About
In this post, you will learn how to create a ROS2 launch file.
The problem: we have a ROS2 C++ package but it has no launch file. Yes, we can run the code of this package using ros2 run..., but we also want to be able to run it using ros2 launch...
Let’s get to work right away and solve this problem!
Step 1: Get the package without a launch file
Remember we have a package…get a copy of the ROSject containing this package using this link: http://www.rosject.io/l/bce4ffd/.
If you would like to learn how we created this ROSject and package, please see this post: https://www.theconstruct.ai/ros2-tutorials-create-a-ros2-package-cpp/.
Step 2: Recompile the package
Open the ROSject. This might take a few moments, please be patient.
Once the ROSject opens, fire up a Shell from the Tools menu and run the following commands to recompile the package.
user:~$ source /opt/ros/crystal/setup.bash ROS_DISTRO was set to 'melodic' before. Please make sure that the environment does not mix paths from different distributions. user:~$ cd ros2_ws user:~/ros2_ws$ colcon build --symlink-install Starting >>> ros2_cpp_pkg Finished <<< ros2_cpp_pkg [21.9s] Summary: 1 package finished [22.0s] user:~/ros2_ws$ source install/setup.bash # source the workspace ROS_DISTRO was set to 'crystal' before. Please make sure that the environment does not mix paths from different distributions. ROS_DISTRO was set to 'melodic' before. Please make sure that the environment does not mix paths from different distributions. user:~/ros2_ws$
Step 3: Create a ROS2 launch file for this package
Create a launch directory within the package and create a launch file within the directory.
On the Shell, run:
user:~/ros2_ws$ cd src/ros2_cpp_pkg/ user:~/ros2_ws/src/ros2_cpp_pkg$ mkdir launch user:~/ros2_ws/src/ros2_cpp_pkg$ cd launch && touch ros2_cpp_code.launch.py user:~/ros2_ws/src/ros2_cpp_pkg$
Fire up an IDE from the Tools menu, find the launch file just created and paste the following code into it:
"""Launch the cpp_code executable in this package"""
from launch import LaunchDescription
import launch_ros.actions
def generate_launch_description():
return LaunchDescription([
launch_ros.actions.Node(
# the name of the executable is set in CMakeLists.txt, towards the end of
# the file, in add_executable(...) and the directives following it
package='ros2_cpp_pkg', node_executable='cpp_code', output='screen'),
])
Now, we need to tell ROS to recognize the launch file. Open up CMakeLists.txt in the IDE and add the following lines at the bottom of it.
# install the launch directory
install(DIRECTORY
launch
DESTINATION share/${PROJECT_NAME}/
)
Don’t forget to save all the changes!!
Step 4: Compile and source the workspace again.
Again? Yes, again! We need to.
user:~/ros2_ws/src/ros2_cpp_pkg/launch$ cd /home/user/ros2_ws user:~/ros2_ws$ colcon build --symlink-install Starting >>> ros2_cpp_pkg Finished <<< ros2_cpp_pkg [3.35s] Summary: 1 package finished [3.48s] user:~/ros2_ws$ source install/setup.bash # source the workspace user:~/ros2_ws$
Step 5: Launch the package using the new launch file and be happy!
You’ve been working very hard, time to eat the fruit of your labor!
The format for the command is ros2 launch <package_name> <launch_file_name>.
We are launching the C++ code in src/ros2_cpp_code.cpp, and the output should be something like this:
user:~/ros2_ws$ ros2 launch ros2_cpp_pkg ros2_cpp_code.launch.py [INFO] [launch]: process[cpp_code-1]: started with pid [13071] [INFO] [ObiWan]: Help me Obi-Wan Kenobi, you're my only hope [INFO] [launch]: process[cpp_code-1]: process has finished cleanly user:~/ros2_ws$
Sweet! Done!!
Extra 1: ROSject link
Get the ROSject containing all code used in the post in the following link: http://www.rosject.io/l/bcf8985/.
Extra 2: ROS2 Full Course for Beginners
Extra 3: Video
Prefer to watch a video demonstrating the steps above? We have one for you below!
Related Resources
ROS2 Basics
Feedback
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If you want to learn about other ROS topics, please let us know in the comments area and we will do a video/post about it 🙂
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