#!/bin/sh# valid-date -- Validates a date, taking into account leap year rules.exceedsDaysInMonth(){ 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(){ 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 scriptif [ $# -ne 3 ] ; then echo "Usage: $0 month day year" >&2 echo "Typical input formats are 8 3 2002" >&2 exit 1fi# Normalize date and split back out returned valuesif [ $? -eq 1 ] ; then exit 1    # error condition already reported by normdatefimonthnoToName(){ # Sets the variable 'month' to the appropriate value case $1 in  01|1 ) monthd="Jan"  ;; 02|2 ) monthd="Feb"  ;;  03|3 ) monthd="Mar"  ;; 04|4 ) monthd="Apr"  ;;  05|5 ) monthd="May"  ;; 06|6 ) monthd="Jun"  ;;  07|7 ) monthd="Jul"  ;; 08|8 ) monthd="Aug"  ;;  09|9 ) monthd="Sep"  ;;   10) monthd="Oct"  ;;    11) monthd="Nov"  ;;   12) monthd="Dec"  ;;  * ) echo "$0: Unknown numeric month value $1" >&2; exit 1  esac  return 0}monthnoToName $1month="$monthd" day="$2" year="$3" 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 fifiecho "Valid date: $newdate"exit 0