Script to Convert Excel tab to CSV

cd /applmgr/custom/inbound/data
for i in *.xls
do
echo $i
newfile=$i.csv
awk 'BEGIN {
        FS = "\t"
        OFS = ","
     }
     {
        $1 = $1
        for (i = 1; i <= NF; i++) {
           if ($i == "") {
              $i = "null"
           }
        }
        print $0
     }' $i > $newfile
done