How to enforce only the use of one flag in a shell script?

Question

I currently have a script that is using short flags -M, -m, and -b. I only want to make use of one argument, so I just have it break on the matching flag. I would like to change it so that it returns a non-zero code if more than one flag is used, and I'd like to support long flags (--major, --minor, --bug). How can I modify this block of code I use now to achieve that?

while getopts ":Mmb" increment; do
  case "${increment}" in
    M)
      result=1
      break
      ;;
    m)
      result=2
      break
      ;;
    b)
      result=3
      break
      ;;
  esac
done

Answer 1

Perhaps:

has_M=0
has_m=0
has_b=0

while getopts ":Mmb" increment; do
  case "${increment}" in
    M) has_M=1 ;;
    m) has_m=1 ;;
    b) has_b=1 ;;
  esac
done
shift $((OPTIND - 1))

if (( has_M + has_m + has_b > 1 )); then
    echo "only one of -M -m -b is allowed" >&2
    exit 1
fi

if ((has_M)); then
    :# do -M stuff
elif ((has_m)); then
    :# do -m stuff
elif ((has_b)); then
    :# do -b stuff
fi