Why launch files automate startup in ROS - See It in Action

Why launch files automate startup

📖 Scenario: You are working with a robot that needs several programs (nodes) to start together. Manually starting each program one by one is slow and can cause mistakes.

🎯 Goal: Build a simple ROS launch file that starts two nodes automatically, so the robot programs run together with one command.

📋 What You'll Learn

Create a launch file named robot_start.launch

Include two nodes: talker and listener

Set the package for both nodes to rospy_tutorials

Use the launch tag as the root element

Use the node tag to define each node

💡 Why This Matters

🌍 Real World

Robots often need many programs running together. Launch files let engineers start all needed programs quickly and correctly.

💼 Career

Knowing how to write launch files is essential for robotics developers to manage complex robot systems efficiently.

1

Create the launch file skeleton

Create a file named robot_start.launch and write the root <launch> tag to start the file.

ROS

Hint

The launch file must start with <launch> and end with </launch>.

2

Add the talker node

Inside the <launch> tags, add a <node> tag for the talker node with pkg="rospy_tutorials" and type="talker.py".

ROS

Hint

Use the node tag with attributes pkg, type, and name.

3

Add the listener node

Add another <node> tag inside <launch> for the listener node with pkg="rospy_tutorials" and type="listener.py".

ROS

Hint

Remember to add the listener node with the same attributes but different type and name.

4

Complete the launch file

Ensure the launch file has both talker and listener nodes inside the <launch> tags and save it as robot_start.launch.

ROS

Hint

Check that both nodes are inside the launch tags and the file is saved correctly.

Practice

1. Why do ROS launch files help automate the startup process of a robot system?

easy

A. They automatically fix errors in the robot's hardware.

B. They replace the need for writing any code in ROS.

C. They start multiple nodes and set parameters together with one command.

D. They only work for simulation, not real robots.

Solution

1. Step 1: Understand what launch files do

   Launch files group many commands to start nodes and set parameters in one place.

2. Step 2: Compare options to this function

   Only They start multiple nodes and set parameters together with one command. correctly describes this automation; others are incorrect or unrelated.

3. Final Answer:

   They start multiple nodes and set parameters together with one command. -> Option C

4. Quick Check:

   Launch files automate startup = A [OK]

Hint: Launch files bundle commands to run nodes and set parameters [OK]

Common Mistakes:

• Thinking launch files replace all coding
• Believing launch files fix hardware issues
• Assuming launch files only work in simulation

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

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall ROS launch file syntax for nodes

   The correct tag is <node> with attributes pkg, type, and name.

2. Step 2: Check each option's correctness

   uses correct tags and attributes; others use wrong tags or attribute names.

3. Final Answer:

   <node pkg="turtlesim" type="turtlesim_node" name="sim" /> -> Option A

4. Quick Check:

   Correct node tag and attributes = A [OK]

Hint: Use syntax in launch files [OK]

Common Mistakes:

• Using <launch> instead of <node> for nodes
• Wrong attribute names like package or executable
• Using <start> tag which does not exist

3. Given this launch file snippet:

<launch>
  <node pkg=\"demo_pkg\" type=\"demo_node\" name=\"demo\" />
  <param name=\"speed\" value=\"5\" />
</launch>

What happens when you run this launch file?

medium

A. The demo_node starts with parameter speed set to 5.

B. Only the parameter speed is set; no nodes start.

C. The launch file will cause a syntax error and not run.

D. The demo_node starts but speed parameter remains default.

Solution

1. Step 1: Analyze the launch file content

   It starts demo_node from demo_pkg with name demo and sets parameter speed to 5.

2. Step 2: Understand launch file behavior

   Both node start and parameter setting happen together when launched.

3. Final Answer:

   The demo_node starts with parameter speed set to 5. -> Option A

4. Quick Check:

   Node runs + param set = D [OK]

Hint: Launch files run nodes and set parameters simultaneously [OK]

Common Mistakes:

• Thinking parameters don't apply to nodes
• Assuming syntax error due to param tag
• Believing node starts without parameters set

4. You wrote this launch file but it fails to start nodes:

<launch>
  <node pkg=\"my_pkg\" exec=\"my_node\" name=\"node1\" />
</launch>

What is the error and how to fix it?

medium

A. Node name cannot be 'node1'; use 'node' instead.

B. Attribute 'exec' is wrong; use 'type' instead.

C. Missing closing tag for <launch>.

D. Package name 'my_pkg' is invalid; must be 'pkg'.

Solution

1. Step 1: Identify incorrect attribute in node tag

   The attribute 'exec' is not valid; ROS launch files use 'type' to specify executable.

2. Step 2: Correct the attribute to fix the launch file

   Replace 'exec' with 'type' to properly start the node.

3. Final Answer:

   Attribute 'exec' is wrong; use 'type' instead. -> Option B

4. Quick Check:

   Use 'type' attribute for node executable = C [OK]

Hint: Use 'type' not 'exec' for node executable in launch files [OK]

Common Mistakes:

• Confusing 'exec' with 'type' attribute
• Forgetting to close launch tags
• Changing node names unnecessarily

5. You want to start two nodes and set a shared parameter using one launch file. Which approach best automates this startup?

hard

A. Start each node manually in separate terminals and set parameters individually.

B. Use a script to start nodes but set parameters only after nodes run.

C. Write separate launch files for each node and run them one after another.

D. Create a launch file that includes both nodes and sets the parameter once.

Solution

1. Step 1: Understand launch file purpose

   Launch files automate starting multiple nodes and setting parameters together.

2. Step 2: Evaluate options for automation

   Create a launch file that includes both nodes and sets the parameter once. uses one launch file to start both nodes and set parameter once, saving time and avoiding errors.

3. Step 3: Compare with other options

   Options A, C, and D require manual steps or multiple files, less efficient and error-prone.

4. Final Answer:

   Create a launch file that includes both nodes and sets the parameter once. -> Option D

5. Quick Check:

   One launch file for all nodes and params = B [OK]

Hint: Use one launch file to start all nodes and set shared params [OK]

Common Mistakes:

• Running nodes manually instead of using launch files
• Splitting nodes into multiple launch files unnecessarily
• Setting parameters after nodes start causing delays