A ROS 2 system typically consists of many nodes running across many different processes (and even different machines). While it is possible to run each of these nodes separately, it gets cumbersome quite quickly.

The launch system in ROS 2 is meant to automate the running of many nodes with a single command. It helps the user describe the configuration of their system and then executes it as described. The configuration of the system includes what programs to run, where to run them, what arguments to pass them, and ROS-specific conventions which make it easy to reuse components throughout the system by giving them each a different configuration. It is also responsible for monitoring the state of the processes launched, and reporting and/or reacting to changes in the state of those processes.

All of the above is specified in a launch file, which can be written in Python, XML, or YAML. This launch file can then be run using the ros2 launch command, and all of the nodes specified will be run.

Motivation:

In most of the introductory tutorials, you have been opening new terminals for every new node you run. As you create more complex systems with more and more nodes running simultaneously, opening terminals and reentering configuration details becomes tedious.

Launch files allow you to start up and configure a number of executables containing ROS 2 nodes simultaneously.

Running a single launch file with the ros2 launch command will start up your entire system - all nodes and their configurations - at once.

Running a Launch File:

Open a new terminal and run:

ros2 launch turtlesim multisim.launch.py

This command will run the following launch file:

# turtlesim/launch/multisim.launch.py

from launch import LaunchDescription
import launch_ros.actions

def generate_launch_description():
    return LaunchDescription([
        launch_ros.actions.Node(
            namespace= "turtlesim1", package='turtlesim', executable='turtlesim_node', output='screen'),
        launch_ros.actions.Node(
            namespace= "turtlesim2", package='turtlesim', executable='turtlesim_node', output='screen'),
    ])

Note: The launch file above is written in Python, but you can also use XML and YAML to create launch files. You can see a comparison of these different ROS 2 launch formats in Using Python, XML, and YAML for ROS 2 Launch Files.

This will run two turtlesim nodes:

You can find more information on ROS 2 launch in the ROS 2 launch tutorials.

The significance of what you’ve done so far is that you’ve run two turtlesim nodes with one command. Once you learn to write your own launch files, you’ll be able to run multiple nodes - and set up their configuration - in a similar way, with the ros2 launch command.

Write the launch file

Let’s put together a ROS 2 launch file using the turtlesim package and its executables. As mentioned above, this can either be in Python, XML, or YAML.

• launch/turtlesim_mimic_launch.py

  
  from launch import LaunchDescription
  from launch_ros.actions import Node
  
  def generate_launch_description():
      return LaunchDescription([
          Node(
              package='turtlesim',
              namespace='turtlesim1',
              executable='turtlesim_node',
              name='sim'
          ),
          Node(
              package='turtlesim',
              namespace='turtlesim2',
              executable='turtlesim_node',
              name='sim'
          ),
          Node(
              package='turtlesim',
              executable='mimic',
              name='mimic',
              remappings=[
                  ('/input/pose', '/turtlesim1/turtle1/pose'),
                  ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
              ]
          )
      ])