rsync - Nov 2008 - --link-dest but only if rsync detects differences

I'm currently using rsync and --link-dest to give me something like a
poor-man's incremental snapshot to disk.  But I really only want to generate
a new backup if rsync detects differences, otherwise I don't need a new
backup.

Currently I do something like this:

rsync -a --delete --link-dest=../backup.previous/  source/ backup.next/
diff -r backup.previous backup.next && rm -r backup.next

Is there a way to tell rsync to not even bother creating backup.next if
there are no differences?  It looks like --compare-dest will also recreate
the directory structure when there are no differences.  The closest I can
come up with other than cleaning up after the fact is first running rsync
with -n --stats and conditionally running rsync again if the output from the
first run indicates new files (which doesn't account for deletions).  It
seems there should be a better/cleaner way but I haven't found it from the
man pages.  I'm currently using rsync 2.6.9.

Is it possible to make rsync perform --link-dest if and only if it detects
differences?

Thanks for any guidance,
~ Daniel
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Maybee somone has a better approach than the "--dry-run " option?

On Thu, 2008-11-13 at 16:17 -0500, Daniel Jarboe wrote:
> I'm currently using rsync and --link-dest to give me something like a
> poor-man's incremental snapshot to disk.  But I really only want to
> generate a new backup if rsync detects differences, otherwise I don't
> need a new backup.
> 
> Currently I do something like this:
> 
> rsync -a --delete --link-dest=../backup.previous/  source/
> backup.next/
> diff -r backup.previous backup.next && rm -r backup.next
> 
> Is there a way to tell rsync to not even bother creating backup.next
> if there are no differences?  It looks like --compare-dest will also
> recreate the directory structure when there are no differences.  The
> closest I can come up with other than cleaning up after the fact is
> first running rsync with -n --stats and conditionally running rsync
> again if the output from the first run indicates new files (which
> doesn't account for deletions).
I don't believe there is any better way than the two you have suggested:
run and delete if there were no changes, or check first with -n.  Note
that with the first approach, you don't need a separate diff command;
you can just use -i and check whether the itemize output is nonempty.

Matt

On 13-Nov-2008, at 14:17, Daniel Jarboe wrote:
> I'm currently using rsync and --link-dest to give me something like a
> poor-man's incremental snapshot to disk.  But I really only want to  
> generate
> a new backup if rsync detects differences, otherwise I don't need a  
> new
> backup.

But with link-dest you are not creating a new backup; you are only  
creating new LINKS to the same backup.

If the new backup has no changes from the last backup, it takes up no  
space, so why does it matter?  You simply expire your old backup off  
automatically with a rotate script.

Here's what my backup folder looks like:

   2 drwxr-xr-x  23 root  wheel       1024 Nov 14 00:54 serv1.daily.0
   2 drwxr-xr-x  23 root  wheel       1024 Nov 13 01:05 serv1.daily.1
   2 drwxr-xr-x  23 root  wheel       1024 Nov 12 00:57 serv1.daily.2
   2 drwxr-xr-x  23 root  wheel       1024 Nov 11 00:58 serv1.daily.3
   2 drwxr-xr-x  23 root  wheel       1024 Nov 10 00:58 serv1.daily.4
   2 drwxr-xr-x  23 root  wheel       1024 Nov  9 00:55 serv1.daily.5
   2 drwxr-xr-x  23 root  wheel       1024 Nov  8 00:56 serv1.daily.6
   2 drwxr-xr-x  24 root  wheel       1024 Oct  1 00:21 serv1.monthly.0
   2 drwxr-xr-x  24 root  wheel       1024 Sep  1 00:22 serv1.monthly.1
   2 drwxr-xr-x  24 root  wheel       1024 Aug  1 00:22 serv1.monthly.2
   2 drwxr-xr-x  24 root  wheel       1024 Jul  1 00:21 serv1.monthly.3
   2 drwxr-xr-x  24 root  wheel       1024 Jun  1 00:22 serv1.monthly.4
   2 drwxr-xr-x  24 root  wheel       1024 May  1 00:27 serv1.monthly.5
   2 drwxr-xr-x  23 root  wheel       1024 Nov 15 00:52 serv1.sday.0
   2 drwxr-xr-x  23 root  wheel       1024 Nov 14 18:55 serv1.sday.1
   2 drwxr-xr-x  23 root  wheel       1024 Nov 14 12:58 serv1.sday.2
   2 drwxr-xr-x  23 root  wheel       1024 Nov 14 06:53 serv1.sday.3
   2 drwxr-xr-x  23 root  wheel       1024 Nov  1 00:51 serv1.weekly.0
   2 drwxr-xr-x  23 root  wheel       1024 Oct 25 00:51 serv1.weekly.1
   2 drwxr-xr-x  23 root  wheel       1024 Oct 18 00:52 serv1.weekly.2
   2 drwxr-xr-x  23 root  wheel       1024 Oct 11 00:48 serv1.weekly.3

sday.3 becomes daily.0 and daily.6 becomes weekly.0 and weekly.3  
becomes monthly.0

-- 
Please to meet you, Rose.  Now run for your life!