options with optional values using getopt

Question

I have the following function and would like to look into how options with optional values can be set up using getopt.

Would like to have -w taking on default values

region -w -- "Text"

And -w 1 that takes the optional numeric value.

region -w 1 -- "Text"

The function is here

region ()
{
  # Process command line options
  shortopts="hw::"
  longopts="help,version,warning:"

  opts=$(getopt -o "$shortopts" -l "$longopts"  \
            -n "$(basename $0)" -- "$@")
  if [ $? -eq 0 ]; then
    eval "set -- ${opts}"
    while [ $# -gt 0 ]; do
      case "$1" in
      -h|-\?|--help)
        printf "%s\n" "Todo."
        shift
        break
        ;;
      -w|--warning)
        case "$2" in
          "1") local -r warn="first";  shift 2  ;;
            *) local -r warn="all";    shift 2  ;;
         esac
         local -r f=1
         ;;
      --)
         local -r f=1
         shift
         break
         ;;
      esac
    done
  else
    shorthelp=1 # getopt returned (and reported) an error.
  fi
    
}

Note that you're missing the double colon for "warning" in the longopts — glenn jackman
– glenn jackman, Commented Jul 22, 2021 at 17:55
For other readers of this question, the canonical bash example script for getopt — glenn jackman
– glenn jackman, Commented Jul 22, 2021 at 18:11

glenn jackman · Accepted Answer · 2021-07-23 13:35:15Z

You're probably not going to like this answer: for providing an arg to an option that optionally takes a arg, you're not allowed to use a space:

$ getopt -o 'w::' -- -w -- arg
 -w '' -- 'arg'

$ getopt -o 'w::' -- -w 1 -- arg
 -w '' -- '1' 'arg'

$ getopt -o 'w::' -- -w1 -- arg
 -w '1' -- 'arg'

Similarly, for option long argument, you must use =

$ getopt -o '' -l 'warning::' -- --warning -- arg
 --warning '' -- 'arg'

$ getopt -o '' -l 'warning::' -- --warning 1 -- arg
 --warning '' -- '1' 'arg'

$ getopt -o '' -l 'warning::' -- --warning=1 -- arg
 --warning '1' -- 'arg'

From the man page:

A simple short option is a '-' followed by a short option character[...] If the option has an optional argument, it must be written directly after the option character if present.

A long option normally begins with '--' [...] If the option has an optional argument, it must be written directly after the long option name, separated by '=', if present (if you add the '=' but nothing behind it, it is interpreted as if no argument was present; this is a slight bug, see the BUGS).

Just a code review comment about code layout: in an if-else-fi construct, put the short branch first. Here, by the time I get to else, I have to scan up to remember what the if condition was.

That also lets you write in an "assertion" style and out-dent the "main" code of the function:

region ()
{
  # Process command line options
  shortopts="hw::"
  longopts="help,version,warning:"

  opts=$(getopt -o "$shortopts" -l "$longopts"  \
            -n "$(basename $0)" -- "$@")
  if [ $? -ne 0 ]; then
    shorthelp=1 # getopt returned (and reported) an error.
    return
  fi

  eval "set -- $opts"
  # ...

Also, you can check the exit status directly and still assign to the variable (I'm also removing the basename call here)

  if ! opts=$(getopt -o "$shortopts" -l "$longopts" -n "${0##*/}" -- "$@")
  then
    shorthelp=1 # getopt returned (and reported) an error.
    return
  fi

With -w I set level=1. Otherwise, for -wNUM I set level=NUM

  local level=0
  #...
      case "$1" in
      -w|--warning)
         level=${2:-"1"}
         ;;

That uses the ${var:-default} form of parameter expansion.

I can deal with your answer. Was more interested in figuring out how the optional argument for -w could be specified or not. — Pietru
– Pietru, Commented Jul 22, 2021 at 19:10
Would like -w to work like this. With -w I set level=1. Otherwise, for -wNUM I set level=NUM. — Pietru
– Pietru, Commented Jul 23, 2021 at 8:21
Am scrutinising tho source code for getopt, when an option accepts an argument optionally. If the user does not supply the optional argument value, it looks like a null argument value is returned d->optarg = NULL;. Does this mean that me get '' for the argument value in such case of no optianal argument value being passed? — Pietru
– Pietru, Commented Jul 23, 2021 at 9:31

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