Lab: Integrating Launch Files into a ROS 2 Package

turtle_bringup — RA305 Robotics Programming

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Overview

By the end of this lab you will be able to:

• Create a minimal ament_python ROS 2 package that contains only launch files (no executable nodes)
• Build and install the package with colcon
• Launch the full turtlesim system with a single ros2 launch command
• Declare and override launch arguments from the command line

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Package Structure

After creating and populating the package, your workspace should look like this:

turtle_ws/
└── src/
    └── turtle_bringup/
        ├── launch/
        │   ├── turtle_bringup.launch.py          ← Part 1
        │   └── turtle_bringup_args.launch.py     ← Part 2
        ├── resource/
        │   └── turtle_bringup                    ← empty marker file (required)
        ├── turtle_bringup/
        │   └── __init__.py                       ← empty (required by Python)
        ├── package.xml
        ├── setup.cfg
        └── setup.py

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Prerequisites

Install xterm (needed for the teleop terminal window):

sudo apt install xterm

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Part 1 — Get the Package

Create your workspace folder and clone the repository directly into the src directory. All files, except for the launch file we just created, are already in place.

Step 1: Create the workspace and src directory

mkdir -p ~/turtle_ws/src

Step 2: Navigate into the src directory

cd ~/turtle_ws/src

Step 3: Clone the repository

git clone https://github.com/KSU-Chad/turtle_bringup.git

Step 4: Confirm the structure

ls turtle_bringup/

You should see: package.xml resource/ setup.cfg setup.py turtle_bringup/

ls turtle_bringup/launch/

You should see: turtle_bringup_args.launch.py

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Copy the launch file you created in the previous activity into the 'turtle_ws/src/turtle_bringup/launch folder'

cd
mv launch/turtle_bringup.launch.py turtle_ws/src/turtle_bringup/launch/

Part 2 — Build and Source

Step 5: Build the package

cd ~/turtle_ws
colcon build

You should see output ending in:

Starting >>> turtle_bringup
Finished <<< turtle_bringup [...]
Summary: 1 package finished [...]

Step 6: Source the install directory

This step is required every time you open a new terminal after building.

source ~/turtle_ws/install/setup.bash

Common mistake: Forgetting to source after building. If ros2 launch reports "Package 'turtle_bringup' not found", you need to source.

Step 7: Verify the package is visible

ros2 pkg list | grep turtle_bringup

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Part 3 — Launch File 1: Hardcoded Parameters

Step 7: Launch the system

ros2 launch turtle_bringup turtle_bringup.launch.py

You should see:

• The turtlesim window open with a blue background
• A separate xterm window open with the teleop node running

Click the xterm window to give it keyboard focus, then use arrow keys to drive the turtle.

Step 8: Inspect the running system

In a new terminal (remember to source first):

# How many nodes are running?
ros2 node list

# What topics exist?
ros2 topic list

# Confirm the background parameters were applied
ros2 param get /sim background_r
ros2 param get /sim background_g
ros2 param get /sim background_b

Questions to answer in your lab notebook:

1. Run ros2 node list while the simulation is running. Write down the node names you see. In the launch file, find the name= argument for each node — what does changing that argument appear to do to the node name?
2. Run ros2 param get /sim background_b. Write down the value. Now look at the launch file where background_b is set. Do the two numbers match?

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Part 4 — Launch File 2: Command-Line Arguments

Step 9: View available arguments without launching

ros2 launch turtle_bringup turtle_bringup_args.launch.py --show-args

You should see the three arguments (bg_r, bg_g, bg_b) with their defaults and descriptions.

Step 10: Launch with defaults (same blue background)

ros2 launch turtle_bringup turtle_bringup_args.launch.py

Step 11: Launch with a custom color

Try each of these and observe the simulator background:

# Red background
ros2 launch turtle_bringup turtle_bringup_args.launch.py bg_r:=255 bg_g:=0 bg_b:=0

# Green background
ros2 launch turtle_bringup turtle_bringup_args.launch.py bg_r:=0 bg_g:=200 bg_b:=0

# Orange background
ros2 launch turtle_bringup turtle_bringup_args.launch.py bg_r:=255 bg_g:=165 bg_b:=0

Question: What happens if you pass bg_r:=999? (Try it.) Write down what you observe.

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Part 5 — Concepts Check

Answer these in your lab notebook before leaving:

1. After building the package, open a new terminal without sourcing install/setup.bash and try to run ros2 launch turtle_bringup turtle_bringup.launch.py. Write down the exact error message you get.

2. Look at turtle_bringup_args.launch.py. Find the three DeclareLaunchArgument blocks. What is the default_value for bg_b? Now run the launch file without passing any arguments — what color is the background? Do they match?

3. Run ros2 launch turtle_bringup turtle_bringup_args.launch.py --show-args. Write down what the output looks like.

4. Remove prefix='xterm -e' from your local copy of turtle_bringup.launch.py, rebuild, and relaunch. Try using the arrow keys.

5. You used bg_r:=255 bg_g:=0 bg_b:=0 to get a red background. Without running it first, predict what values would give you a purple background. Then try it and see if you were right.

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Troubleshooting

Symptom	Likely Cause	Fix
Package 'turtle_bringup' not found	Haven't sourced install	source ~/turtle_ws/install/setup.bash
No executable found	Launch file not installed	Check data_files in setup.py, rebuild
Teleop window opens but arrow keys do nothing	xterm doesn't have focus	Click the xterm window
xterm: command not found	xterm not installed	sudo apt install xterm
Background stays grey after setting parameters	turtlesim reads bg params on init only	Kill and relaunch — params can't be hot-reloaded on this node

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