Shell script for calculateing factorial with either reading user input from terminal or passed in arguments

factorial.sh

#!/bin/bash

# Mainly a demonstration of various ways to do the same thing.
# Using passed in argument or read from terminal.
# Factorial calculation by reading data a file by another program
# and use the output from that program as a input of this program.

calculateFactorial(){

	#check if any command line arguments were passed otherwise read input.	
	if [ -z $1 ]; then
		echo "Enter number to find factorial:"
		read n
	else
		n=$(( $1 ))
	fi
	
	#variables
	f=1
	i=1
	
	#factorial calculation
	if [ $n -eq 0 ]; then
		echo "1"
	else
		while [ $i -le $n ]
		do
			f=$(( $f * $i ))
			i=`expr $i + 1`
		done
	
		echo "Factorial of $n is: $f."
	fi 
}

calculateFactorial $1