Shell Program to Calculate the Factorial of a Number

Here we are going to see to calculate the factorial of a number. Factorial of a non-negative integer is the multiplication of all integers smaller than or equal to n. 

For example factorial of 5 is 5*4*3*2*1 which is 120.

Method 1: Using Recursive

Factorial can be calculated using the following recursive formula.

n! = n*(n-1)! \\n! = 1 \hspace{1 mm}if\hspace{1 mm} n = 0\hspace{1 mm} or\hspace{1 mm} n = 1 

Below is  implementation of factorial:

#!/bin/bash
# Recursive factorial function

factorial()
{
    product=$1
           
    # Defining a function to calculate factorial using recursion
    if((product <= 2)); then
        echo $product
    else
        f=$((product -1))
        
# Recursive call

f=$(factorial $f)
f=$((f*product))
echo $f
fi
}

# main program
# reading the input from user
echo "Enter the number:"   
read num

# defining a special case for 0! = 1
if((num == 0)); then   
echo 1
else
#calling the function
factorial $num
fi

Output:

Enter the number
5
120

Enter the number
3
24

Enter the number
6
720

Method 2: Using for loop

Approach:

• Get a number
• Use for loop to compute the factorial by using the below formula
• fact(n) = n * n-1 * n-2 * ...
• Display the result.

Below is the Implementation using for loop:

# shell script for factorial of a number
# factorial using for loop

echo "Enter a number"

# Read the number
read num                     

fact=1                    

for((i=2;i<=num;i++))
{
  fact=$((fact * i)) 
}

echo $fact

Output:

Enter a number
5
120

Enter a number
7
5040

Enter a number
4
24

Method 3: using do-while loop

• Get a number
• Use do-while loop to compute the factorial by using the below formula
• fact(n) = n * n-1 * n-2 * .. 1
• Display the result.

Below is the Implementation using a while loop.

# shell script for factorial of a number
# factorial using while loop

echo "Enter a number"

# Read the number
read num                
fact=1

# -gt is used for '>' Greater than sign
while [ $num -gt 1 ]  
do
  fact=$((fact * num))  
  num=$((num - 1))     
done

# Printing the value of the factorial
echo $fact            

Output:

Enter a number
10
3628800

Enter a number
2
2

Enter a number
9
362880