用rsync有很多优点，比如rsync只会复制差异性内容而不是全部内容，所以对于需要在两个目录（不一定是不同的机器上）保持一致时用rsync再方便不过了，下面这篇文章介绍了一些rsync的一些巧妙的应用：

Rsync Examples over SSH

backups over ssh and secure local backup tips and tricks

Rsync is arguably one of the most powerful tools you can have in your arsenal as a systems administrator or user. It does not matter if you take care of one system or thousands, rsync can make your life easier. You can backup files, transfer data from one system to another or make sure the data in two locations are identical. Rsync can do this for you without you worrying about what data has changed; it will take care of the details.

History of rsync...

rsync is a software application for Unix systems which synchronizes files and directories from one location to another while minimizing data transfer using delta encoding when appropriate. An important feature of rsync not found in most similar programs/protocols is that the mirroring takes place with only one transmission in each direction. rsync can copy or display directory contents and copy files, optionally using compression and recursion.

The rsync utility uses an algorithm (invented by Australian computer programmer Andrew Tridgell) for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a different version of the same structure.

The recipient splits its copy of the file into fixed-size non-overlapping chunks, of size S, and computes two checksums for each chunk: the MD4 hash, and a weaker 'rolling checksum'. It sends these checksums to the sender.

The sender then compares its rolling checksums with the set sent by the recipient to determine if any matches exist. If they do, it verifies the match by computing the MD4 checksum for the matching block and by comparing it with the MD4 checksum sent by the recipient.

The sender then sends the recipient those parts of its file that didn't match any of the recipient's blocks, along with assembly instructions on how to merge these blocks into the recipient's version to create a file identical to the sender's copy.

If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files. Wikipedia, rsync

How does rsync help me?