Is there a way to record a single message?

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The command rosbag record -h will output a description of the optional arguments. Take a look at the one called --limit.

Hello Kansai,

I know that if I have a node that publishes messages (in particular Image messages) I can record these messages with rosbag record into a rosbag.

Yes, you can record the image message through ros bag file. You can do so by the following the command. Even the message is single I think it can record it.

rosbag record -o subset <Topic-Name-Image>

And if you want to record multiple topic then you can

rosbag record -o subset <topic-name-image 1=""> <topic-name-image 2="">

The -O argument tells rosbag record to log to a file named subset.bag.

Play back recording with:

rosbag play <bagfile-name>

You can check your recording in Rviz just by subscribing to your recording node.

For exporting single image here is a good document http://wiki.ros.org/rosbag/Tutorials/...