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I also have a daily Backuppc backup. Backuppc is a pig to set up, but, when it works, it works well. It has the advantage (and possible pitfall), that it backs up only one copy of a file - so if you have 2 Pis and 2 linux machines, and they all have the same version of a library file installed, only one copy will be saved. This saves tons of space. The disadvantage of course is that if that copy of the file gets corrupted, then all the copies will be corrupted.
I use backuppc for other things here in the office. However I wanted something much smaller and simpler to work only with the Pi.
Hmm, once set up, the backuppc pi backup is quick, reliable and only backs up what you want it to, and what has changed. If you have already got a backuppc server set up, all you'd have to set up is the client config. To be honest I'd have thought that's a lot easier than writing your own backup system!
I'm coming into this late but......
I have my own incremental backup system which I use across several computers (desktop, two laptops and a Raspbery Pi). I wrote it myself because I was using rdiff-nackup and wanted to improve on that.
My backup is written (mostly) in python and uses rsync in server mode for the underlying file copying.
It does daily (can be other intervals, I do some more frequent ones across disks on my desktop) incremental backups of a selection of directory trees to a 'remote' backup system in the garage. It comprises only a hundred lines or so of code so is simple and easy to maintain. The resulting backups are all simply copies of the original diretory trees and files can be restored simply by copying them back.
Configuration is pretty simple comprisng a standard rsync '--files-from' file list and a standard rsync server setup on the backup machine. The python scripts that do the 'clever' stuff are linked from the rsync server configuration as 'pre-xfer' and 'post-xfer' scripts.
I just looked, the python scripts are 112 lines and 41 lines, 72 lines of those are comments.
rsync in server mode is a very powerful beastie! :-)