rsync: "-c" option clarification

Kevin Korb kmk at
Thu Mar 23 20:10:17 UTC 2017

Before anyone yells at me, yes, you can use rsync's --checksum to detect
(and fix) files that are incorrect despite having correct timestamps and
sizes.  This would mean that a previous rsync had been corrupted not the
current one.  But it is important to note that this would only be
reported to you if you also use --itemize-changes and what to look for
(a file with a c but not an s or a t).

It is also worth noting that single file compression tools (like gzip)
automatically set the original mtime when compressing or decompressing.
If you decompress then recompress such a file you can cause a case of a
file with matching mtime+size but not matching checksum due to gzip's
metadata even though the uncompressed result is identical.  I would not
consider this to be a case worth updating the remote copy but I am sure
someone will disagree.

On 03/23/2017 03:49 PM, Kevin Korb via rsync wrote:
> The -c option causes rsync to checksum EVERY file on both ends BEFORE
> rsync does anything else.  It checksums files that are on only 1 end.
> It checksums files that are different sizes.  It will not catch a
> hardware problem preventing rsync from writing a file correctly.
> On 03/23/2017 03:12 PM, steven banville via rsync wrote:
>> Hi
>> I am using "rsync" to send files from a source machine to a remote
>> machine as one typically does.  I would like to clarify that the "-c"
>> option will cause the checksum on the receiving end to be created by
>> reading the already written file and NOT the data stream on the
>> receiving end.  This would help in catching disk I/O errors if the
>> checksum is done on the file on disk.
>> I understand if the size and (or date?) don't match, the checksum is not
>> needed on the receiving end.
>> I may be missing something but it wasn't entirely clear to me that the
>> checksum is done based on the file on disk.
>> Thanks,
>> -Steve

	Kevin Korb			Phone:    (407) 252-6853
	Systems Administrator		Internet:
	FutureQuest, Inc.		Kevin at  (work)
	Orlando, Florida		kmk at (personal)
	Web page:
	PGP public key available on web site.

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