Bash getopts: recognizing negative options (-x- or +x)?

Question

(Similar to this, but in bash.)

I have an existing bash script that uses the builtin getopts to recognize -s (flag only - no argument). I find I use that option every time, so I would like to make it default unless -s- or +s is specified on the command line. However, although ksh getopts can handle +s, I can't find that capability in the bash getopts manual.

My current workarounds are:

1. to use s: so I can recognize -s- by $OPTARG="-"; or
2. to replace -s with a different option, e.g., -d (for "don't").

However, #1 has the problem that it swallows the next argument if I accidentally specify -s, and #2 has the problem that it uses a different switch letter than the one I already have in my muscle memory. I am hoping there might be a straightforward way to parse -s- or +s in bash.

• The util-linux getopt(1) also doesn't appear to have +s. It can handle optional arguments, so might be able to take -s-. However, I've not used getopt(1) before so would appreciate pointers on how not to shoot myself in the foot.
• BashFAQ 035 says "parse it yourself." if you have a routine already written that does +s or -s-, I'd love to see it.

My current arg-parsing loop is very basic:

while getopts "nthps" opt ; do
    case "$opt" in
        <other cases cut>
        (s)     saw_s="yes"
                ;;
    esac
done
shift $((OPTIND-1))

Answer 1

The sequence of negative flags (+abcdef +g) differs from a normal sequence (-abcdef -g) with just the plus character. So you can simply revert the flag value for + prefix.

The second form of negative sequence is as simple. Just strip the last character (-), and negate the normal flag value.

Example

The following script accepts all of the mentioned formats, e.g. -ns -n -s -n- -ns- +ns +n +s.

arglist='ns'

while (( $# )); do
  arg="$1"

  # Parse -abcdef- (negative form of -abcdef) options
  if [ "${arg:0:1}" = '-' -a "${arg#${arg%?}}" = '-' ]; then
    flag_val=no
    arg="${arg%?}" # -abcdef- becomes -abcdef
  elif [ "${arg:0:1}" = '+' ]; then
    flag_val=no
    arg="${arg/#+/-}"
  else
    flag_val=yes
  fi

  # OPTIND is the index of the next argument to be processed.
  # We are going to parse "$arg" from the beginning, so we need
  # to reset it to 1 before calling getopts.
  OPTIND=1

  while getopts "$arglist" opt "$arg"; do
    case "$opt" in
      s) saw_s="$flag_val" ;;
      n) saw_n="$flag_val" ;;
    esac
  done

  shift
done

# Set default values
: ${saw_s:=yes}
: ${saw_n:=no}

printf "saw_s='%s'\nsaw_n='%s'\n" "$saw_s" "$saw_n"

Testing

$ ./pargs.sh
saw_s='yes'
saw_n='no'
$ ./pargs.sh -s
saw_s='yes'
saw_n='no'
$ ./pargs.sh +s
saw_s='no'
saw_n='no'
$ ./pargs.sh -s-
saw_s='no'
saw_n='no'
$ ./pargs.sh -s- +s -s
saw_s='yes'
saw_n='no'
$ ./pargs.sh -s +s
saw_s='no'
saw_n='no'
$ ./pargs.sh +s -s
saw_s='yes'
saw_n='no'
$ ./pargs.sh -s -s-
saw_s='no'
saw_n='no'
$ ./pargs.sh -sn
saw_s='yes'
saw_n='yes'
$ ./pargs.sh -sn -s-
saw_s='no'
saw_n='yes'
$ ./pargs.sh -sn +s
saw_s='no'
saw_n='yes'
$ ./pargs.sh +sn
saw_s='no'
saw_n='no'
$ ./pargs.sh -sn-
saw_s='no'
saw_n='no'
$ ./pargs.sh -sn- -n
saw_s='no'
saw_n='yes'