rsync --compare-dest does not work

I have these directories SRC, DST, COMPARE:

• /opt/SRC

  file1
  file2
  file3

• /opt/DST

  file1
  file2

• /opt/COMPARE

  empty

I wish:

$ ls /opt/COMPARE
file3

But when I run :

rsync -avz --compare-dest=/opt/COMPARE /opt/SRC/ /opt/DST

The /opt/COMPARE directory is still empty, why?

2 Answers

I believe that you are applying the --compare-dest incorrectly.

From man rsync

 --compare-dest=DIR This option instructs rsync to use DIR on the destination machine as an additional hierarchy to compare destination files against doing transfers (if the files are missing in the destination directory). If a file is found in DIR that is identical to the sender’s file, the file will NOT be transferred to the destination direc‐ tory. This is useful for creating a sparse backup of just files that have changed from an earlier backup. This option is typically used to copy into an empty (or newly created) directory. Beginning in version 2.6.4, multiple --compare-dest directories may be provided, which will cause rsync to search the list in the order specified for an exact match. If a match is found that differs only in attributes, a local copy is made and the attributes updated. If a match is not found, a basis file from one of the DIRs will be selected to try to speed up the transfer. If DIR is a relative path, it is relative to the destination directory. See also --copy-dest and --link-dest. NOTE: beginning with version 3.1.0, rsync will remove a file from a non-empty des‐ tination hierarchy if an exact match is found in one of the compare-dest hierar‐ chies (making the end result more closely match a fresh copy).

If I am reading this correctly, --compare-dest used used to stop the sending certain files. This would allow you to create a new copy destination directory, containing only files that have changed since the last copy. Sort of an incremental copy, if you will.

1. rsync will compare the files in /opt/SRC to the files in /opt/COMPARE. If file exists in /opt/SRC and doesn't exist in /opt/COMPARE, or different version of it exists in both, it will copy it to /opt/DST.
2. rsync is not only inspecting content when deciding if files are "identical". For this to work correctly, you should sometimes use flags like "--no-times" (this oneworked for me in one occasion), or "--no-perms", "--no-owner", "--no-group, "--checksum", etc.
3. it is a good idea to use "--dry-run" to test the behavior, without actually copy.