i still don't get what you are trying exactly. If you have build a sketch, and the sketch contains all functionality you need (eg. "string parsing"), why do you want to log in as root on the galileo to call some functions? When you compile a sketch from the Arduino IDE and you run it from there, the binary of the sketch is transfered to the Galileo and executed there under Linux. There is no need to login as root. When you want that the sketch communicates to some other programs on The Galileo it pretty much depends on the application needs, what's most suited.
Concerning the image: where did you get the image from? the source should mention, how it was built. When you have just the SPI Linux, than you have a pretty minimal Linux. If you have the Galileo booting from the SD-Card, you might have a potentially a build environment there. To check this, log in on the galileo, type in "which gcc" and "which make", and if the shell finds it, you should be able to compile the sketches on the Galileo.
