this post was submitted on 07 Dec 2023
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Does cp -v print out the file name when it starts copying it or when it's done? (lemmy.ml)
submitted 11 months ago* (last edited 11 months ago) by HiddenLayer5@lemmy.ml to c/linux@lemmy.ml
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So if I had a cp -v operation fail, is the last file name it printed out the last successful file copy, or is it the failed partially copied file? If you had to ensure all files are copied correctly without overwriting anything, would deleting the last filename that was printed from the destination folder delete the partially copied file that the operation failed on?

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[–] SpaceCadet@feddit.nl 27 points 11 months ago* (last edited 11 months ago) (2 children)

Just use rsync -va

NO STOP!

The default quick check algorithm of rsync is not safe for this. It only checks filesize and modification time to determine if files are equal. After a b0rked copy, these are not to be trusted.

You should add the -c flag so that files are properly checksummed, unfortunately if you have slow storage on either end, this often negates the speed advantage of rsync.

For example, consider this example:

mkdir source
mkdir destination
echo "hello" > source/file.txt
echo "world" > destination/file.txt
touch -r source/file.txt destination/file.txt
rsync -avh source/ destination/
cat source/file.txt
cat destination/file.txt

Contrary to what you might expect, the rsync command copies nothing and the output at the end will show:

hello
world

If you change the rsync command in the example above to rsync -c -avh source/ destination/, it will work as expected.

[–] taladar@sh.itjust.works 1 points 11 months ago

True if the initial state is unknown but if you do your initial copy and all the later syncs with rsync it is not really necessary since rsync puts the partial files in a temporary location (there are same parameters to control the details of that too).

[–] damium@programming.dev 1 points 11 months ago

My memory of the cp command is that attributes such as file times were transferred at the last step. I think this would make rsync safe in most situations where a system crash wasn't involved.