Write a shell script to find factorial of given number n - OS

fact=1
echo "Enter number to find factorial : "
read n
a=$n
#if enter value less than 0
if [ $n -le 0 ]
then
echo "invalid number"
exit
fi
#factorial logic
while [ $n -ge 1 ]
do
fact=`expr $fact \* $n`
n=`expr $n - 1`
done
echo "Factorial for $a is $fact"