In a directory in Unix convert Tab Delimeted file to Comma delimeted

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