Synchronize the files from One folder of a Server to other

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http://unix.stackexchange.com – I need a script in which Files of One server will be copied to other folder of another server ONLY AND ONLY IF the file on SOURCE SERVER file doesn't exist on DESTINATION SERVER. Further Detail This will be running all time on a SOURCE server if the user create the FILE on SOURCE SERVER and when the file is completed and saved it(the Command or script) first check on DESTINATION SERVER if the file exist on DEST. SERVER if not then copy if to DEST. SERVER How to do that I have no idea of scripting. (HowTos)