Using rsync for versioned backups without --backup

[Roland Smith]

On Sun, May 24, 2009 at 11:39:57PM -0700, Kelly Jones wrote:
> I want to use rsync to backup a large file (say 1G) that changes a
> little each day (say 1M), but I also want the ability to re-create
> older versions of this file.
> 
> I could use --backup, but that would create a 1G file each day, even
> though I only "really" need the 1M that's changed.
> 
> How do I tell rsync: "while updating, also store the changes you'd
> need to convert today's backup into yesterday's backup"?

I don't think rsync can do that. Essentially it is a file copying tool.

You could use diff if it is a text-only file, or xdelta if it is a
binary file. But both would require you to keep at least two subsequent
versions of the file so a diff can be generated.

You'd need to do something like this every day:

  diff -u foo-yesterday foo >diff-20090525
  # save the diff somewhere
  rm foo-yesterday
  cp foo foo-yesterday


Another possibility is to control the file with a revision control
system. If the file is plain text, rcs(1) will work. If it is a binary
file use a system like devel/git that handles binary files well. 

Say that your file is called <bar>. Since git tracks directory contents,
best put it in a separate directory, and put that under git control:

  mkdir ~/foo
  mv bar ~/foo/
  cd ~/foo
  git init
  git add bar
  git commit -a -m "Initial commit"

So now you can start changing the file. Next day you see if it has
changed, and if so, check in the changes:

  cd ~/foo
  git status
  git commit -m "Changes 2009-05-25" bar

If you now make a backup of ~/foo/.git, you can always restore every
checkin you did of <bar>.

Roland
-- 
R.F.Smith                                   http://www.xs4all.nl/~rsmith/
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