ROS 2 Launch File Documentation: sensor_launch.py

This page provides a complete explanation of the sensor_launch.py file used to launch two ROS 2 nodes a publisher and a subscriber in the sensor_data_evaluation package.

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File Location

We placed the launch file at:

sensor_data_evaluation/launch/sensor_launch.py

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Purpose

The launch file automates the execution of two nodes:

• publisher_node: publishes messages
• subscriber_node: subscribes and listens to messages

This simplifies testing and development in a typical ROS 2 Publisher/Subscriber setup.

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Prerequisites

Before using this launch file, ensure:

1. The ROS 2 package sensor_data_evaluation exists.

2. It builds two executables:

   • publisher_node
   • subscriber_node

3. The CMakeLists.txt includes this to install the launch/ folder:

   install(DIRECTORY launch DESTINATION share/${PROJECT_NAME})

4. To build the package:

   colcon build --packages-select sensor_data_evaluation

5. Then source the workspace with:

   source install/setup.bash

Launch File Content

Create the file at sensor_data_evaluation/launch/sensor_launch.py with the following content:

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='sensor_data_evaluation',
            executable='publisher_node',
            name='publisher',
        ),
        Node(
            package='sensor_data_evaluation',
            executable='subscriber_node',
            name='subscriber',
        )
    ])

Line-by-Line Breakdown

Line	Description
from launch import LaunchDescription	Imports the class for defining a launch description.
from launch_ros.actions import Node	Imports the Node class to define ROS 2 nodes in launch files.
generate_launch_description()	Standard function ROS 2 looks for in a launch file.
LaunchDescription([...])	Returns a list of nodes/actions to be launched.
Node(...)	Specifies each node with: • package: the ROS 2 package • executable: compiled node binary • name: the runtime node name

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How to Run the Launch File

To start both nodes via the launch file:

ros2 launch sensor_data_evaluation sensor_launch.py

Expected behavior:

• publisher_node and subscriber_node will both be started.

• You should see output in the terminal for both nodes.

• Use ros2 node list to verify that the nodes are running.

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How to Test Nodes Individually

You can run each node on its own using:

ros2 run sensor_data_evaluation publisher_node
ros2 run sensor_data_evaluation subscriber_node

Clean and Rebuild

If needed, clean and rebuild the workspace:

rm -rf build/ install/ log/
colcon build
source install/setup.bash

Verification Checklist

• Launch file is placed in launch/ inside the package sensor_data_evaluation.

• It is installed via CMakeLists.txt.

• Launching works with ros2 launch sensor_data_evaluation sensor_launch.py.

• Nodes appear with ros2 node list.

References

• ROS 2 Launch Tutorial: https://docs.ros.org/en/foxy/Tutorials/Launch-Files/Creating-Launch-Files.html
• ROS 2 Nodes: https://docs.ros.org/en/ros2_documentation/kilted/Concepts/Basic/About-Nodes.html
• colcon build: https://colcon.readthedocs.io