Shell script to find factorial of an number

SCRIPT:

#!/bin/bash
input=$1
fact_op=1
while [ $input -gt 0 ]
do
    echo "$fact_op  $input"
    fact_op=$(( $fact_op * $input ))
    input=$(( $input - 1 ))
done
echo "factorial of value $1 is $fact_op"

OUTPUT:
sujin@sujin:~/work/shell$ ./factorial.sh 5
1  5
5  4
20  3
60  2
120  1
factorial of value 5 is 120