Integrating launch files into ROS 2 packages

Goal: Add a launch file to a ROS 2 package

Tutorial level: Intermediate

Time: 10 minutes

Prerequisites

You should have gone through the tutorial on how to create a ROS 2 package.

As always, don’t forget to source ROS 2 in every new terminal you open.

Background

In the previous tutorial, we saw how to write a standalone launch file. This tutorial will show how to add a launch file to an existing package, and the conventions typically used.

Tasks

1 Create a package

Create a workspace for the package to live in:

LinuxmacOSWindows

mkdir -p launch_ws/src
cd launch_ws/src

Python packageC++ package

ros2 pkg create --build-type ament_python --license Apache-2.0 py_launch_example

2 Creating the structure to hold launch files

By convention, all launch files for a package are stored in the launch directory inside of the package. Make sure to create a launch directory at the top-level of the package you created above.

Python packageC++ package

For Python packages, the directory containing your package should look like this:

src/
  py_launch_example/
    launch/
    package.xml
    py_launch_example/
    resource/
    setup.cfg
    setup.py
    test/

To enable colcon to locate and utilize our launch files, we need to inform Python’s setup tools of their presence. To achieve this, open the setup.py file, add the necessary import statements at the top, and include the launch files into the data_files parameter of setup:

import os
from glob import glob
# Other imports ...

package_name = 'py_launch_example'

setup(
    # Other parameters ...
    data_files=[
        # ... Other data files
        # Include all launch files.
        (os.path.join('share', package_name, 'launch'), glob(os.path.join('launch', '*launch.[pxy][yma]*')))
    ]
)

3 Writing the launch file

Python launch fileXML launch fileYAML launch file

Inside your launch directory, create a new launch file called my_script_launch.py. _launch.py is recommended, but not required, as the file suffix for Python launch files. However, the launch file name needs to end with launch.py to be recognized and autocompleted by ros2 launch.

Your launch file should define the generate_launch_description() function which returns a launch.LaunchDescription() to be used by the ros2 launch verb.

import launch
import launch_ros.actions

def generate_launch_description():
    return launch.LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp',
            executable='talker',
            name='talker'),
  ])

4 Building and running the launch file

Go to the top-level of the workspace, and build it:

colcon build

After the colcon build has been successful and you’ve sourced the workspace, you should be able to run the launch file as follows:

Python packageC++ package

Python launch fileXML launch fileYAML launch file

ros2 launch py_launch_example my_script_launch.py

Documentation

The launch documentation provides more details on concepts that are also used in launch_ros.

Additional documentation/examples of launch capabilities are forthcoming. See the source code (https://github.com/ros2/launch and https://github.com/ros2/launch_ros) in the meantime.