while IFS=',' read -r f1 f2 f3 f4
do
echo $(grep -m1 $f1 $input2) | while IFS=',' read -r g1 g2 g3 g4 g5 g6
do
echo "$f1,$f2,$f3,$f4,$g2,$g4,$g6,$[$g2-$f3]" >> $output;
done