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.