Shell Script to find factorial of a number

Program

#! /bin/bash
echo "Enter a number"
read num
fact=1
n=$num;
while [ $num -ge 1 ]
do
	fact=`expr $fact \* $num`
	num=`expr $num - 1`
done
echo "Factorial of $n is $fact"

Output

$ sh factorial-of-number.sh 
Enter a number
7
Factorial of 7 is 5040