ROS 2 Launch Files 

In the previous sections, you must have noticed that we need to manually run each ROS 2 node and load parameters, which can be a tedious process.
ROS 2 Launch files simplify this by allowing us to run multiple nodes, load configuration files, and set parameters using a single command.

Create a ROS 2 Launch file 

ros2 launch Command 

ros2 launch is a tool for launching multiple ROS 2 nodes either locally or remotely.
It uses .py or .launch.xml files to define which nodes to run and which parameters to set.

Usage:

ros2 launch <package_name> <launch_file>

<package_name> is the name of the ROS 2 package.
<launch_file> is the name of the launch file you want to execute.

Steps to Create a Launch File 

After creating a package, create a launch folder in your package to store the launch files.

cd ~/ros2_ws/src/<package_name>
mkdir launch

Create a launch file by navigating into the launch directory and using a Python file or .launch.xml file for launch configuration.

cd launch
touch <file_name>.launch.py

For example, you can name the launch file my_launch_file.launch.py.

Basic Launch File Structure in ROS 2 

In ROS 2, launch files can be written in Python or XML. Here’s an example of a Python-based launch file structure:

from launch import LaunchDescription
from launch_ros.actions import Node

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

package: The package that contains the node.
executable: The name of the node executable (without extension).
name: A user-defined name for the node.
output: Set to screen to show output in the terminal.

Steps to Load Configuration Files (YAML) in ROS 2 

You can load YAML files in a ROS 2 launch file using the launch.actions.SetParameterFile or ros2 param commands.

Here’s an example of loading parameters from a YAML file:

from launch import LaunchDescription
from launch_ros.actions import Node
from launch.actions import SetParameterFile

def generate_launch_description():
    return LaunchDescription([
        SetParameterFile(
            node_name='my_node',
            parameter_file='$(find my_package)/config/config.yaml'
        ),
        Node(
            package='my_package',
            executable='my_node',
            name='my_node_name',
            output='screen',
        ),
    ])

Running a Shell Script in a ROS 2 Launch File 

You can execute shell scripts within a ROS 2 launch file using the ExecuteProcess action.

from launch import LaunchDescription
from launch.actions import ExecuteProcess

def generate_launch_description():
    return LaunchDescription([
        ExecuteProcess(
            cmd=['bash', '$(find my_package)/scripts/my_script.sh'],
            output='screen'
        )
    ])

Example 1: Launch Two ROS 2 Nodes 

Launch two ROS 2 nodes: talker and listener from the demo_nodes_cpp and demo_nodes_py packages, respectively.

Create a launch file talker_listener.launch.py in the launch folder of your package.

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='demo_nodes_cpp',
            executable='talker',
            name='talker_node',
            output='screen',
        ),
        Node(
            package='demo_nodes_py',
            executable='listener',
            name='listener_node',
            output='screen',
        )
    ])

ros2 launch <package_name> talker_listener.launch.py

Example 2: Launch Turtlesim with Custom Background Color 

Launch the turtlesim_node and turtle_teleop_key nodes.
Change the background color of the Turtlesim simulator from blue to forest green.

Create a launch file turtlesim_custom.launch.py in the launch folder of your package.

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='turtlesim',
            executable='turtlesim_node',
            name='sim_node',
            output='screen',
            parameters=[{
                'background_r': 34,
                'background_g': 139,
                'background_b': 34,
            }]
        ),
        Node(
            package='turtlesim',
            executable='turtle_teleop_key',
            name='teleop_node',
            output='screen',
        )
    ])

ros2 launch <package_name> turtlesim_custom.launch.py

Example 3: Load YAML Configuration 

Load parameters from config.yaml and launch the node node_param_get_set.

Create a launch file load_yaml.launch.py in the launch folder of your package.

from launch import LaunchDescription
from launch.actions import SetParameterFile
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        SetParameterFile(
            parameter_file='$(find my_package)/config/config.yaml'
        ),
        Node(
            package='my_package',
            executable='node_param_get_set',
            name='param_node',
            output='screen',
        ),
    ])

ros2 launch <package_name> load_yaml.launch.py