Rscript on ubuntu
The answers posted so far are generally useful, but they don't directly answer the question. I recently had the same question and discovered there is no rscript binary for Ubuntu. The r binary itself is used to execute scripts in batch mode as opposed to the separate rscript binary that I was using in OS X.
It appears you may be able to get an rscript binary from other sources (see http://craig-russell.co.uk/2012/05/08/install-r-on-ubuntu.html#.UwKWzkJdW2Q for example), but I'm not sure why you would need that when simple running "r script.r" from the command line works just fine.