How to sync 2 folders with all subfolders without overwriting

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http://stackoverflow.com – I'm in linux (Ubuntu 12.04). I have 2 folders, folder A and folder B. (both have the same subfolders). I would like to copy all files from folder B (and from all subfolders) which are not in folder A and paste them in folder A in the appropiate subfolder, using a single terminal command. Without overwriting. Thanks! (HowTos)