Launch Files & Parameters

Introduction

So far, you've run one node at a time:

ros2 run package node_name

But real robots have many nodes running together (sensor drivers, controllers, planning, perception, etc.). Launch files start them all with one command:

ros2 launch package launch_file.launch.xml

This chapter teaches:

• Writing launch files (XML syntax)
• Starting multiple nodes with arguments
• Configurable parameters via launch files
• Organizing your ROS 2 project

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Learning Outcomes

By the end of this chapter, you will:

1. Write launch files in XML
2. Start multiple nodes with one command
3. Pass parameters to nodes via launch files
4. Understand ROS 2 package structure
5. Organize multi-node systems
6. Debug launch file issues

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Part 1: ROS 2 Package Structure

Minimum Package Structure

my_robot_package/
├── package.xml              # Package metadata
├── setup.py                 # Install configuration
├── setup.cfg                # Install settings
├── resource/my_robot_package/
├── my_robot_package/        # Python source code
│   ├── __init__.py
│   ├── robot_node.py        # Your node code
│   └── utils.py
└── launch/                  # Launch files
    └── robot.launch.xml

package.xml Template

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>my_robot_package</name>
  <version>0.0.1</version>
  <description>My robot controller package</description>
  <maintainer email="you@example.com">Your Name</maintainer>
  <license>MIT</license>

  <buildtool_depend>ament_python</buildtool_depend>

  <depend>rclpy</depend>
  <depend>std_msgs</depend>
  <depend>geometry_msgs</depend>

  <test_depend>pytest</test_depend>

  <exec_depend>ros2launch</exec_depend>
</package>

Key fields:

• <name> - package name
• <depend> - packages this depends on
• <exec_depend> - runtime dependencies
• Ensure ROS 2 can find your packages

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Part 2: Simple Launch File

Example 1: Launch a Single Node

File: launch/simple.launch.xml

<?xml version="1.0"?>
<launch>
  <!-- Start a single node -->
  <node pkg="my_robot_package" exec="robot_node.py" name="my_robot"/>
</launch>

Run it:

ros2 launch my_robot_package simple.launch.xml

Example 2: Launch Multiple Nodes

File: launch/multi_node.launch.xml

<?xml version="1.0"?>
<launch>
  <!-- Sensor node -->
  <node pkg="my_robot_package" exec="sensor_node.py" name="sensor">
    <!-- Redirect output to screen (useful for debugging) -->
    <output>screen</output>
  </node>

  <!-- Controller node -->
  <node pkg="my_robot_package" exec="controller_node.py" name="controller">
    <output>screen</output>
  </node>

  <!-- Motor driver node -->
  <node pkg="my_robot_package" exec="motor_node.py" name="motor">
    <output>screen</output>
  </node>
</launch>

Run it:

ros2 launch my_robot_package multi_node.launch.xml

All three nodes start with one command! Ctrl+C stops all of them.

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Part 3: Parameters in Launch Files

Example 3: Pass Parameters to Nodes

File: launch/robot_with_params.launch.xml

<?xml version="1.0"?>
<launch>
  <!-- Define arguments (input to launch file) -->
  <arg name="speed" default="0.5"/>
  <arg name="debug" default="false"/>

  <!-- Start node with parameters -->
  <node pkg="my_robot_package" exec="robot_node.py" name="robot">
    <param name="motor_speed" value="$(var speed)"/>
    <param name="debug_mode" value="$(var debug)"/>
  </node>
</launch>

Run with custom values:

# Use defaults
ros2 launch my_robot_package robot_with_params.launch.xml

# Override arguments
ros2 launch my_robot_package robot_with_params.launch.xml speed:=1.0 debug:=true

Example 4: Parameter File

For complex configurations, use a YAML file:

File: config/robot_params.yaml

robot:
  motor_speed: 0.5
  wheel_diameter: 0.1
  max_speed: 2.0
  debug_mode: false

sensor:
  lidar_range: 10.0
  lidar_update_rate: 10

control:
  loop_rate: 100
  kp: 1.0
  ki: 0.1
  kd: 0.01

File: launch/robot_with_config.launch.xml

<?xml version="1.0"?>
<launch>
  <!-- Load all parameters from YAML file -->
  <node pkg="my_robot_package" exec="robot_node.py" name="robot">
    <param from="$(find-pkg-share my_robot_package)/config/robot_params.yaml"/>
  </node>
</launch>

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Part 4: Advanced Launch Features

Groups and Conditionals

<?xml version="1.0"?>
<launch>
  <arg name="use_sim" default="true"/>
  <arg name="robot_name" default="robot1"/>

  <!-- Conditional: use different nodes based on argument -->
  <group if="$(var use_sim)">
    <!-- Simulation nodes -->
    <node pkg="gazebo_ros" exec="gazebo" args="--verbose" name="gazebo">
      <output>screen</output>
    </node>
    <node pkg="my_robot_package" exec="sim_interface.py" name="interface">
      <output>screen</output>
    </node>
  </group>

  <group unless="$(var use_sim)">
    <!-- Real robot nodes -->
    <node pkg="my_robot_package" exec="hardware_interface.py" name="hardware">
      <output>screen</output>
    </node>
  </group>

