I've been playing with the Galileo and written my first little program. It doesn't do much, but it does let you send commands to the Unix part of the system from the USB debugging connection.
The program just assembles a command message and sends it to the system. It appends a redirect so that the result of the command is sent back to the debug serial port.
To send commands to your Galileo just open the Serial Monitor from the Tools menu and type the command in the send window. Put a * on the end to mark the end of the command and press enter to send it.
It is not particularly elegant, but it is my first program and it does work. If you want to send a * to the command line you have to change the code. But I'm sure you can do that.
You can download the sketch from here.