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19. ROS 2 Launch Startup File Configuration

1. Introduction to Launch​

Until now, every time we launched a ROS node, we had to open a new terminal and run a command. With so many nodes in a robotic system, doing this every time is cumbersome.

ROS 2 provides launch files, which allow you to:

  • Start multiple nodes at once\
  • Configure parameters\
  • Manage system startup more efficiently

ROS 2 launch files can be written in:

  • XML\
  • YAML\
  • Python

This tutorial uses Python launch files, which provide the most flexibility.

Why Use Python Launch Files?​

  • Access to full Python standard libraries\
  • Direct access to ROS 2 launch APIs\
  • More powerful logic and conditional execution

2. Writing a Single Node Launch Program​

2.1 Create a Package​

ros2 pkg create learn_launch --build-type ament_python

2.2 Create the Launch File​

Create a launch folder inside the package and add:

single_node_launch.py

Paste the following code:

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    node = Node(
        package='pkg_helloworld_py',
        executable='helloworld',
        output='screen'
    )
    return LaunchDescription([node])

2.3 Configure setup.py​

Add the launch file path so it gets installed:

import os
from glob import glob

(
    os.path.join('share', package_name, 'launch'),
    glob(os.path.join('launch', '*launch.py'))
)

2.4 Compile the Package​

colcon build --packages-select learn_launch

2.5 Run the Launch File​

ros2 launch learn_launch single_node_launch.py

2.6 Source Code Analysis​

Import Libraries​

from launch import LaunchDescription
from launch_ros.actions import Node

Define Launch Logic​

def generate_launch_description():
    node = Node(
        package='pkg_helloworld_py',
        executable='helloworld'
    )
    return LaunchDescription([node])

Key Parameters:

  • package: ROS 2 package name\
  • executable: Node executable name

3. Writing a Launch Program for Multiple Nodes​

3.1 Create the Multi-Node Launch File​

Create:

multi_node_launch.py

Add the following content:

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    publisher_node = Node(
        package='pkg_topic',
        executable='publisher_demo',
        output='screen'
    )

    subscriber_node = Node(
        package='pkg_topic',
        executable='subscriber_demo',
        output='screen'
    )

    return LaunchDescription([
        publisher_node,
        subscriber_node
    ])

3.2 Compile the Package​

colcon build --packages-select learn_launch

3.3 Run the Multi-Node Launch File​

ros2 launch learn_launch multi_node_launch.py

This documentation is maintained by HemiHex for ROS 2 launch system configuration.