Shell Programming and Scripting

 
\abc1   12345
\abc2   12327
 
\abc1   12345
\abc2   12330

 
<current _date & time >
\abc1   <blank since there is no change>
\abc2    3   (This is obtained by subtracting 12330 - 12327)

grep -vE ^[:blank:]*$ input | tail -4 >/tmp/last4lines
check(){
if [[ $1 != $3 ]] 
then
echo "ERROR : cannot compare different first fields"
else
let res=$4-$2
[[ $res = 0 ]] && echo "$a" || echo "$a $res"
fi
}
sed '1~2' /tmp/last4lines | xargs | read a b c d
eval check $a $b $c $d >>output
sed '2~2' /tmp/last4lines | xargs | read a b c d
eval check $a $b $c $d >>output