Subscribing to /rosout topic

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That should be pretty easy to do in Python (and only slightly more difficult in C++). For the subscription part, you can start with the subscriber example, but adapt it to subscribe to rosgraph_msgs/Log messages on /rosout_agg instead of std_msgs/String messages on /chatter. There's some more details about rosout on the wiki page. For the file writing part, you can either use an XML or HTML writer in Python (a search engine will turn up many) or use regular file I/O and hand-code the HTML.