Launch Files

Introduction¶

As we know from the work we've done in the Simulation Labs, ROS applications can be executed in two different ways:

Using the ros2 run command:
```
ros2 run {Package name} {Node name}
```

Using the ros2 launch command:

ros2 launch {Package name} {Launch file}

The ros2 launch command, used in combination with launch files, offers a few advantages over ros2 run, for example:

Multiple nodes can be executed simultaneously.
From within one launch file, we can call other launch files.
We can pass in additional arguments to launch things conditionally, or to change the launch behaviours.

Points 1 and 2 above are explored in Part 3 (Exercises 1 & 2). In this section we'll explore Point 3 further¹.

Identifying Launch Arguments¶

We can use the -s option with ros2 launch to discover the additional arguments that can be supplied to any given launch file. Take the waffle.launch.py launch file from tuos_simulations, for example:

ros2 launch tuos_simulations waffle.launch.py -s

You should be presented with a range of arguments here, starting with:

$ ros2 launch tuos_simulations waffle.launch.py -s
Arguments (pass arguments as '<name>:=<value>'):
urdf_file_name : turtlebot3_waffle.urdf

    'with_gui':
        Select whether to launch Gazebo with or without Gazebo Client (i.e. the GUI).
        (default: 'true')

Scroll to the bottom of the list, and you should see the following:

    'x_pose':
        Starting X-position of the robot
        (default: '0.0')

    'y_pose':
        Starting Y-position of the robot
        (default: '0.0')

    'yaw':
        Starting orientation of the robot (radians)
        (default: '0.0')

Using these arguments, we can control the position and orientation of the Waffle when it is spawned into the simulated world. Try this:

ros2 launch tuos_simulations waffle.launch.py x_pose:=1 y_pose:=0.5

The robot should spawn into an empty world, but at coordinate position \(x=1.0\), \(y=0.5\), rather than \(x=0\), \(y=0\), as would normally be the case.

Launching Launch Files from Within Launch Files!¶

This was covered in Part 3 Exercise 2, where we learnt how to launch an "Empty World" simulation from within our own launch file (and also launch a velocity control node from one of our own packages alongside this).

Passing Launch Arguments¶

How do we pass an argument to a launch file (tuos_simulations/waffle.launch.py, for example) that we are executing from within another launch file?

As per Part 3 Exercise 2, a basic launch file would look like this (in this case configured to launch tuos_simulations/waffle.launch.py):

launch_args_example.launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

import os
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
from ament_index_python.packages import get_package_share_directory

def generate_launch_description():
    return LaunchDescription([
        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(
                os.path.join(
                    get_package_share_directory("tuos_simulations"), 
                    "launch", "waffle.launch.py"
                )
            )
        )
    ])

To launch this and supply the x_pose and y_pose launch arguments to it as well, we need to add the following:

launch_args_example.launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

import os
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
from ament_index_python.packages import get_package_share_directory

def generate_launch_description():
    return LaunchDescription([
        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(
                os.path.join(
                    get_package_share_directory("tuos_simulations"), 
                    "launch", "waffle.launch.py"
                )
            ),
            launch_arguments={ # (1)!
                'x_pose': '1.0',
                'y_pose': '0.5' # (2)!
            }.items() 
        )
    ])

Arguments are passed to the launch file via the launch_arguments option of IncludeLaunchDescription().
Arguments are passed as a dictionary, which can contain multiple key value pairs separated by commas: dict = {key1:value1, key2:value2, ... }.

In this case, keys are the names of the launch arguments to be passed to the waffle.launch.py launch file, and values are the actual values we want to assign to those arguments (and which can be changed as required).

For more advanced launch file features, have a look at this guide. ↩