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I am curious about how Ubuntu One automatic file sync works under the hood. When I add or change a file in my file manager, how does it know that the change occurred and it needs to sync it? This will likely need a somewhat technical answer.

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Wait for a few months. U1 code is going to be open sourced soon. BTW U1 is practically dead. – Registered User Apr 5 '14 at 4:36
My guess is that U1 has a daemon that runs md5 checksums on files and folders and that reacts when an md5 checksum is different. It might even be git behind the scenes. Just guesses here. – don.joey Apr 5 '14 at 6:18
Did you google this? – don.joey Apr 5 '14 at 6:18
@AdityaPatil I think this question is more about the client, rather than the server. And yes, I'm already aware that U1 is being discontinued, which is partially why I'm asking, in case I decide to try my hand at making an alternative. – Christopher Kyle Horton Apr 5 '14 at 19:25
up vote 4 down vote accepted

The Linux kernel supports something called Inotify. Using it, you can tell the kernel to notify you when something happens to a file or folder. So you can do something like "Whenever a new file is placed in this folder, or whenever a file in this folder has been changed, run the following action".

You can be notified when a file is created, read, written to, opened, closed, moved (or renamed), deleted, etc.

I don't know for a fact that this is how Ubuntu One file sync works, but I think it's a good assumption. If you're interested in the techniques, I suggest reading up on Inotify.

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The client is open source, and yes, inotify is how that works on Linux. On MacOS and Windows, there is a layer of abstraction in the client which provides the native file notification information to the client by translation into inotify events. – dobey Apr 6 '14 at 19:59

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