Your first ROS 2 node

The ROS graph

A ROS 2 system is a set of nodes: programs that each do one job. Nodes talk over topics, named channels that carry messages of one type. A node publishes on a topic, and every node subscribed to it gets each message. Publishers and subscribers never know about each other: ROS connects them by the topic's name and type.

This arm already runs the nodes a real arm's driver would:

Node	Topic	Message type	What it is
/joint_state_broadcaster	publishes /joint_states	sensor_msgs/msg/JointState	where every joint is, 50 times a second
/forward_position_controller	subscribes to /forward_position_controller/commands	std_msgs/msg/Float64MultiArray	targets [base, shoulder, elbow, wrist] in radians
/robot_state_publisher	publishes /robot_description	std_msgs/msg/String	the arm's description

A node is a class

class Waver(Node):
    def __init__(self):
        super().__init__("waver")
        self.pub = self.create_publisher(Float64MultiArray, "/forward_position_controller/commands", 10)
        self.timer = self.create_timer(0.05, self.tick)

Your class extends rclpy's Node, and super().__init__("waver") gives it its name. The 10 is the publisher's queue depth, a quality-of-service (QoS) setting. A timer calls self.tick every 0.05 s; self is how the methods share state. Message fields are typed as strictly as in rclpy: msg.data = [0.0, 0.3, 1.2, 1.0] works, an integer like 0 raises, and numpy arrays need .tolist().

main()

rclpy.init() starts ROS, rclpy.spin(node) runs its callbacks until shutdown, then destroy_node() and rclpy.shutdown() clean up. On a robot, ros2 run my_pkg waver calls main().

Here vs a real robot

• Everything runs in one process on simulated time, as with use_sim_time, and callbacks take no time.
• spin() returns after 8 s, as if you'd pressed Ctrl+C. On a robot Ctrl+C raises KeyboardInterrupt, so real code wraps spin in try.
• There's no ros2 command line: the checks play ros2 node list and ros2 topic hz.

Your task

Finish Waver: at 20 Hz, publish [0.6·sin(πt), 0.3, 1.2, 1.0], where t is the seconds since the node started. The base waves ±0.6 rad every 2 s while the other joints hold still. Log "Waving" once when the node starts, with self.get_logger().info.