OPT; do
    case "${OPT}" in
      o) OPTION="${OPTARG}" ;;
      ?) exit_error ;;
    esac
  done

  shift $(( OPTIND - 1 ))

  OPERAND="$1"
}

validate_command_line() {
  if [ -z "${OPERAND}" ]; then
    exit_error 'missing operand'
  fi
}

exit_error() {
  [ -z "$1" ] || echo "${COMMAND}: $1"
  exit 1
} >&2

print_items() {
  printf 'option: %s\n' "${OPTION}"
  printf 'operand: %s\n' "${OPERAND}"
}

init_variables
parse_command_line "$@"
validate_command_line

print_items