Shell Programming and Scripting

#!/bin/bash

#####
## VARIABLES
#####
status=0
iyear=${birthday:0:4}
imonth=${birthday:4:2}
iday=${birthday:6:2}
curdate=$(date +"%Y%m%d")
cal $imonth $iyear 2> /dev/null | grep $iday > /dev/null && daterr=0 || daterr=1

while [ $status -eq 0 ] 
do

[ $# = 0 ] && read -p "Please enter your birthday in the format YYYYMMDD : " birthday || birthday=$1
#####
## DATE VERIFICATION
#####

# DATE MUST BE 8 CHARACTERS
if [ ${#birthday} -ne 8 ]
then
	echo "You entered ${#birthday} characters, please enter your birthdate in the form YYYYMMDD" 
	ErrMsg="You entered ${#birthday} characters, please enter your birthdate in the form YYYYMMDD"
	status=0
	continue

# DATE CANNOT CONTAIN LETTERS
elif echo "$birthday" | grep "[^0-9]" > /dev/null
then
	echo "Your entry contains non numeric characters, please enter your birthdate in the form YYYYMMDD : "
	status=0
	continue

# DATE MUST ACTUALLY EXIST
elif [ $dateerr -eq  1 ]
then
	echo "You entered $birthday which is an invalid date, please enter your birthday in the form YYYYMMDD : "
	status=0
	continue

# DATE MUST NOT BE IN THE FUTURE
elif [ $iyear$imonth$iday -gt $curdate 2> /dev/null ]
then
	echo "You entered $birthday, which is a future date, please enter your birthday in the form YYYYMMDD : "
	status=0
	continue
else
	status=1
	break
fi
done