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I have one big folder "X" in my home directory, and outdated copy of this on a flash drive -- Y. What is the analog of rm Y; cp X Y, but without overwriting already existing files?

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migrated from Apr 8 '12 at 13:18

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up vote 18 down vote accepted

There are many more properties to rsync than the previous answer mentions. You should look into them yourself:

man rsync

But for your problem I'd suggest:

rsync -a --delete X Y

This way, the receiver (Y) will delete any file it has that is not in X. Be sure you do it right though. Used incorrectly as root can easily kill your system... :)

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Agree that you should be very careful with this. Try adding --dry-run to the command to see what will happen without actually making any changes. If it goes well do the same command without --dry-run – squarecandy Sep 23 '15 at 4:17

Take a look at rsync(1). It's intended to do pretty much exactly what you want.

rsync -av X Y

It supports remote copying, through its own protocol or ssh, but it can also be used locally.

Depending on the exact behaviour you want you may also want to pass the --update option. This will tell rsync not to overwrite files which already exist on the target and are newer than the ones in the source.

If you want to remove extra files (i.e. files which exist in Y, but not in X) you can add the --delete option.

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But it won't remove files, than removed in X, while rm Y; cp X Y will. – Lescott Apr 8 '12 at 11:26
In order to remove files that removed from X, use --delete option. – Onur Yılmaz Jul 30 '15 at 5:00

Try using rsnyc, possibly with the following command:

rsync -avzu /home/x /y

-u = do not overwrite a file at the destination, if it is modified

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But it won't remove files, than removed in X, while rm Y; cp X Y will. – Lescott Apr 8 '12 at 11:27

I think some of the functionality that you are looking for in option -u (update) actually exists in this option:

"--ignore-existing" - skip updating files that exist on receiver

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