Hi,
On Fri, Oct 27, 2017 at 05:27:22PM -0400, H wrote:
> What is the best tool to compare file hashes in two different drives/directories such as after copying a large number of files from one drive to another? I used cp -au to copy directories, not rsync, since it is between local disks.
[snip]
> Are there other tools for this automatic compare where I am really looking for a list of files that exist in only one place or where checksums do not match?
rsync obviously offers the 'exist in only one place' feature but also offers checksum comparisons (in version 3 and higher, I understand)...
-c, --checksum
This changes the way rsync checks if the files have been changed
and are in need of a transfer. Without this option, rsync uses
a "quick check" that (by default) checks if each file’s size and
time of last modification match between the sender and receiver.
This option changes this to compare a 128-bit checksum for each
file that has a matching size. Generating the checksums means
that both sides will expend a lot of disk I/O reading all the
data in the files in the transfer (and this is prior to any
reading that will be done to transfer changed files), so this
can slow things down significantly.
The sending side generates its checksums while it is doing the
file-system scan that builds the list of the available files.
The receiver generates its checksums when it is scanning for
changed files, and will checksum any file that has the same size
as the corresponding sender’s file: files with either a changed
size or a changed checksum are selected for transfer.
Note that rsync always verifies that each transferred file was
correctly reconstructed on the receiving side by checking a
whole-file checksum that is generated as the file is trans‐
ferred, but that automatic after-the-transfer verification has
nothing to do with this option’s before-the-transfer "Does this
file need to be updated?" check.
For protocol 30 and beyond (first supported in 3.0.0), the
checksum used is MD5. For older protocols, the checksum used is
MD4.
Rich.
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