Simple incremental backup

This is a very simple space-preserving incremental backup script inspired by the book Linux Server Hacks (Ch.03). The gist of it is that time-based backups are done by hardlinking files, which reduces the space consumption (especially for files that do not change).

I use this every day to backup my laptop to an external USB harddisk at work, and once a week I do the same thing at home. It’s also capable of doing the same thing for a remote host (i.e. backup the remote host to a local disk, not the other way round!) which I use to backup my home PC to the same external disk that I use for my laptop.

The included example configuration and ‘excludes’ files are real. But the scripts have been modified for this first public release to be a little bit more clean in the way configuration is done. Basically this is now in the file /etc/simplebackup.conf, which is sourced from the backup script as bash source.

The accompanying keepers.pl script is ‘my first Perl program’ (maybe ‘my last Perl program’) to flag certain backups for deletion. It uses the same configuration file but it requires quotes around the values (in the future I will probably port the backup script to Perl as well and get rid of this ugly syntax constraint).

Algorithm for backup maintenance

Nearly every day I make a backup, which is a verbatim copy of (a part) of my system in a subdirectory whose name is today’s date, like 2010-01-13

This accumulates over time so I need to judiciously clean up old backups to prevent the storage space to fill up.

The general strategy is to keep more of the recent backups, and fewer of the older ones; the average time between retained backups should increase as you look back in time. A simple scheme could be to keep all daily backups of the last seven days, one backup per week less than a month old, and one backup per month less than a year old. These will have to be discreet points in time, i.e. not depend on the current day of the week. So let’s say we keep the first backup of every week (Mo-Su) for the last 4 weeks and the first backup of every month for the last 6 months.

Algorithm:

1. sort the existing backups by time, most recent first.
2. Take the next backup date of the list
3. is it less than 7 days old? Yes: flag as ‘keep’ (mark the week) and go back to step 2.
4. If it is less than 30 days old, then if this is the first backup of the week (check the mark) it is in (starting on Monday), flag it as keep and go to step 2. Else discard
5. If it is the first backup of the month keep it