What's the right way to call rosbag in a launch file?

Hi there, Since the Python API for rosbag is not really documented, I've decided to just start a couple bags from a launch file. I want to record some topics from my robot, and some from my desktop system. Here's what I have so far: I've tried a few different things based on forum posts and one other question on this site, to no avail. The errors I've been getting are along the lines of: `Error writing: Error opening file: /cmd_vel.bag.active` and `[robot_bag-1] process has died [pid 21226, exit code 1 ...`