UNIX for Advanced & Expert Users

#!/usr/bin/ksh
function juliandate {
day=$1
month=$2
year=$3
 ((standard_jd = day - 32075 + 1461 * (year + 4800 - (14 - month)/12)/4 + 367 * (month - 2 + (14 - month)/12*12)/12 - 3 * ((year + 4900 - (14 - month)/12)/100)/4))
 ((jd = standard_jd-2400001))
return $jd
}
######
#MAIN#
######
userid=$USER
 if [ "$userid" = "root" ]
 then
   awk -F ":" '{print $1}' /etc/shadow  > userlist.txt
   while read User
   do
     chage -l $User > userdetail.txt
     Password_Expiry_Date=`awk '/Password Expires/' userdetail.txt | awk -F ":" '{print $2}'`
     Warning_days=`awk '/Warning/' userdetail.txt | awk -F ":" '{print $2}'`
     P_Expiry_Date=$(date -d "$Password_Expiry_Date" +%Y-%m-%d)
     Expiry_Date=`echo $P_Expiry_Date | awk -F "-" '{print $3}'`
     Expiry_Month=`echo $P_Expiry_Date | awk -F "-" '{print $2}'`
     Expiry_Year=`echo $P_Expiry_Date | awk -F "-" '{print $1}'`
     Today=$(date -d "`date`" +%Y-%m-%d)
     Date=`echo $Today| awk -F "-" '{print $3}'`
     Month=`echo $Today | awk -F "-" '{print $2}'`
     Year=`echo $Today | awk -F "-" '{print $1}'`
     juliandate $Date $Month $Year
     julianday1=$?
     juliandate $Expiry_Date $Expiry_Month $Expiry_Year
     julianday2=$?
     diff_days=`expr $julianday2 - $julianday1`
     if [ $diff_days -le $Warning_days ]
     then
       if [ $diff_days -eq 0 ]
       then
         mailx -s "RENEW PASSWORD TODAY ITSELF" $User@mail.com  < message1.txt
       elif [ $diff_days -lt 0 ]
       then
          mailx -s "Your Password Expired" $User@mail.com < message2.txt
       else
          echo "Your password is going to expire in $diff_days day(s)" > message3.txt
          mailx -s "Renew Password" $User@mail.com < message3.txt
         fi
     fi
   done < userlist.txt
 else
   echo "You need root permission to run this script"
 fi