What does the -: following the last opt char do?

Question

I saw a getopts usage like this:

while getopts ":hv-:" optchar; do
...
done

What is the use of the last -:, with this placement, what the - and : have to do with the parsing behavior?

Answer 1

The :-: at the end of the optstring (":hv-:") is used to handle long options (options starting with --). The placement of - and : after the v does not directly impact the behavior of the v option.

Here's the breakdown:

: at the beginning of the optstring suppresses error messages for invalid options.
h and v are valid short options.
The hyphen (-) after v indicates that v can take an optional argument.
The double hyphen (--) is used to indicate the end of options and to handle long options.
The last colon (:) at the end is a part of the getopts syntax and is associated with handling the long options.

The getopts command uses the -- to distinguish between short and long options. When a long option is encountered, it is handled in the --) case inside the getopts loop. The colon (:) after the - in :-: is associated with long options that require arguments. For example, if you have a long option like --input=file.txt, the : after - allows you to specify that input requires an argument.

Here's a brief example:

while getopts ":hv-:" optchar; do
  case "${optchar}" in
    h)
      echo "Short option h detected."
      ;;
    v)
      echo "Short option v detected."
      ;;
    -)
      case "${OPTARG}" in
        h)
          echo "Long option --h detected."
          ;;
        v)
          echo "Long option --v detected."
          ;;
        *)
          echo "Invalid long option: --${OPTARG}"
          ;;
      esac
      ;;
    \?)
      echo "Invalid option: ${OPTARG}"
      ;;
  esac
done

In this example, short options -h and -v are handled, and long options --h and --v are also handled. The :-: in the optstring enables the handling of long options and their potential arguments.