#!/bin/bash
# nextday
# given day month year, returns values for the next day in day month year

day=$1
mon=$2
year=$3

# check that year entry value is valid
if [ $year -lt 1 ]
then
    echo "Invalid year = $year"
    exit
fi

# check that month entry value is valid
if [ $mon -lt 1 ]
then
    echo "Invalid month = $mon"
    exit
fi

if [ $mon -gt 12 ]
then
    echo "Invalid month = $mon"
    exit
fi

# check for days in month
ndmon=`monthdays $mon $year`

# check that day entry value is valid
if [ $day -lt 1 ]
then
    echo "Invalid day = $day"
    exit
fi

if [ $day -gt $ndmon ]
then
    echo "Invalid day = $day"
    exit
fi

# subtract a day and check
let day=day-1
if [ $day -lt 1 ]
then
    let mon=$mon-1
    if [ $mon -lt 1 ]
    then
        mon=12
        let year=$year-1
    fi
    day=`monthdays $mon $year`
fi

# return new date values
echo "$day $mon $year"