Bash getopts optional arg

gistfile1.txt

optional_arg() {
  eval local next_token=\$$((OPTIND + 1))
  if [[ -n $next_token && $next_token != -* ]]; then
    OPTIND=$((OPTIND + 1))
    OPTARG=$next_token
    [[ -n "$1" ]] &&
      eval $1=\$next_token
  else
    OPTARG=""
  fi
}