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Chapter 1 Exercises: ROS 2 Basics

Total Time: 15-20 minutes Level: Beginner to Intermediate Created by: exercise-designer skill

Exercise 1 (Beginner): Modify the Hello Node​

Objective: Get comfortable editing and running ROS 2 code

Difficulty: ⭐ Beginner | Time: 5-7 minutes

Task​

Modify hello_node.py to:

  1. Change the message from "Hello, ROS 2!" to include your name
  2. Change the logging level from info() to warning()
  3. Change the timer interval from 1.0 second to 2.0 seconds
  4. Rebuild and run the modified code

Instructions​

  1. Open code-examples/ros2_packages/hello_world_py/hello_world_py/hello_node.py in your editor
  2. Find the timer_callback method
  3. Change self.get_logger().info(...) to self.get_logger().warning(...)
  4. Change the message text
  5. Find the self.create_timer(1.0, ...) line
  6. Change 1.0 to 2.0
  7. Save the file
  8. Rebuild: colcon build --packages-select hello_world_py
  9. Run: ros2 run hello_world_py hello_node

Expected Output​

[WARN] [hello_node]: Hello, my_name! Message #0
[WARN] [hello_node]: Hello, my_name! Message #1
(after 2 seconds)
[WARN] [hello_node]: Hello, my_name! Message #2

Hints​

Details

💡 Hint 1: Where to edit? Look in the timer_callback() method where self.get_logger().info() is called.

Details

💡 Hint 2: How to change logging level? Replace .info() with .warning() on the same line.

Details

💡 Hint 3: How to change timer interval? In __init__, find self.create_timer(1.0, ...) and change 1.0 to 2.0

Validation Checklist​

  • Code builds without errors: colcon build succeeds
  • Node runs: ros2 run hello_world_py hello_node starts
  • Messages appear every 2 seconds (not 1 second)
  • Messages use [WARN] level (not [INFO])
  • Message text includes your name

Solution​

See below for the complete solution code.

Exercise 2 (Beginner/Intermediate): Create Your Own Node​

Objective: Write a complete ROS 2 node from scratch

Difficulty: ⭐⭐ Beginner/Intermediate | Time: 7-10 minutes

Task​

Create a new ROS 2 node named greeter_node that:

  1. Says "Hello, friend!" every 3 seconds
  2. Logs the current message count
  3. Stops gracefully when you press Ctrl+C

Instructions​

  1. Create a new directory:

    mkdir -p code-examples/ros2_packages/greeter_py/greeter_py

  2. Create package.xml with basic ROS 2 package metadata

  3. Create setup.py with entry point for your node

  4. Create greeter_py/greeter_node.py with:

    • A GreeterNode class inheriting from Node
    • A timer that fires every 3 seconds
    • A callback that logs the greeting

  5. Build and test:

    colcon build --packages-select greeter_py
    source install/setup.bash
    ros2 run greeter_py greeter_node

Expected Output​

[INFO] [greeter_node]: Hello, friend! Count: 0
[INFO] [greeter_node]: Hello, friend! Count: 1
[INFO] [greeter_node]: Hello, friend! Count: 2
^C
[INFO] [greeter_node]: Goodbye!

Hints​

Details

💡 Hint 1: Starting from scratch? Copy the hello_world_py structure as a template, then modify it for your needs.

Details

💡 Hint 2: How to add graceful shutdown? In the main() function after rclpy.shutdown(), add a goodbye message.

Details

💡 Hint 3: Package.xml and setup.py? See code-examples/ros2_packages/hello_world_py/ for examples.

Validation Checklist​

  • New package created with correct structure
  • package.xml includes dependencies
  • setup.py has correct entry point
  • Node builds without errors
  • Node runs and logs messages every 3 seconds
  • Ctrl+C stops the node cleanly

Solution​

See below for complete solution code.

Exercise 3 (Intermediate): Two-Node System​

Objective: Create two nodes that communicate (preparation for pub/sub)

Difficulty: ⭐⭐ Intermediate | Time: 8-12 minutes

Task​

Create two nodes:

  1. Sender Node: Publishes numbers 1-5, one per second
  2. Receiver Node: (Placeholder) Will receive in Chapter 1 Section 2

For now, just create the sender. In Section 2, you'll add the receiver.

