Launch file XML syntax in ROS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a ROS launch file?

A ROS launch file is used to start multiple ROS nodes and set parameters in one place, making it easier to manage complex robot systems.

beginner

What is the root element of a ROS launch file written in XML?

The root element is <launch>. It wraps all other elements like nodes, parameters, and includes.

beginner

How do you define a node in a ROS launch file XML?

Use the <node> tag with attributes like name, pkg (package), and type (executable). Example: <node name="talker" pkg="rospy_tutorials" type="talker"/>

beginner

What is the use of the <param> tag in a ROS launch file?

The <param> tag sets parameters on the ROS parameter server. You can specify name and value to configure nodes.

intermediate

How can you include another launch file inside a ROS launch file?

Use the <include> tag with the 'file' attribute pointing to the other launch file's path. This helps organize launch files modularly.

Which XML tag is used to start a ROS node in a launch file?

A <node>

B <start>

C <process>

D <run>

The tag is used to define and start a ROS node in a launch file.

What attribute specifies the package of a node in a ROS launch file?

A pkg

B type

C package

D name

The 'pkg' attribute specifies the ROS package where the node executable is located.

How do you set a parameter named 'speed' with value '5' in a launch file?

A <set name="speed" value="5"/>

B <param speed="5"/>

C <param name="speed" value="5"/>

D <value name="speed">5</value>

The tag with name and value attributes sets parameters on the ROS parameter server.

Which tag allows you to reuse another launch file inside your launch file?

A <use>

B <import>

C <load>

D <include>

The tag is used to include another launch file.

What is the root element of a ROS launch file?

A <roslaunch>

B <launch>

C <root>

D <start>

The root element of a ROS launch file is .

Describe the structure of a basic ROS launch file using XML tags.

Think about how you start nodes and set parameters inside the launch file.

Explain how you would organize multiple nodes and parameters in a ROS launch file for a robot system.

Consider how to keep the launch file clean and reusable.

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