Subscribing a topic from a bag file is the same as subscribing to a ROS topic.

In one terminal, please play your bag file as shown below:

rosbag play file.bag

Please see the wiki page for more info on the rosbag play command. Next, in another terminal, please subscribe to your recorded topic. Again, please see the wiki page on how to write a subscriber.

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By the way, are you trying to extract messages from a bag file? If yes, I recommend you look at the rosbag/Code API page.

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http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28python%29

This tutorial breaks down a bit on how topics are shared between a publishing source and a receiving subscriber.

It's the same thing when trying to subscribe a topic from a bagfile. Only this time, your publisher is the bagfile being played. As long as you reference the correct topic you're trying to subscribe to, you will be able to extract it.

To play the bagfiles, you can use:

rosbag play filename.bag

This will playback the bag file and publish all the topics recorded within the file. If you only want specific topics to be published, you can use:

rosbag play filename.bag --topics /topic1 /topic2 /topicEtc

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