add calculator.bash

I am doing an open course and this looks like something I might want to look back later

bash/calculator.bash

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+
+# Ensure we are given three parameters
+if [ $# -ne 3 ]; then
+	echo "Please run this script like: ./$0 <operand1> <operator> <operand2>"
+	echo "where operator is either +, -, * or /. You may need to escape or"
+	echo "single quote it."
+	exit 1
+fi
+
+# Store the operands and operator
+operand1=$1
+operator=$2
+operand2=$3
+
+# The main case/switch which was required in the exercise
+case $operator in
+'+')
+	result=$((operand1 + operand2))
+	operatorWord="plus"
+	;;
+'-')
+	result=$((operand1 - operand2))
+	operatorWord="minus"
+	;;
+'*')
+	result=$((operand1 * operand2))
+	operatorWord="multiplied by"
+	;;
+'/')
+	if [ "$operand2" -eq 0 ]; then
+		echo "The laws of mathematics don't allow division by zero."
+		exit 1
+	else
+		result=$((operand1 / operand2))
+		operatorWord="divided by"
+	fi
+	;;
+# Let's not forget default case either
+*)
+	echo "Please use operator +, -. * or /"
+	exit 1
+	;;
+esac
+
+# and finally the output in format wanted by the task
+echo "$operand1 $operatorWord $operand2 equals $result"