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Currently I have two directories A/ and B/ which are identical in every respect, with the exception of the timestamps. Therefore if I run the command :

rsync --dry-run -crvv A/ B/

then all files are marked "uptodate", whereas the command :

rsync --dry-run -rvv A/ B/

shows that all files are to be copied over from A/ to B/.

My question is this : given that I know the files are identical (in respect to contents), then is there any way (via rsync or otherwise) to set the timestamps for files in B/ to be identical to the timestamps of the files in A/, without copying over all the files from A/ to B/ ?


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If you know the files are identical, then why must you go out of your way to not copy over them to get the date/time stamp you require? – Tim Dec 28 '11 at 16:54
@Tim: maybe there's a lot of data. – Richard May 24 at 3:14

Using -t (preserve timestamps) and --size-only will only compare files on size. If the size matches, rsync will not copy the file but since -t is specified, it will update the timestamp on the destination file without recopying it.

Make sure to not use -u (update) as this will skip files that already exist and completely skip updating the timestamp.

I had the problem of originally not using rsync to copy a bunch of files to a new drive, and therefore the timestamps were updated to current time. I used the command below to sync everything correctly in a decent amount of time:

rsync -vrt --size-only /src /dest
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+1 Nice recipe! – leonbloy Mar 2 '13 at 3:56
I also suggest adding --existing to prevent accidental copies of new files. – leden Mar 26 at 23:15

Using --size-only will cause rsync to skip comparing file timestamps (and therefore file contents) if the sizes match. Combining this with --times will clone the timestamps across to the target tree.

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Well, you could certainly write a script that reads the timestamp from one side and then uses touch to set it on same file on the other side.

But that would likely take you much longer than simply letting rsync try to copy all the files. Very little data will actually be transferred (only block hashes if the files are truly identical). But the full contents of every file will have to be read on each side at the least. So of you are limited by disk bandwidth or IOPS it could take a while. But still probably less time than writing and testing a script to do it.

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Thanks, I was unaware of this feature of rsync (it is discussed in wikipedia too). – artella Dec 28 '11 at 18:36

I think rsync is the right tool to make this but if you want some script to do this than someting this can give a good start.

You can get a file list with timestamps (acces times) like this:

find . -printf '"%p" %a\n' -type f > /tmp/times

And get the appropriate time update commands from it:

 while read line; do echo touch -a -d \"${line#* }\" ${line%% *}; done < /tmp/times

It isn't a complete script but a good start place!:)

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Thanks. I think I will probably stick with Rsync for the moment, but I will definitely go out to the web and learn the different components of your script, as it may be useful in the future. – artella Dec 28 '11 at 18:38

The rsync manual: man rsync lists a -t and -N switch, they might be worth fiddling with.

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