Launch Files¶

In this exercise, we will explore starting groups of nodes at once with launch files.

Motivation¶

The ROS architecture encourages engineers to use ‘’nodes’’ as a fundamental unit of organization in their systems, and applications can quickly grow to require many nodes to operate. Opening a new terminal and running each node individually quickly becomes unfeasible. It’d be nice to have a tool to bring up groups of nodes at once. ROS ‘’launch’’ files are one such tool.

Reference Example¶

Launching a ROS System

Further Information and Resources¶

ROS2 python launch Node action

Scan-N-Plan Application: Problem Statement¶

In this exercise, you will:

Create a new package, myworkcell_support.
Create a directory in this package called launch.
Create a file inside this directory called workcell.launch that:
1. Launches fake_ar_publisher
2. Launches vision_node

You may also choose to launch myworkcell_core node with the others or keep it separate. We often configure systems with two main launch files. In this example, fake_ar_publisher and vision_node are “environment nodes”, while myworkcell_node is an “application” node.

“Environment” Launch File - driver/planning nodes, config data, etc.
“Application” Launch File - executes a sequence of actions for a particular application.

Scan-N-Plan Application: Guidance¶

In your workspace, create the new package myworkcell_support with a dependency on myworkcell_core. Rebuild and source the workspace so that ROS can find the new package:
```
cd ~/ros2_ws/src
ros2 pkg create myworkcell_support --dependencies myworkcell_core
cd ~/ros2_ws
colcon build
source ~/ros2_ws/install/setup.bash
```
Create a directory for launch files (inside the new myworkcell_support package):
```
cd src/myworkcell_support
mkdir launch
```

Create a new file, workcell.launch.py (inside the launch directory) with the following skeleton:

import launch
import launch_ros

def generate_launch_description():
    return launch.LaunchDescription([
        # launch actions here...
    ])

In the space marked by the comment about ‘launch actions’, insert lines to bring up the nodes outlined in the problem statement. See the reference documentation for more information:

launch_ros.actions.Node(
    node_name='fake_ar_publisher_node',
    package='fake_ar_publisher',
    node_executable='fake_ar_publisher_node',
),
launch_ros.actions.Node(
    node_name='vision_node',
    package='myworkcell_core',
    node_executable='vision_node',
),

There are other options you can set for these Node actions; node_name, package, and node_executable are the required ones.

Try to run the launch file:
```
ros2 launch myworkcell_support workcell.launch.py
```
You should see an error that the launch file was not found. This is because it only exists in the src/ directory, and the ros2 tool will only work with files in the install/ directory. Therefore, the launch file must be installed.
Add an installation rule to CMakeLists.txt, after the find_package section, and before the BUILD_TESTING section:
```
install(DIRECTORY launch DESTINATION share/${PROJECT_NAME}/)
```
Now build the workspace to install the launch file and try to run it again:
```
colcon build
ros2 launch myworkcell_support workcell.launch.py
```
Note: Both nodes were automatically started. Press Ctrl+C to close all nodes started by the launch file.
Notice that none of the usual messages were printed to the console window. Launch files will suppress console output below the ERROR severity level by default. To restore normal text output, add the output='screen' argument to each of the nodes in your launch files:
```
launch_ros.actions.Node(
    node_name='fake_ar_publisher_node',
    package='fake_ar_publisher',
    node_executable='fake_ar_publisher_node',
    output='screen',
),
launch_ros.actions.Node(
    node_name='vision_node',
    package='myworkcell_core',
    node_executable='vision_node',
    output='screen',
),
```
- Without the output='screen' argument the logging statements are redirected to a log file for each launch file run.
- Important: Remember that ros2 works on the launch file in the install/ directory and so you won’t see the new behavior without running colcon build again to reinstall the file. To avoid having to do this for every change during development, you can run colcon build with a --symlink-install option which will install a link to the file in the src/ directory so any changes will be seen immediately.