Chapter-12.2-Launch-Files-(Python).md

Chapter 12.2. Launch Files (Python)

Up to this point, we have been running all of our ROS2 Python nodes independently — with each node needing its own dedicated terminal to run. We have also had to open extra terminals for actions such as — setting the value of a parameter.

Launch File is a feature in ROS2 that allows us to launch multiple nodes as well as additional independent terminal commands - all at the same time - from a single terminal - using one single bash command.

In this lesson, we will create a launch file to launch our rpm_publisher.py node along with some additional independent bash commands.

1. Open your ros2_py_udemy_tutorial workspace folder in VS Code.

2. Create a new folder named launch inside your udemy_ros2_pkg package folder.

   

3. Right-click on launch → New File → Name it rpm_pub_node.launch.py — this is a python file (.py) and inside this file, we are gonna add some python code. (The .launch part is added by convention as a tribute to the ROS1 launch files which ended with .launch . You can also name this file as rpm_pub_node_launch.py and that is perfectly acceptable.)

   

4. Add the following code to the newly created rpm_pub_node.launch.py file:

   # Necessary Imports 
   from launch import LaunchDescription
   from launch_ros.actions import Node     # This node class is different from the one we 
                                           # use for making publisher-subscriber nodes. 
   from launch.actions import ExecuteProcess
   
   # Below function is used by 'ros2 launch' command while running this launch file.
   def generate_launch_description():
       
       # This function returns a LaunchDescription object 
       # LaunchDescription object takes a list as its constructor argument.
       # This list consists of all the nodes and independent terminal commands that we want to 
       # run at once with this launch file.
       return LaunchDescription([
           
           # Details of an indivisual node we want to run 
           # are provided as arguments inside the Node object.
           # This 'Node' class is different from the one we 
           # use for making publisher-subscriber nodes.
           Node(
               package="udemy_ros2_pkg",                       # name of the package
               executable="rpm_publisher.py",                  # name of the executable
               name="rpm_pub_node",                            # name of the node
               # parameters=[ {"param_name":param_value} ]     # Setting value(s) for any parameter(s) of this node      
           ),
           
           # An ExecuteProcess object takes an Independent Teminal Command
           # that we want to run with this launch file.
           ExecuteProcess(
               cmd=["ros2", "topic", "list"],   # command : ros2 topic list
               output="screen"                 # Here we are telling to show the output 
                                               # of this command on the screen
           )
       ])

5. Save the files and head over to the CMakeLists.txt file.

6. Add the following code to the CMakeLists.txt file :

   cmake_minimum_required(VERSION 3.8)
   project(udemy_ros2_pkg)
   
   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)
   # Adding the below 2 dependencies for configuring our python 
   # scripts stored inside /scripts folder into this package.
   find_package(ament_cmake_python REQUIRED) 
   find_package(rclpy REQUIRED)
   
   ament_python_install_package(scripts)
   # The above line of code is used to specify that our package contains Python scripts
   # inside a "Python Package Folder" named "scripts". 
   # It is a command provided by the ROS2 build system (ament) to configure 
   # the installation of Python packages.
   # A package folder is simply a python files folder containing a __init__.py file inside it.
   # Make sure that the "scripts" folder has atleast a blank file named __init__.py inside it. 
   
   # Specifying our python scripts.
   install(PROGRAMS
     scripts/publisher.py
     scripts/subscriber.py
     scripts/rpm_publisher.py
     scripts/rpm_subscriber.py
     DESTINATION lib/${PROJECT_NAME}
   )
   
   install(DIRECTORY
           launch
           DESTINATION share/${PROJECT_NAME}/
   )
   
   if(BUILD_TESTING)
     find_package(ament_lint_auto REQUIRED)
     # the following line skips the linter which checks for copyrights
     # comment the line when a copyright and license is added to all source files
     set(ament_cmake_copyright_FOUND TRUE)
     # the following line skips cpplint (only works in a git repo)
     # comment the line when this package is in a git repo and when
     # a copyright and license is added to all source files
     set(ament_cmake_cpplint_FOUND TRUE)
     ament_lint_auto_find_test_dependencies()
   endif()
   
   ament_package()

7. Save all the files and rebuild the workspace.

8. To run the launch file from the terminal → Open a new terminal in the workspace folder and run the following bash commands:

   source install/setup.bash
   # ros2 launch <package_name> <file_name>
   ros2 launch udemy_ros2_pkg rpm_pub_node.launch.py

   

   Observe the highlighted portion of the output below:

   

   The [INFO] [rpm_publisher.py-1] message are states that rpm_publisher.py node has been started successfully.

   Next in the queue - [INFO] [ros2-2] states that the ros2 topic list command has been ran successfully.

   Next in the queue, there are two [ros2-2] messages which give the output the ros2 topic list command.

   And finally, the last [INFO][ros2-2] message states that the second activity has completed successfully.

9. To check if our rpm_publisher node is running properly - run the following commands from a new terminal:

   #Checking the list of active ros2 nodes
   ros2 node list
   
   #Checking the list of active ros2 topics
   ros2 topic list
   
   #To see the messages published by rpm_publisher node through rpm topic
   ros2 topic echo rpm