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I have directory a and directory b. They are big. b is almost identical to a. "almost" means that 4-5 files differ, and I don't know which they are. I want to copy b over a, but only the files that differ. i'm in bash.

(no, I can't simply delete a and replace it with b, because 1) a is version-controlled 2) a full copy (or a mv) would take too much. I want to copy only the files that differ).

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Is rsync an option? – Bobby Feb 3 '11 at 11:43
It is, but I don't know much about rsync. – janesconference Feb 3 '11 at 11:44
up vote 13 down vote accepted

You can use rsync to do this, the command I use is rsync -tr "folder to copy from" "folder to copy to"

e.g. rsync -tr /home/me/stuff/* /home/me/otherstuff/

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BEWARE of this command. It will delete all the files which wont differ. I ran it and LOST almost everything!! – Arvind K. Apr 12 at 8:15
@Arvind I don't know what command you used that caused you to lose your data but it wasn't rsync -tr. Read the man page. – Tog Jun 4 at 19:34

It is also possible to do this with good old cp:

Thanks to srcspider for reminding me to use -T!

cp -ruT old-dir new-dir

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+1 I get so used to synching across machines I forget the elegant ways. – Tog Feb 3 '11 at 12:11
does not work for me :( – janesconference Feb 3 '11 at 12:51
That's actually good gnu cp. ;-) – Keith Feb 3 '11 at 19:58
On OS X, the -u option does not exist. I simply used cp -r old-dir new-dir, since I did not have to compare dates. – Jean-François Beauchamp Nov 12 '12 at 2:11
Does not work. I am not sure if this is some name convention but cp -ru folder-A folder-B will simply copy folder-A inside folder-B not copy contents of A inside B and overwrite, which would be the desired effect. – srcspider Mar 21 '13 at 16:16

Another good option is Unison (, particularly if there isn't really a "source" and a "destination". Each directory is a root and Unison syncs them and keeps metadata for future syncs. It offers both a command-line and a GUI option that can easily be scheduled via cron as well.

I use it to make a backup of my Dropbox to my local NAS appliance which can't run a Dropbox client.

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It CAN be done with good ol' cp, though with a slightly different format than stated above. Here's how I did it:

cp -ru --target-directory="destination_path" source_path/*
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You can diff the 2 directories.

diff -r dir1 dir2

it will show you the list of files that are differnet

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