Rsync checksums

jw schultz jw at
Wed Jan 29 01:56:38 EST 2003

On Tue, Jan 28, 2003 at 04:13:10PM +0200, Niko Nyman wrote:
> on 28.1.2003 15:06, jw schultz at jw at wrote:
> >> Would it be possible to archive files on a removable drive, save the
> >> checksums for the archived files, and later compare existing files against
> >> the saved checksums without plugging in the removable drive?  I imagine in
> >> theory this would allow for an easy incremental-backup-to-CD application
> >> based on rsync to be written for OS X -- would be useful as there seems to
> >> be no good backup software based on non-proprietary formats.
> > 
> > Your particular idea is far outside the scope of rsync.
> > List regulars may ignore the rest of this email, i'm
> > feeling a bit too gracious.
> > 
> > There are any number of approaches to this.
> > Most backup systems don't use checksums to decide what to
> > write to an incremental anyway.  The most use timestamps.
> I might have been going too far with the checksum idea.. So if I backup
> using  rsync -a  it compares date and size of the files?

and ownership and permissions. -a includes -p -g and -o
among other options.

> I wasn't perfectly sure if your example script did this but I need all file
> "metadata" to stay intact: permissions, timestamps etc must stay unmodified
> at the destination.  Rsync does this and I'm happy using it when running
> backups to my external hard disk.
> As rsync has the capability to compare source files on my internal HDs
> against the destination files on the external HD I'm still curious if there
> would be a way to make rsync compare source files timestamps (and sizes?)
> against a list of destination file information, instead of files on a
> mounted disk.

Rsync is a syncronization tool.  Think of it as an optimized
network oriented cp.

There have been discussions of rsync caching block and or
file checksums but that is for the future, if ever, and
would still be with the actual files online.

Rsync is ill suited for generating changed file lists for
the sort of incrmental/diferentail/cumulative backups you
are talking about.

	J.W. Schultz            Pegasystems Technologies
	email address:		jw at

		Remember Cernan and Schmitt

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