ROS 2 Lyrical Luth · Lesson 08 of 86

Message types: standard and custom

std_msgs, sensor_msgs, geometry_msgs, writing .msg files and building a custom interface package.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain message types: standard and custom in your own words
Use it in a small robotics example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

Every topic has a message type that defines its fields. Use the standard ones first: std_msgs (numbers, strings), geometry_msgs (Pose, Twist), sensor_msgs (LaserScan, Image, Imu). Look up a type with ros2 interface show.

When no standard type fits, define your own in a .msg file inside a dedicated interface package built with rosidl_default_generators. Keep interfaces in their own package so every other package can depend on them without circular dependencies.

Guided lesson

Work through these steps

1.
Source the ROS 2 Lyrical environment in a fresh terminal.
2.
Read the message types: standard and custom example and identify what each line does.
3.
Build only the affected package, source the workspace overlay, and run the example.
4.
Inspect the result from a second terminal with the ros2 CLI or RViz2 and compare it with the expected result.

Worked example

Read it before you run it

ros2 interface show geometry_msgs/msg/Twist
# Print the fields of a standard message (linear and angular velocity).
ros2 pkg create --build-type ament_cmake robot_interfaces
# Create a package dedicated to interface definitions.
mkdir robot_interfaces/msg
# Message files live in msg/.
printf 'builtin_interfaces/Time stamp\nuint8 mode\nuint16 fault_code\n' > robot_interfaces/msg/RobotStatus.msg
# Define RobotStatus with a timestamp, a mode and a fault code.
# In CMakeLists.txt: find_package(rosidl_default_generators REQUIRED)
# Generator dependency for C++ and Python code.
# rosidl_generate_interfaces(${PROJECT_NAME} "msg/RobotStatus.msg" DEPENDENCIES builtin_interfaces)
# Generate the language bindings for the file.
ros2 interface show robot_interfaces/msg/RobotStatus
# After building, confirm the new type exists.

What the example is doing

1.The ros2 commands query the running graph; they do not prove that a node's internal behavior is correct.
2.Build from the workspace root, then source install/setup.bash so the shell can discover the new package.
3.Use a second sourced terminal to inspect nodes and interfaces while the example runs.

Watch it run · Messages between nodes

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Copy the starter into your ROS 2 package, replace every TODO, then build with colcon build and run it from a sourced terminal.

# robot_interfaces/msg/RobotStatus.msg
builtin_interfaces/Time stamp
# TODO: add a uint8 field named mode and a uint16 field named fault_code

Expected output

uint8 mode
uint16 fault_code

Stuck? Show the full solution (try it yourself first)

# robot_interfaces/msg/RobotStatus.msg
builtin_interfaces/Time stamp
uint8 mode
uint16 fault_code

Expected result

How you know it worked

ROS_DISTRO is lyrical in every terminal used for the exercise.
The complete package from the exercise builds and the expected graph entity appears in a second terminal.

Practical exercise

Apply the lesson

Create robot_interfaces with a RobotStatus message (stamp, mode, fault_code). Publish it from C++ and echo it with the CLI. Then add a field, rebuild, and note which packages had to rebuild.

Before you continue

ROS_DISTRO reports lyrical in every terminal you used
The example builds and runs without errors
You checked the result with a ros2 command, RViz2 or a test, not just by reading the code

If something goes wrong

ros2 cannot find a package or executable.

Source /opt/ros/lyrical/setup.bash, build from the workspace root, then source install/setup.bash in that terminal.

Nodes run but cannot discover each other.

Confirm both terminals use Lyrical, the same ROS_DOMAIN_ID, and compatible network/discovery settings.