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I'm going to try and change your whole direction here, because you're trying to use ROS in a very strange way that it was not intended for.

To start your ROS system you should use a launch file, this will start the roscore automatically. This launch file will also start the serial_node that you need to start. You'll also want to launch the node which controls your robot.

Although you can publish messages using the ROS command line tools, this is not the way it should be done if you're making your own nodes. You should create a publisher as described in this tutorial and publish the messages directly from your script, not via the rostopic command line tool.

I can't think of a good reason why you'd ever need to run a ROS command line utility from inside a ROS node.

Hope this helps.