07-valid-date.sh

#!/bin/bash
# valid-date--Validates a date, taking into account leap year rules.

PATH=.:$PATH

exceedsDaysInMonth()
{
  # Given a month name and day number in that month, this function will 
  #   return 0 if the specified day value is less than or equal to the 
  #   max days in the month; 1 otherwise.

  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: Unknown month name $1" >&2; exit 1
  esac
   if [ $2 -lt 1 -o $2 -gt $days ] ; then
     return 1
   else
     return 0   # the day number is valid 
   fi 
}

isLeapYear()
{    
  # This function returns 0 if the specified year is a leap year; 
  #   1 otherwise.
  # The formula for checking whether a year is a leap year is: 
  #   1. Years not divisible by 4 are not leap years.
  #   2. Years divisible by 4 and by 400 are leap years.
  #   3. Years divisible by 4, not divisible by 400, and divisible by
  #   100, are not leap years.
  #   4. All other years divisible by 4 are leap years.

  year=$1
  if [ "$((year % 4))" -ne 0 ] ; then
    return 1 # nope, not a leap year
  elif [ "$((year % 400))" -eq 0 ] ; then
    return 0 # yes, it's a leap year
  elif [ "$((year % 100))" -eq 0 ] ; then
    return 1
  else
     return 0
  fi 
}

# BEGIN MAIN SCRIPT
# =================

if [ $# -ne 3 ] ; then
 echo "Usage: $0 month day year" >&2
 echo "Typical input formats are August 3 1962 and 8 3 2002" >&2
 exit 1
fi

# Normalize date and store the return value to check for errors.

newdate="$(normdate "$@")"
echo $newdate
if [ $? -eq 1 ] ; then
 exit 1        # Error condition already reported by normdate
fi

# Split the normalized date format, where
# first word = month, second word = day, third word = year.

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

# Now that we have a normalized date, let's check to see if the
  # day value is legal and valid (e.g., not Jan 36).

if ! exceedsDaysInMonth $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 29 days" >&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