ROS2 Topic Echo: How to Handle Multiple Message Types

You're encountering an issue where the 'ros2 topic echo /scan' command fails because the topic '/scan' carries multiple message types: 'radar_msgs/msg/RadarScan' and 'sensor_msgs/msg/LaserScan'. This guide explains how to handle this situation.

Understanding the Problem

When using 'ros2 topic echo', ROS2 needs to know the specific message type to interpret the data correctly. Since the '/scan' topic publishes multiple types, the command cannot automatically determine which one to display.

Solution: Specifying the Message Type

You can specify the desired message type using the following format:

ros2 topic echo /scan <message_type>

For instance, to view 'sensor_msgs/msg/LaserScan' data:

ros2 topic echo /scan sensor_msgs/msg/LaserScan

To view 'radar_msgs/msg/RadarScan' data:

ros2 topic echo /scan radar_msgs/msg/RadarScan

Additional Notes

• You can use 'ros2 topic info /scan' to get information about the topic, including its message types and publisher/subscriber counts.
• Even if only one node publishes to the topic, it might still publish different message types.