A bag is a file created by ROS with the .bag format to save all of the information of the messages, topics, services, and others. You can use this data later to visualize what has happened; you can play, stop, rewind, and perform other operations with it.

The bag file can be reproduced in ROS just as a real session can, sending the topics at the same time with the same data. Normally, we use this functionality to debug our algorithms.

To use bag files, we have the following tools in ROS:

• rosbag: This is used to record, play, and perform other operations
• rqt_bag: This is used to visualize data in a graphic environment
• rostopic: This helps us see the topics sent to the nodes