I'd like to have a copy of my .emacs file in my Dropbox folder but I'd also like to have this copy in an automatic way, I mean every time I change my .emacs in the home directory I want it to be instantly updated in the Dropbox directory (this necessity is because I want the .emacs to be equal in my windows and ubuntu partitions and I think Dropbox is a good tool for that). First I used the simple:

$ cp .emacs Dropbox/.emacs

then I tried using rsync (or gsync), well it works but I cannot grasp the difference between rsync and cp for this very simple backup. In fact, even if I have activated the option RSYNC_ENABLE=true in the rsync file, every time I modify the .emacs in my home I have to run the command rsync (or use gsync), as a cp, without having it automatically. How can I have it automatically?

Probably I'm missing something from the very basics of rsync.

Thanks for your help