Launch file XML syntax in ROS - Mini Project: Build & Apply

Launch file XML syntax

📖 Scenario: You are setting up a ROS (Robot Operating System) launch file to start nodes for a simple robot application.

🎯 Goal: Create a basic ROS launch file using XML syntax that defines a node to be launched with specific parameters.

📋 What You'll Learn

Create the root tag

Add a tag with the exact attributes: pkg="turtlesim", type="turtlesim_node", name="simulator"

Add a tag inside the to set the parameter background_r to 255

Close all XML tags properly

💡 Why This Matters

🌍 Real World

ROS launch files are used to start multiple robot software nodes with specific configurations easily.

💼 Career

Understanding launch file syntax is essential for robotics engineers and developers working with ROS to automate robot system startup.

1

Create the root <launch> tag

Write the opening <launch> tag to start the ROS launch file.

ROS

Hint

The root tag for a ROS launch file is <launch>.

2

Add a <node> tag with attributes

Inside the <launch> tag, add a <node> tag with attributes pkg="turtlesim", type="turtlesim_node", and name="simulator".

ROS

Hint

Use the <node> tag with the exact attributes inside <launch>.

3

Add a <param> tag inside the <node>

Inside the <node> tag, add a <param> tag with attributes name="background_r" and value="255".

ROS

Hint

The <param> tag sets parameters for the node. Use the exact attribute names and values.

4

Close all XML tags properly

Close the <node> and <launch> tags to complete the XML structure.

ROS

Hint

Every opening tag must have a matching closing tag in XML.

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