Shell Programming and Scripting

#!/bin/bash

d1=20180113
d2=20170113
printf "Dates are d1=%d d2=%d\n" $d1 $d2
# different versions of if-then-else 
[ $d1 -gt $d2 ] && echo 'greater' || echo 'not greater'

if [ $d1 -eq $d2 ] ; then
  echo 'equal'
else
  echo 'not equal'
fi
$ ./dcomp.shl
Dates are d1=20180113 d2=20170113
greater
not equal

secs=$(date -d 20110101 +%s)  # epoch seconds for Jan 1 2011
days=$(( secs % 86400 ))
echo "epoch days=$days"

echo "ENTER FIRST DATE IN YYYYMMDD format"
read startdate



startyear=${startdate:0:4}
startmonth=${startdate:4:2}
startday=${startdate:6:2}




echo The date you Entered is $startyear"-"$startmonth"-"$startday


echo "ENTER SECOND DATE IN YYYYMMDD format"
read enddate
endyear=${enddate:0:4}
endmonth=${enddate:4:2}
endday=${enddate:6:2}



echo The date you Entered is $endyear"-"$endmonth"-"$endday



days_months=(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)

echo ${days_months[*]}
################################################ CALCULATE YEARS#######################################################################################
if [ $startyear -eq $endyear ]
then
yeardifference=0;
echo $yeardifference
else
         if[ $startyear % 4 == 0 && $startyear % 100 != 0 || $startyear % 400 == 0 ]then
         if[ $endyear % 4 == 0 && $endyear % 100 != 0 || $endyear % 400 == 0 ]then
                if [ $startyear -gt $endyear ]then
                yeardifference=$((($startyear - $endyear) * 365 + 2))
                echo $yeardifference
        else
                yeardifference=$((($endyear - $startyear) * 365 + 2))
                echo $yeardifference
                fi
        fi
        fi

fi