Bash Scripting: A Simple Arithmetic Calculator (+, -, *, /, %)

Welcome back to our shell scripting series! After tackling conditional logic with if statements, it's time to explore one of the most fundamental parts of any programming language: arithmetic operations.

Today, we'll write a simple, clean, and robust shell script that acts as a basic calculator. It will ask the user for two numbers and then perform all five basic arithmetic operations: Addition (+), Subtraction (-), Multiplication (*), Division (/), and Modulus (%).

This is a fantastic exercise to learn about user input, variables, and a special, modern syntax for doing math in Bash: $((...)).

Let's get started!

The Shell Script

First, here is the complete, commented script. This script is "robust" because it includes a special check to prevent a "division by zero" error, a common bug that can crash scripts.

#!/bin/bash

# This script performs all 5 basic arithmetic operations
# on two user-provided numbers.

# 1. Prompt the user for two numbers
# 'read -p' combines echo and read into one line
read -p "Enter the first number (a): " num1
read -p "Enter the second number (b): " num2

echo "------------------------------------"
echo "You entered: a = $num1 and b = $num2"
echo "Calculating..."
echo "------------------------------------"

# 2. Perform and Display Safe Operations (Add, Subtract, Multiply)
# We use '$((...))' for arithmetic expansion.
sum=$((num1 + num2))
diff=$((num1 - num2))
product=$((num1 * num2))

echo "$num1 + $num2 = $sum"
echo "$num1 - $num2 = $diff"
echo "$num1 * $num2 = $product"

# 3. Perform and Display Unsafe Operations (Divide, Modulus)
# We must check for division by zero!

if [ $num2 -eq 0 ]; then
    # -eq means "is equal to"
    echo "$num1 / $num2 = Error: Division by zero is not allowed."
    echo "$num1 % $num2 = Error: Division by zero is not allowed."
else
    # This block only runs if $num2 is not zero
    quotient=$((num1 / num2))
    remainder=$((num1 % num2))
    
    echo "$num1 / $num2 = $quotient (Note: This is Integer Division)"
    echo "$num1 % $num2 = $remainder (This is the Remainder)"
fi

echo "------------------------------------"
echo "Calculation complete."

---

How It Works: A Step-by-Step Breakdown

Let's break down the key concepts in this script.

• read -p "..." num1
  The -p flag with the read command is a shortcut. It stands for "prompt." It prints the string in quotes and then waits to read the user's input into the specified variable (num1), all in one clean line.


• sum=$((num1 + num2))
  This is the modern, preferred way to perform math in Bash.
  • $((...)): This syntax is called "arithmetic expansion." It tells Bash to treat everything inside the double parentheses as a math problem.
  • It's much cleaner than the older expr command and handles spaces and operators more intuitively.


• product=$((num1 * num2))
  A key advantage of $((...)) is that you don't need to escape the multiplication symbol (*). In other parts of the shell, * is a wildcard, but inside arithmetic expansion, it's just "multiply."


• if [ $num2 -eq 0 ]; then ... fi
  This is our crucial error-handling block.
  • if [ ... ]: This is a conditional test.
  • $num2 -eq 0: This checks if the value of num2 is "equal to" 0. For numbers, we use operators like -eq (equal), -ne (not equal), -gt (greater than), and -lt (less than).
  • If the test is true (num2 is 0), it prints our error message.


• else
  This block runs only if the if test was false (meaning num2 is not 0, and it's safe to divide).


• quotient=$((num1 / num2))
  This performs Integer Division. Bash, by default, does not handle decimal numbers (floating points). This means 10 / 3 will result in 3, not 3.333.... This is an important concept to remember!


• remainder=$((num1 % num2))
  The % (modulus) operator gives you the remainder of an integer division. For example, 10 % 3 results in 1 (because 10 divided by 3 is 3 with a remainder of 1).

---

How to Run the Script

Ready to try it? Here's how to run it on any Linux or macOS machine.

Step 1: Create the file
Open your terminal and use nano to create a new file:

nano calculator.sh

Step 2: Paste the code
Copy the entire shell script from the first section and paste it into the nano editor.

Step 3: Save and Exit
(In nano: Press Ctrl+O, Enter to save. Press Ctrl+X to exit.)

Step 4: Make the script executable
We must give the file permission to run:

chmod +x calculator.sh

Step 5: Run it!
Now you can run your script:

./calculator.sh

Example Output (Two Tests)

Here are two examples of the script in action.

Test 1: Normal Operation (e.g., 10 and 3)

$ ./calculator.sh
Enter the first number (a): 10
Enter the second number (b): 3
------------------------------------
You entered: a = 10 and b = 3
Calculating...
------------------------------------
10 + 3 = 13
10 - 3 = 7
10 * 3 = 30
10 / 3 = 3 (Note: This is Integer Division)
10 % 3 = 1 (This is the Remainder)
------------------------------------
Calculation complete.

Test 2: Division by Zero (e.g., 10 and 0)

$ ./calculator.sh
Enter the first number (a): 10
Enter the second number (b): 0
------------------------------------
You entered: a = 10 and b = 0
Calculating...
------------------------------------
10 + 0 = 10
10 - 0 = 10
10 * 0 = 0
10 / 0 = Error: Division by zero is not allowed.
10 % 0 = Error: Division by zero is not allowed.
------------------------------------
Calculation complete.

---

Challenge Yourself: Get Decimal Results

You've now mastered basic arithmetic and error handling in Bash! You might be wondering, "How do I get decimal answers (like 10 / 3 = 3.33)?"

Bash can't do this by itself, but it can call another program that can! The tool for this is bc (Basic Calculator).

Challenge: Try to figure out how to use bc in your script. Here's a hint:

echo "scale=2; 10 / 3" | bc

Happy scripting!