Split R file into multiple files.

I need help splitting 2-3 R programs into smaller sub-programs such that each function in the file becomes a stand-alone program that can be called from a shell script. Conceptually: I have one program that has N stand-alone functions each of which produces one or more files as output. If we produce M files as output, then I'd like to produce M functions each of which produces exactly one output file. These functions are fairly straight-forward, very linear logic -- almost no conditionals etc. and total file size is