How to keep two folders automatically synchronized?

Question

I would like to have a synchronized copy of one folder with all its subtree.

It should work automatically in this way: whenever I create, modify, or delete stuff from the original folder those changes should be automatically applied to the sync-folder.

Which is the best approach to this task?

BTW: I'm on Ubuntu 12.04

Final goal is to have a separated real-time backup copy, without the use of symlinks or mount. I used Ubuntu One to synchronize data between my computers, and after a while something went wrong and all my data was lost during a synchronization.

So I thought to add a step further to keep a backup copy of my data:

• I keep my data stored on a "folder A"
• I need the answer of my current question to create a one-way sync of "folder A" to "folder B" (cron a script with rsync? could be?). I need it to be one-way only from A to B any changes to B must not be applied to A.
• The I simply keep synchronized "folder B" with Ubuntu One
  
  In this manner any change in A will be appled to B, which will be detected from U1 and synchronized to the cloud. If anything goes wrong and U1 delete my data on B, I always have them on A.

Inspired by lanzz's comments, another idea could be to run rsync at startup to backup the content of a folder under Ubuntu One, and start Ubuntu One only after rsync is completed.

What do you think about that? How to know when rsync ends?

Answer 1

You can use inotifywait (with the modify,create,delete,move flags enabled) and rsync.

while inotifywait -r -e modify,create,delete,move /directory; do     rsync -avz /directory /target done 

If you don't have inotifywait on your system, run sudo apt-get install inotify-tools

Answer 2

You need something like this: https://github.com/axkibe/lsyncd It is a tool which combines rsync and inotify - the former is a tool that mirrors, with the correct options set, a directory to the last bit. The latter tells the kernel to notify a program of changes to a directory ot file. It says:

It aggregates and combines events for a few seconds and then spawns one (or more) process(es) to synchronize the changes.

But - according to Digital Ocean at https://www.digitalocean.com/community/tutorials/how-to-mirror-local-and-remote-directories-on-a-vps-with-lsyncd - it ought to be in the Ubuntu repository!

I have similar requirements, and this tool, which I have yet to try, seems suitable for the task.