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ROS 2 Refresher (Chapter 1 Reference)

Use this refresher when: Lessons reference ROS 2 concepts from Chapter 1

Quick Reference: ROS 2 Core Concepts​

Nodes​

A node is an executable that uses ROS 2 to communicate with other nodes. Each node should have a single, modular purpose.

import rclpy
from rclpy.node import Node

class MyNode(Node):
    def __init__(self):
        super().__init__('my_node_name')
        self.get_logger().info('Node started!')

def main():
    rclpy.init()
    node = MyNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

Topics​

Topics are named buses for nodes to exchange messages asynchronously (publish/subscribe).

# Publisher
self.publisher = self.create_publisher(String, 'topic_name', 10)

# Subscriber
self.subscription = self.create_subscription(
    String, 'topic_name', self.callback, 10)

Messages​

Messages are structured data types. Common types include:

  • std_msgs/String - Simple text
  • geometry_msgs/Twist - Velocity commands
  • sensor_msgs/Image - Camera images
  • sensor_msgs/LaserScan - LIDAR data

Services​

Services are synchronous request/response communication:

# Service client
self.client = self.create_client(ServiceType, 'service_name')

Launch Files​

Launch files start multiple nodes with parameters:

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(package='my_package', executable='my_node'),
    ])

Essential Commands​

# List running nodes
ros2 node list

# List topics
ros2 topic list

# Echo topic data
ros2 topic echo /topic_name

# Get topic info
ros2 topic info /topic_name

# Run a node
ros2 run package_name node_name

# Launch multiple nodes
ros2 launch package_name launch_file.py

Need More Detail?​

See Chapter 1: ROS 2 Fundamentals for complete coverage.