Instructions​

  1. Create a new package sender_receiver_py
  2. Create sender_node.py that:
    • Is a ROS 2 node named sender
    • Publishes to topic /numbers (placeholder - not connected yet)
    • Sends integers 1 through 5
    • Waits 1 second between sends
  3. Build and test: 
    colcon build --packages-select sender_receiver_py
    ros2 run sender_receiver_py sender_node

Expected Output​

[INFO] [sender]: Sending number: 1
[INFO] [sender]: Sending number: 2
[INFO] [sender]: Sending number: 3
[INFO] [sender]: Sending number: 4
[INFO] [sender]: Sending number: 5
[INFO] [sender]: Done!

Hints​

Details

💡 Hint 1: Where to start? Start with hello_node.py structure, change the messages to numbers.

Details

💡 Hint 2: Counting up to 5? Modify the timer callback to check if self.i < 5, then increment.

Details

💡 Hint 3: Done message? After 5 messages, print "Done!" and you can stop the timer or node.

Validation Checklist​

  • Package builds successfully
  • Node runs and prints 5 numbers in sequence
  • Numbers print one per second
  • Node logs "Done!" at the end
  • Node exits cleanly

Solution​

See below for complete solution code.

Common Mistakes & Troubleshooting​

"ModuleNotFoundError: No module named 'rclpy'"​

Problem: Code can't find ROS 2 libraries

Solution:

source /opt/ros/humble/setup.bash  # Source ROS 2
colcon build                        # Build your package
source install/setup.bash           # Source build output

"Node not found" error​

Problem: ros2 run can't find your node

Solution:

  1. Check package name: ros2 pkg list | grep your_package
  2. Check entry point in setup.py
  3. Rebuild: colcon build
  4. Source again: source install/setup.bash

Build fails with "Missing dependency"​

Problem: package.xml doesn't list all dependencies

Solution:

  1. Add to <exec_depend> in package.xml: 
    <exec_depend>rclpy</exec_depend>
    <exec_depend>std_msgs</exec_depend>
  2. Rebuild: colcon build

Complete Solutions​

Exercise 1 Solution: Modified hello_node.py​

# File: hello_world_py/hello_world_py/hello_node.py
import rclpy
from rclpy.node import Node

class HelloNode(Node):
    def __init__(self):
        super().__init__('hello_node')
        # Changed timer interval from 1.0 to 2.0
        self.timer = self.create_timer(2.0, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        # Changed from .info() to .warning() and message text
        self.get_logger().warning(f'Hello, friend! Message #{self.i}')
        self.i += 1

def main(args=None):
    rclpy.init(args=args)
    node = HelloNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Exercise 2 Solution: greeter_node.py​

# File: greeter_py/greeter_py/greeter_node.py
import rclpy
from rclpy.node import Node

class GreeterNode(Node):
    def __init__(self):
        super().__init__('greeter_node')
        self.timer = self.create_timer(3.0, self.timer_callback)
        self.count = 0

    def timer_callback(self):
        self.get_logger().info(f'Hello, friend! Count: {self.count}')
        self.count += 1

def main(args=None):
    rclpy.init(args=args)
    node = GreeterNode()
    rclpy.spin(node)
    node.get_logger().info('Goodbye!')
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Exercise 3 Solution: sender_node.py​

# File: sender_receiver_py/sender_receiver_py/sender_node.py
import rclpy
from rclpy.node import Node

class SenderNode(Node):
    def __init__(self):
        super().__init__('sender')
        self.timer = self.create_timer(1.0, self.timer_callback)
        self.count = 0

    def timer_callback(self):
        if self.count < 5:
            self.get_logger().info(f'Sending number: {self.count + 1}')
            self.count += 1
        else:
            self.get_logger().info('Done!')
            # Could call rclpy.shutdown() here to stop

def main(args=None):
    rclpy.init(args=args)
    node = SenderNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Next Steps​

After completing these exercises: ✅ You understand how to create and modify ROS 2 nodes ✅ You can build and run ROS 2 packages ✅ You know how to structure ROS 2 code

Next, you'll learn how to make these nodes communicate via topics and services.

👉 Next: Section 2: Topics - Publish and Subscribe

Exercises Status: Ready ✅ Skills Used: exercise-designer Solutions Provided: Yes ✅