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I found the "rsync --link-dest" behaves differently depending on how I created the comparison directory. If the comparison directory was created by a "cp" command, the "rsync --link-desk" does the hard-link properly. However, if the content of the comparison directory was created manually, even the content is the same, the same rsync command will not do hard-link but copy. Here is an example:

$ mkdir /tmp/src
$ echo "test" > /tmp/src/file
$ cp -a /tmp/src /tmp/dest1
$ rsync -ai --link-dest=/tmp/dest1 /tmp/src/ /tmp/dest2
$ ls -l /tmp/dest2/
total 4
-rw-rw-r-- 2 user user 5 Apr  7 23:24 file

In the example above, "rsync --link-dest" worked as expected. If we create the content of comparison directory "/tmp/dest1" manually, the same command does not do hard-link:

$ rm -rf /tmp/src /tmp/dest1 /tmp/dest2
$ mkdir /tmp/src
$ echo "test" > /tmp/src/file
$ cat /tmp/src/file > /tmp/dest1/file
$ md5sum /tmp/src/file /tmp/dest1/file
d8e8fca2dc0f896fd7cb4cb0031ba249  /tmp/src/file
d8e8fca2dc0f896fd7cb4cb0031ba249  /tmp/dest1/file
$ rsync -ai --link-dest=/tmp/dest1 /tmp/src/ /tmp/dest2
cd..t...... ./
>f..t...... file
$ ls -l  /tmp/dest2
total 4
-rw-rw-r-- 1 user user 5 Apr  7 23:31 file

This time the "rsync --link-dest" command copied the file in the source directory to the destination directory instead of hard-linking it, even the content and the directory layout between the source and comparison directories are the same.

So what is different between "cp" and manually create the content to the "rsync --link-dest" command? I am using rsync 3.1.2 and the same behaviour can be observed on both Ubuntu 18.10 and MacOS 10.14.3. Thanks.

  • “So what is different between "cp" and manually [creating] the content [with cat >] …?” cp -a will set the modification time of the new file to the same as the original file; cat > will not. – Scott Apr 8 at 2:25
  • @scott, thank you very much for pointing that out. Yes, the "cp -a" copy the modification time as well, I can see that with the "stat" command. – Jason Umi Apr 8 at 7:18

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