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📘 ROS 2 Basics & Messaging — Study Notes

A structured guide to understanding ROS 2 (Humble) fundamentals — including nodes, topics, publishers/subscribers, remapping, and Docker setup — organized in the recommended order of learning.

1️⃣ ROS 2 Installation & Environment Setup

Installation (Ubuntu 22.04)

sudo apt update && sudo apt upgrade
sudo apt install ros-humble-desktop

Environment Setup

source /opt/ros/humble/setup.bash
# Make permanent
echo "source /opt/ros/humble/setup.bash" >> ~/.bashrc

Verify Installation

ros2 --version
ros2 run demo_nodes_cpp talker

2️⃣ Understanding Nodes

What is a Node?

A Node is an independent process that performs computation in a ROS 2 graph. Nodes communicate through topics, services, and actions.

Publisher Node  --->  Topic  --->  Subscriber Node

Each node can:

  • Publish data (e.g., from a sensor)
  • Subscribe to data (e.g., to control a motor)
  • Provide or call services
  • Perform tasks in coordination with others

🔗 Related diagrams:

3️⃣ Topics & Messaging System

Listing and Inspecting Topics

ros2 topic list             # show all active topics
ros2 topic info /topic_name # show details (type, QoS, publishers, subscribers)

Viewing Topic Data

ros2 topic echo /topic_name # view published messages

Publishing Messages

ros2 topic pub /topic_name <msg_type> '{data}'
ros2 topic pub -r 1 /topic_name <msg_type> '{data}'  # publish at 1 Hz

Monitoring Topics

ros2 topic hz /topic_name  # check publishing frequency
ros2 topic bw /topic_name  # check bandwidth usage
ros2 topic find <type>     # find topics of a specific message type

Example — Check topic rate and bandwidth

ros2 topic hz /turtle1/pose
ros2 topic bw /turtle1/pose

4️⃣ Remapping (2.1)

Remapping lets you change default node or topic names at runtime without editing source code.

Example — renaming a node:

ros2 run turtlesim turtlesim_node --ros-args --remap __node:=my_turtle

Now, running ros2 node list shows:

/my_turtle
/turtlesim
/teleop_turtle

Remapping is useful when running multiple instances of the same node or avoiding naming conflicts.

5️⃣ Debugging & Visualization

  • ros2 node list — list all running nodes
  • ros2 node info <node> — get details about a node
  • rqt_graph — visualize node-topic connections
  • ros2 topic echo /topic_name — observe message data
  • ros2 topic hz /topic_name — verify publishing rate

These tools are key for debugging publisher/subscriber connectivity.

6️⃣ Using Docker with ROS 2

Common Docker Commands

docker ps -a                 # list containers
docker start <name>          # start container
docker stop <name>           # stop container
docker compose down          # remove containers
docker run -it osrf/ros:humble-desktop # interactive run

Run ROS 2 GUI Apps (with X11)

docker run -it --rm --net=host \
  --env="DISPLAY" \
  --volume="$HOME/.Xauthority:/root/.Xauthority:rw" \
  osrf/ros:humble-desktop

Add --gpus all for GPU support if available.

7️⃣ Clean Up & Exit

Stop all running nodes:

Ctrl + C  # in each terminal

Stop Docker containers:

docker compose down
docker stop <container_name>

🔗 Helpful References