How to make differential backup in linux?

Question

I have to write a script to take differential backup (of a directory). Differential backup:

A differential backup backs up only the files that changed since the last full back. For example, suppose you do a full backup on Sunday. On Monday you back up only the files that changed since Sunday, on Tuesday you back up only the files that changed since Sunday, and so on until the next full backup.

Suppose for Example:

Monday - Performed a full backup and save the file set.

Thursday - Want to Perform a differential backup using the same file set. All files that have changed since the full backup are backed up in the differential backup.

From now on, if I require I will use the script so please suggest to proceed or how to write the script to automate this.

Answer 1

Not quite what you asked for, but very similar in effect (i.e., you "pay" storage only for files that actually have changed):

Using rsync, creating hard links for unchanged files.

The big advantage is that each "snapshot" is a full-fledged backup in its own right, i.e. on recovery you only have to restore that one snapshot (instead of recovering a base and its increments).

There is good documentation on that approach available at www.mikerubel.org/computers/rsync_snapshots/

Answer 2

There is no need to write script when tools exist that do just this.

SVN, GIT

You can setup svn and commit your directory. Every time you commit you save difference. Save = Backup in this case.

You can go back in time and restore snapshots of previous commits.

This solution is discussed at length here

Answer 3

man dump

 -level#
              The dump level (any integer). A level 0, full backup, guarantees
              the  entire  file  system  is copied (but see also the -h option
              below). A level number above 0, incremental backup,  tells  dump
              to copy all files new or modified since the last dump of a lower
              level. The default level is 0. Historically only levels 0  to  9
              were  usable  in  dump,  this  version is able to understand any
              integer as a dump level.

It is worth reading about the "Tower of Hanoi" incremental backup scheme and understanding why and how that is used.

See also http://surf.ml.seikei.ac.jp/~nakano/dump-restore/dump-restore-mini-HOWTO.en.html#ss3.3

And http://en.wikipedia.org/wiki/Incremental_backup which says

A Unix example would be:

rsync -e ssh -va --link-dest=$dst/hourly.1 $remoteserver:$remotepath $dst/hourly.0

The use of rsync's --link-dest option is what makes this command an example of incremental backup.