2.4 Launch Files

Launch files in ROS 2 are used to start multiple instances of nodes using a single terminal command. These files save you time by eliminating the need to open X terminals to ros2 run X nodes.

Creating a Launch File

1. In your first ROS 2 package called tutorial, create a new directory called launch. Inside this launch subdirectory, create a new file called example.launch.yaml.

2. Inside of example.launch.yaml, add the following code:

launch:

- node:
    pkg: "tutorial"
    exec: "publisher"
    name: "tutorial_publisher"
    namespace: ""

- node:
    pkg: "tutorial"
    exec: "subscriber"
    name: "tutorial_subscriber"
    namespace: ""

info

YAML launch files in ROS 2 are structured so that you may specify multiple nodes defined using their executable and package. You may also rename the node from inside the launch file, or add a namespace to the node.

3. Now open your setup.py file. Inside this file, you must specify to build the launch files inside the launch subdirectory.

Add the following imports at the top of the file:

import os
from glob import glob

Then, in the data_files argument in the setup call, add the following line:

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

such that the argument looks like:

data_files=[
    ('share/ament_index/resource_index/packages',
        ['resource/' + package_name]),
    ('share/' + package_name, ['package.xml']),
    (os.path.join('share', package_name, 'launch'), glob('launch/*'))
],

4. Compile the package and test your launch file:

cd ~/auvc_ws && colcon build --packages-select tutorial
source install/setup.zsh

ros2 launch tutorial example.launch.yaml

In the terminal, you should see that both the publisher and subscriber nodes have started running. When you shutdown the launch file, all associated nodes will also stop running.

Check-off

Review with a TA or instructor to check-off this section.