Parse Command Line Positional Arguments and Flags with Bash

#!/usr/bin/env bash

set -o errexit
set -o pipefail
set -o nounset

# A help menu showing how to use the script and what the expected output is.
_usage() {
cat << EOF
  ${0} NAME COUNTRY -i|--interests X[,Y] [-c|--cool]

  Output:
    Name: NAME
    Country: COUNTRY
    Interests: X[,Y]
    Cool?: (true|false)
EOF
}

main() {
  # Positional arguments.
  local position=0
  local name=
  local country=

  # Flag arguments.
  local interests=
  local cool="false"

  # Count all of the args passed into the script and continue looping while we have args.
  while [[ "${#}" -gt 0 ]]; do
    # Determine which arg we're working with.
    case "${1}" in
      # Optimize for happiness and support multiple variants.
      -h|--help|help)
        _usage

        # Since there's nothing to do, we're done.
        exit 0
        ;;
      -i|--interests)
        # Set the value so we can use it later in the script.
        interests="${2:-}"

        # Interests requires a value, so we make sure it's not empty.
        # If it's empty we'll let the user know and print a message to STDERR.
        [[ -z "${interests}" ]] && printf "%s must have a value\n\n" "${1}" >&2 && _usage >&2 && exit 1

        # Pop the first 2 elements off the list of arguments in $@, in this case
        # that's the -i|--interests flag and its value. This lets us break out
        # of the while loop because eventually we'll reach 0 args.
        shift 2
        ;;
      -c|--cool)
        # Set the value so we can use it later in the script.
        cool="true"

        # We only need to pop the first element off the list of arguments
        # since this flag doesn't have a value.
        shift
        ;;
      # We've handled all of our flags, now it's onto anything else (the positional args).
      *)
        # Determine which position we're at (this value defaults to 0 earlier in the script).
        case "${position}" in
          0)
            # Set the value so we can use it later in the script.
            name="${1}"

            # We've processed this position so let's increment it by 1 and
            # pop the first element off the list of args so we can continue in the loop.
            position=1
            shift
            ;;
          1)
            # The same as above.
            country="${1}"
            position=2
            shift
            ;;
          2)
            # The user called the script with an unexpected argument, so let's bail.
            printf "Unknown argument: %s\n\n" "${1}" >&2
            _usage >&2
            exit 1
            ;;
        esac
        ;;
    esac
  done

  # Validation.
  [[ -z "${name}" ]] && printf "Requires NAME\n\n" >&2 && _usage >&2 && exit 1
  [[ -z "${country}" ]] && printf "Requires COUNTRY\n\n" >&2 && _usage >&2 && exit 1
  [[ -z "${interests}" ]] && printf "Requires --interests X[,Y]\n\n" >&2 && _usage >&2 && exit 1

  # The script's logic now that the inputs are defined.
  echo "Name: ${name}"
  echo "Country: ${country}"
  echo "Interests: ${interests}"
  [[ "${cool}" == "true" ]] && echo "Cool?: ${cool}"

  return 0
}

main "${@:-}"