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ROS 2 Launch Files

Launch files are Python-based scripts that manage the startup of multiple nodes and configurations simultaneously. They allow users to define which nodes to launch, pass parameters, remap topics, and set arguments, streamlining the deployment and orchestration of complex robotics systems.

Creating Launch File Package

When creating a launch file in ROS 2 you have to use the ros2 pkg create package_name. Once the package is created delete all the folders in the package folder and create a folder called launch inside this folder you can put all the launch files. Finally edit the CMakeLists.txt so that it looks like the one below.

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cmake_minimum_required(VERSION 3.8)
project(my_robot1_description)


if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)

install(
    DIRECTORY launch
    DESTINATION share/${PROJECT_NAME}/
)

ament_package()

Creating Python Launch File

Below is a example of a launch file.

display.launch.py
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from launch import LaunchDescription
from launch_ros.parameter_descriptions import ParameterValue
from launch.substitutions import Command
from launch_ros.actions import Node
from ament_index_python import get_package_share_directory
import os


def generate_launch_description():
    urdf_path = os.path.join(get_package_share_directory('my_robot1_description'), 'urdf', 'my_robot1.urdf')
    robot_description = ParameterValue(Command(['xacro ', urdf_path]), value_type=str)
    rviz_config_path = os.path.join(get_package_share_directory('my_robot1_description'), 'rviz', 'urdf_config.rviz')


    robot_state_publisher_node = Node(
        package="robot_state_publisher",
        executable="robot_state_publisher",
        parameters = [{"robot_description": robot_description}]
    )


    joint_state_publisher_gui_node = Node(
        package="joint_state_publisher_gui",
        executable="joint_state_publisher_gui",
    )


    rviz2_node = Node(
        package="rviz2",
        executable="rviz2",
        arguments=['-d', rviz_config_path]
    )

    return LaunchDescription([
        robot_state_publisher_node,
        joint_state_publisher_gui_node,
        rviz2_node
    ])
display.launch.xml
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<launch>
    <let name="urdf_path" value="$(find-pkg-share my_robot1_description)/urdf/my_robot1.urdf"/>
    <let name="rviz_config_path" value="$(find-pkg-share my_robot1_description)/rviz/urdf_config.rviz"/>

    <node pkg="robot_state_publisher" exec="robot_state_publisher">
        <param name="robot_description" value="$(command 'xacro $(var urdf_path)')"/>
    </node>

    <node pkg="joint_state_publisher_gui" exec="joint_state_publisher_gui" />

    <node pkg="rviz2" exec="rviz2" output="screen" args="-d $(var rviz_config_path)" />

</launch>

Adding configs and UFDRs in Launch File Args

When you need to add a file or a config like when creating a rviz node you can take a few quick steps. First create another folder in the same directory you have the launch file. Next navigate to the CMakeList.txt file and and add a space followed by the name of the folder you create. In this case I am creating an folder called rviz and I will add it to the end of line 12.

CMakeLists.txt
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cmake_minimum_required(VERSION 3.8)
project(my_robot1_description)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)

install(
    DIRECTORY launch rviz
    DESTINATION share/${PROJECT_NAME}/
)

ament_package()

Now once it is added you will need to reference it in you launch file. Doing this is slightly different in python and xml but it is not two hard.

from launch import LaunchDescription
from launch_ros.actions import Node
from ament_index_python import get_package_share_directory
import os


def generate_launch_description():
    rviz_config_path = os.path.join(get_package_share_directory('my_robot1_description'), 'rviz', 'urdf_config.rviz')

    rviz2_node = Node(
        package="rviz2",
        executable="rviz2",
        arguments=['-d', rviz_config_path]
    )
    return LaunchDescription([rviz2_node])

<launch>
    <let name="rviz_config_path" value="$(find-pkg-share my_robot1_description)/rviz/urdf_config.rviz"/>
    <node pkg="rviz2" exec="rviz2" output="screen" args="-d $(var rviz_config_path)" />
</launch>

Using Launch File

To run a launch file use the ros2 launch command. 

ros2 launch package_name file.launch.py