Recording

ROS2 bags are a recorded ‘bag’ of data logged from ROS topics, that can later be played back or analyzed. Refer to ros2 bag --help for help with the CLI.

Playing back

Playing a bag (ros2 bag play <bag>) will publish the recorded messages to the topics as they are. This can be used to test changes to nodes that subscribe to messages multiple times with the same data, without needing to do real flights over and over. E.g., the image recognition node subscribes to /argus/left/image_raw, so a bag containing messages from that topic can be used after the fact to run the image recognition node again on the same data. Then you could also run another node that reads messages published by the image detection, etc.

When doing this mocking, you can play only specific topics from a bag with the --topics flag. This is useful when e.g. a bag contains the input needed to run your node, but also its output, which would mix up the old and new data.

Parsing

There are Python libraries made for parsing bags, one such example is rosbag2_py. LLMs are quite capable of writing simple bag analysis scripts when needed.