debian/ubuntu

rsync is a great tool for synchronizing 2 directories (or files) either one of them is remote and the other is local, or 2 local folders (it doesn't support synchronizing 2 remote servers, yet). the power of rsync comes from that it doesn't transfer anything unless it detects that it's changes since last run. moreover, it transfer the changes in files only (unless you specified otherwise), and it can even compress files before sending which makes it ideal for continuous backups and mirroring