How can I add a help method to a shell script?

How do I check if a -h attribute has been passed into a shell script? I would like to display a help message when a user calls myscript.sh -h.

here's an example for bash:

usage="$(basename "$0") [-h] [-s n] -- program to calculate the answer to life, the universe and everything

where:
    -h  show this help text
    -s  set the seed value (default: 42)"

seed=42
while getopts ':hs:' option; do
  case "$option" in
    h) echo "$usage"
       exit
       ;;
    s) seed=$OPTARG
       ;;
    :) printf "missing argument for -%s\n" "$OPTARG" >&2
       echo "$usage" >&2
       exit 1
       ;;
   \?) printf "illegal option: -%s\n" "$OPTARG" >&2
       echo "$usage" >&2
       exit 1
       ;;
  esac
done
shift $((OPTIND - 1))

To use this inside a function:

• use "$FUNCNAME" instead of $(basename "$0")
• add local OPTIND OPTARG before calling getopts

For example, a function that adds two numbers, or if the -s option is given then subtract two numbers:

add () {
    local OPTIND OPTARG opt operator="+"
    while getopts :hs opt; do
        case $opt in
            s)  operator="-" ;;
            h)  echo "a function to add or subtract two numbers";
                return 1 ;;
            \?) echo "unknown option: -$OPTARG" 1>&2;
                return 2 ;;
        esac
    done
    shift $((OPTIND - 1))
    echo "$(( $1 $operator $2 ))"
}

$ add 15 4
19
$ add -s 15 4
11
$ add -h; echo $?
a function to add two numbers
1
$ add -x; echo $?
unknown option: -x
2