Rsync checksums

Niko Nyman niko at
Wed Jan 29 01:13:10 EST 2003

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?

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.

Just to clarify, when I mentioned creating an "easy incremental backup app"
I didn't mean "easy" as in a few lines of shell code, more like easy for
someone who's willing to write a few hundred lines of objective-C ;)  I
believe quite many of the Utilities shipped with OS X are just GUIs to
robust shell programs so why not create a good backup software GUI for rsync
(I know there is one GUI already but I was talking about a *good* GUI...)

niko at

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