On October 28, 2017 8:10:34 AM EDT, Rich <centos@xxxxxxxxxxxxxx> wrote:
>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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Thank you, this time I used diff.
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