Variables Multiplication for the factorial calculation

Question

I'm making a script that calculates the factorial for a given number, but I'm having some problems with the multiplication.

Note: the factorial for is given by: 9!=9*8*7*6*5*4*3*2*1

Here's my code:

#!/bin/bash

echo "Insert an Integer"

read input

if ! [[ "$input" =~ ^[0-9]+$ ]] ; then
   exec >&2; echo "Error: You didn't enter an integer"; exit 1
fi

function factorial
{
while [ "$input" != 1 ];
do
    result=$(($result * $input))
    input=$(($input-1))
done
}
factorial
echo "The Factorial of " $input "is" $result

it keeps giving me errors of all kinds for different multiplication technics :/

Currently the output is:

joaomartinsrei@joaomartinsrei ~/Área de Trabalho/Shell $ 
./factorial.sh
Insert an Integer
3
./factorial.sh: line 15: * 3: syntax error: operand expected (error token is "* 3")
The factorial of 3 is

Answer 1

The main problem is that you never initialize result (to 1), so this:

result=$(($result * $input))

is equivalent to this:

result=$(( * $input))

which is not a valid arithmetic expression.