入力された日付が有効か確認する

#!/bin/sh
#valid_date.sh

exceedsDayInMonth(){
    #月と日を引数として受け取り、指定された日がその月の範囲に
    #収まっていれば0、そうでない場合は1を返す

    case $(echo $1 | tr '[:upper:]' '[:lower:]') in
        jan* ) days=31 ;; feb* ) days=28 ;;
        mar* ) days=31 ;; apr* ) days=30 ;;
        may* ) days=31 ;; jun* ) days=30 ;;
        jul* ) days=31 ;; aug* ) days=31 ;;
        sep* ) days=30 ;; oct* ) days=31 ;;
        nov* ) days=30 ;; dec* ) days=31 ;;
        *) echo "$0: Unknow month name $1" >&2
        exit 1 ;;
    esac

    if [ $2 -lt 1 -o $2 -gt $days ]; then
        return 1
    else
        return 0
    fi
}

isLeapYear(){
    #うるう年なら0、そうでない場合は1を返す

    year=$1
    if [ "$((year % 4))" -ne 0 ]; then
        return 1
    elif [ "$((year % 400))" -eq 0 ]; then
        return 0
    elif [ "$((year % 100))" -eq 0 ]; then
        return 0
    else
        return 0
    fi
}

if [ $# -ne 3 ]; then
    echo "Usage: $0 month day year" >&2
    echo "Typical input format are August 3 2009 and 8 3 2009" >&2
    exit 1
fi

#日付の形式を整えてから元どうりに分割する 
newdate="$(./normdate.sh "$@")"
if [ $? -eq 1 ]; then
    exit 1
fi

month="$(echo $newdate | cut -d' ' -f1)"
 days="$(echo $newdate | cut -d' ' -f2)"
 year="$(echo $newdate | cut -d' ' -f3)"

#日の値が有効か確認する
if ! exceedsDayInMonth $month "$2" ; then
    if [ "$month" = "Feb" -a "$2" -eq "29" ]; then
        if ! isLeapYear $3 ; then
            echo "$0: $3 is not a leap year, so Feb doesn't have $2 day" >&2
            exit 1
        fi
    else
        echo "$0: bad day value: $month doesn't have $2 days" >&2
        exit 1
    fi
fi
echo "Valid date: $newdate"
exit 0

$ ./valid_date.sh august 3 2009
Valid date: Aug 3 2009

$ ./valid_date.sh 9 31 2009
./valid_date.sh: bad day value: Sep doesn't have 31 days

$ ./valid_date.sh feb 29 2004
Valid date: Feb 29 2004

$ ./valid_date.sh feb 29 2009
./valid_date.sh: 2009 is not a leap year, so Feb doesn't have 29 day