leap year

#!/usr/bin/env bash
#
# TODO

# This option will make the script exit when there is an error
set -o errexit
# This option will make the script exit when it tries to use an unset variable
set -o nounset

main () {
  year=$1
  echo $year
  let "even_4 = $year % 4"
  echo 'foo'
  let "even_100=$year%100"
  let "even_400=$year%400"
  echo 'foo'
  if [ $even_4 -eq 0 ]; then

    if [ $even_100 -eq 0]; then

      if [ $even_400 -eq 0]; then
        # even 100 and 400, so ok
        echo "even 100 and 400"
        return 0
      fi

      # even 100 but not 400, not leap
      echo "even 100 but 400"
      return 1
    fi

    # even 4, so ok
    echo "even 4"
    return 0
  fi

  # not even 4, not leap
  echo "not even 4"
  return 1
}

main "$@"



### exists on the following:
$ bash -x leap.sh 2004
+ set -o errexit
+ set -o nounset
+ main 2004
+ year=2004
+ echo 2004
2004
+ let 'even_4 = 2004 % 4'