ROS 2 Nodes tutorials

A Node in ROS 2 is a process that performs computation. Multiple nodes can be running at the same time and can communicate with each other. Each node in ROS has a unique name, and they can interact by publishing/subscribing to topics, calling services, or using actions.

Each node in ROS should be responsible for a single, modular purpose, e.g. controlling the wheel motors or publishing the sensor data from a laser range-finder. Each node can send and receive data from other nodes via topics, services, actions, or parameters.

A full robotic system is comprised of many nodes working in concert. In ROS 2, a single executable (C++ program, Python program, etc.) can contain one or more nodes.

Example: Creating and Running a Simple Node

Let's create a simple ROS 2 node in Python.

1. Create a Python Script:

   • Open your ROS 2 workspace or create one:

     mkdir -p ~/ros2_ws/src/my_package
     cd ~/ros2_ws/src/my_package

   • Create a new file simple_node.py:

     nano simple_node.py

2. Write a Simple Node:

   • Add this basic code to create a ROS 2 node that logs a message:

     import rclpy
     from rclpy.node import Node
     
     class MyNode(Node):
         def __init__(self):
             super().__init__('my_node')
             self.get_logger().info('Hello, ROS 2!')
     
     def main(args=None):
         rclpy.init(args=args)
         node = MyNode()
         rclpy.spin(node)
         node.destroy_node()
         rclpy.shutdown()
     
     if __name__ == '__main__':
         main()

3. Run the Node:

   • Make the script executable and run it:

     chmod +x simple_node.py
     python3 simple_node.py

   • The output will be:

     [INFO] [<timestamp>] [my_node]: Hello, ROS 2!

   This shows that your node is running and logging messages to the console.