Launch file XML syntax in ROS

Introduction

Launch files help start multiple parts of a robot system easily. They use XML to organize what runs and how.

You want to start several robot programs together with one command.

You need to set parameters for robot parts before running them.

You want to include other launch files inside one main file.

You want to control the order in which robot nodes start.

You want to pass arguments to robot programs when launching.

Syntax

ROS

<launch>
  <node pkg="package_name" type="executable_name" name="node_name" output="screen">
    <param name="param_name" value="param_value" />
  </node>
  <include file="other_launch_file.launch" />
  <arg name="arg_name" default="default_value" />
</launch>

The root tag is always <launch> which wraps all commands.

Each <node> tag runs a program from a package with a name and output option.

Examples

This starts the turtlesim program with the name 'sim' and shows output on screen.

ROS

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim" output="screen" />
</launch>

This defines an argument 'speed' and passes it as a parameter to the move_node program.

ROS

<launch>
  <arg name="speed" default="2.0" />
  <node pkg="my_robot" type="move_node" name="mover" output="screen">
    <param name="speed" value="$(arg speed)" />
  </node>
</launch>

This includes another launch file named bringup.launch to run its contents here.

ROS

<launch>
  <include file="bringup.launch" />
</launch>

Sample Program

This launch file starts the demo_node from demo_pkg. It uses an argument 'robot_name' with a default value. The node receives this as a parameter named 'robot'.

ROS

<launch>
  <arg name="robot_name" default="my_robot" />
  <node pkg="demo_pkg" type="demo_node" name="demo" output="screen">
    <param name="robot" value="$(arg robot_name)" />
  </node>
</launch>

Important Notes

Use output="screen" to see program messages in the terminal.

Arguments let you customize launch files without changing the XML.

Include other launch files to organize complex robot systems.

Summary

Launch files use XML to start and configure robot programs.

They include nodes, parameters, arguments, and other launch files.

This helps run many parts of a robot system easily and clearly.

Practice

1. What is the main purpose of a <launch> file in ROS?

easy

A. To compile the robot's source code

B. To write the robot's main program logic

C. To store sensor data during robot operation

D. To start and configure multiple robot nodes easily

Solution

1. Step 1: Understand the role of launch files

   Launch files in ROS are XML files used to start and configure multiple nodes and parameters together.

2. Step 2: Compare options with launch file purpose

   Only To start and configure multiple robot nodes easily correctly describes this purpose. Other options describe unrelated tasks.

3. Final Answer:

   To start and configure multiple robot nodes easily -> Option D

4. Quick Check:

   Launch files start nodes = A [OK]

Hint: Launch files group node startups in XML [OK]

Common Mistakes:

• Confusing launch files with program code
• Thinking launch files store data
• Assuming launch files compile code

2. Which of the following is the correct way to declare a node in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall correct node tag attributes

   The correct attributes are pkg, type, and name inside a <node> tag.

2. Step 2: Match options with correct syntax

   Only <node pkg="my_pkg" type="my_node" name="node1" /> uses the correct tag and attribute names. Others use wrong attribute names or tags.

3. Final Answer:

   <node pkg="my_pkg" type="my_node" name="node1" /> -> Option B

4. Quick Check:

   Correct node syntax = A [OK]

Hint: Use pkg, type, name in <node> tag [OK]

Common Mistakes:

• Using wrong attribute names like package or exec
• Using incorrect tag names like launch_node
• Confusing name with id or label

3. Given this launch file snippet, what will happen when it runs?

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim" />
  <param name="background_r" value="255" />
</launch>

medium

A. Starts the turtlesim node with a red background color

B. Fails because <param> is outside <node>

C. Starts the turtlesim node with default background color

D. Starts a node named 'background_r' with value 255

Solution

1. Step 1: Understand node and param placement

   The <param> tag outside <node> sets a global parameter, not specific to the node.

2. Step 2: Check if turtlesim uses this param

   The parameter background_r is not recognized globally by turtlesim, so it uses default background color.

3. Final Answer:

   Starts the turtlesim node with default background color -> Option C

4. Quick Check:

   Param outside node is global but unused = C [OK]

Hint: Params outside nodes are global, may not affect node [OK]

Common Mistakes:

• Assuming param inside launch affects node color
• Thinking param outside node causes error
• Confusing param tag as node

4. Identify the error in this launch file snippet:

<launch>
  <node pkg="demo_pkg" type="demo_node" name="demo">
</launch>

medium

A. Missing closing tag for <node>

B. Incorrect attribute name 'type'

C. Missing 'pkg' attribute in <node>

D. Extra closing tag for <launch>

Solution

1. Step 1: Check XML tag completeness

   The <node> tag is opened but not closed properly (no closing tag or self-closing slash).

2. Step 2: Verify other attributes and tags

   Attributes 'pkg' and 'type' are correct. The <launch> tag is properly closed.

3. Final Answer:

   Missing closing tag for <node> -> Option A

4. Quick Check:

   Unclosed node tag = D [OK]

Hint: Every opened tag must be closed or self-closed [OK]

Common Mistakes:

• Forgetting to close <node> tag
• Confusing attribute names
• Miscounting launch tag closures

5. How can you include another launch file named camera.launch inside your main launch file to start its nodes as well?

hard

A.

B.

C.

D.

Solution

1. Step 1: Recall correct include syntax in ROS launch files

   The <include> tag uses the attribute file to specify the file path.

2. Step 2: Understand file path usage with $(find package)

   To locate a launch file inside a package, use $(find package_name)/launch/filename.launch with file attribute.

3. Step 3: Compare options

   Only <include file="$(find package_name)/launch/camera.launch" /> uses the correct attribute file and the proper path syntax with $(find ...).

4. Final Answer:

   <include file="$(find package_name)/launch/camera.launch" /> -> Option A

5. Quick Check:

   Include uses file and $(find) = C [OK]

Hint: Use [OK]

Common Mistakes:

• Using 'launchfile' instead of 'file' attribute
• Not using $(find package_name) for path
• Omitting include tag or wrong syntax