Rsync is a file and directory syncronization tool. It can be used to keep local and/or remote files syncronized. What makes rsync powerful is that it can detect changes within large files and only transfer the parts that are different. This is much more efficient than transfering entire files, especially as the file sizes get bigger.
The rsync utility is part of the default installation in OS X.
Note: examples that use a shell use ssh with public key authentication
To synchronize a local directory with a remote one (pull), use:
rsync -r -a -v -e "ssh -l username" --delete hostname:/remote/dir/ /local/dir/
To synchronize a remote directory with a local one (push), use:
rsync -r -a -v -e "ssh -l username" --delete /local/dir/ hostname:/remote/dir/
To synchronize a local file with a remote one, use:
rsync -a -v -e "ssh -l username" hostname:/filename /local/filename
To synchronize a remote file with a local one, use:
rsync -a -v -e "ssh -l username" /local/filename hostname:/filename
To synchronize a local directory with a remote rsync server:
rsync -r -a -v --delete rsync://rsync-server.com/stage/ /home/stage/
To synchronize a local directory with a local directory (make a backup), use:
rsync -r -a -v --delete /local/dir/ /backup/dir/