  <!-- Common nodes for both paths -->
  <node pkg="my_robot_package" exec="controller.py" name="controller">
    <param name="robot_name" value="$(var robot_name)"/>
  </node>
</launch>

Run with different configurations:

# Use simulation
ros2 launch my_robot_package robot.launch.xml use_sim:=true

# Use real hardware
ros2 launch my_robot_package robot.launch.xml use_sim:=false robot_name:=real_robot_1

Namespaces

Namespaces help organize multiple robots or subsystems:

<?xml version="1.0"?>
<launch>
  <!-- Robot 1 in namespace /robot1 -->
  <group ns="robot1">
    <node pkg="my_robot_package" exec="controller.py" name="controller"/>
    <node pkg="my_robot_package" exec="sensor_node.py" name="sensor"/>
  </group>

  <!-- Robot 2 in namespace /robot2 -->
  <group ns="robot2">
    <node pkg="my_robot_package" exec="controller.py" name="controller"/>
    <node pkg="my_robot_package" exec="sensor_node.py" name="sensor"/>
  </group>
</launch>

Topics are now scoped:

/robot1/sensor/temperature
/robot1/cmd/velocity
/robot2/sensor/temperature
/robot2/cmd/velocity

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Part 5: Complete Launch File Example

<?xml version="1.0"?>
<launch>
  <!-- Arguments -->
  <arg name="robot_name" default="my_robot"/>
  <arg name="simulation" default="true"/>
  <arg name="config_file" default="robot_params.yaml"/>

  <!-- Include another launch file -->
  <include file="$(find-pkg-share my_robot_package)/launch/sensor_stack.launch.xml"/>

  <!-- Load parameters from file -->
  <node pkg="my_robot_package" exec="robot_controller.py" name="controller" output="screen">
    <param name="name" value="$(var robot_name)"/>
    <param from="$(find-pkg-share my_robot_package)/config/$(var config_file)"/>
  </node>

  <!-- Start nodes conditionally -->
  <group if="$(var simulation)">
    <node pkg="gazebo_ros" exec="gazebo" name="gazebo" output="screen"/>
    <node pkg="my_robot_package" exec="sim_bridge.py" name="bridge"/>
  </group>

  <group unless="$(var simulation)">
    <node pkg="my_robot_package" exec="hardware_driver.py" name="hw_driver"/>
  </group>

  <!-- RViz for visualization (optional) -->
  <node pkg="rviz2" exec="rviz2" name="rviz" output="screen">
    <param name="rviz_config" value="$(find-pkg-share my_robot_package)/config/rviz.rviz"/>
  </node>
</launch>

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Part 6: Debugging Launch Files

Common Issues

Error	Cause	Solution
Package not found	Python path wrong	Rebuild: colcon build --symlink-install
Node not found	Wrong executable name	Check setup.py entry_points
Parameter not set	Parameter name mismatch	Use ros2 param list to verify
Port in use	Multiple nodes on same ROS_DOMAIN_ID	Use different ROS_DOMAIN_ID or namespaces

Debug Commands

# Check if launch file syntax is valid
ros2 launch my_robot_package robot.launch.xml --validate

# See what nodes will be started (dry run)
ros2 launch my_robot_package robot.launch.xml --print

# List active nodes after launch
ros2 node list

# Check node parameters
ros2 param list /robot_node

# Check package can be found
ros2 pkg prefix my_robot_package

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Part 7: Best Practices

Launch File Organization

my_robot_package/
├── launch/
│   ├── robot.launch.xml           # Main entry point
│   ├── sensor_stack.launch.xml    # Sensor nodes
│   ├── control_stack.launch.xml   # Control nodes
│   └── sim.launch.xml             # Simulation variant
├── config/
│   ├── robot_params.yaml          # Default parameters
│   ├── robot_params_sim.yaml      # Simulation-specific params
│   └── rviz.rviz                  # RViz configuration
└── my_robot_package/
    ├── robot_node.py
    ├── sensor_node.py
    └── control_node.py

Naming Conventions

✅ Do:

• robot.launch.xml - descriptive names
• motor_speed - lowercase with underscores
• Namespaces for multi-robot: /robot1/sensor

❌ Don't:

• main.launch.xml - too vague
• motorSpeed - inconsistent
• Deeply nested namespaces: /a/b/c/d/e

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Summary

Package Structure:

• package.xml - metadata
• setup.py - Python installation
• launch/ - launch files
• config/ - parameter files

Launch Files:

• XML format
• Start multiple nodes with one command
• Pass parameters to nodes
• Support conditionals and namespaces

Parameters:

• Configure nodes without recompiling
• Via launch file or command line
• Can use YAML files for complex configs

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Next Steps

You've now mastered Module 1 fundamentals! The three labs will cement your knowledge:

1. Lab 1.1: Your first ROS 2 publisher/subscriber
2. Lab 1.2: Implement a service
3. Lab 1.3: Create a complete ROS 2 package

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Navigation

• Previous: Chapter 5: Python with rclpy
• Labs: Lab 1.1: Your First Node