Re: [PLUG] Copying ext2 data partitions

[Bill Jonas <bill@billjonas.com>]

On Mon, Dec 31, 2001 at 08:28:29AM -0500, Michael F. Robbins wrote:
> So is there an option I'm missing, or is there a better way to do this
> alltogether?

Tar is considered by many to be superior to cp for this purpose,
although those views may or may not be valid since GNU cp has become
quite good with preserving permissions, attributes, etc.  Personally, I
would probably do it like this:

# Everything you did up to the cp command, then:
(cd /mnt/home && tar -cpf - .) | (cd /mnt/backup && tar -xpvf -)

Taken from
http://duke.csc.villanova.edu/doc/LDP/HOWTO/Tips-HOWTO-2.html#ss2.6
(main Linuxdoc site is down, this is a mirror).

A quick explanation of the above command: the parentheses create
subshells (bash forks off a copy of itself for each set of parentheses
it finds and executes the commands within the parentheses).  The cd
commands are pretty self-explanatory; the &&'s are used to prevent
trashing the directory structures in case anything goes wrong during the
directory change.  (Not that it would or should, but caution.)  "tar
-cp" means "create, preserving attributes"; "f -" says that the output
file should be standard output; and "." specifies the directory to tar
up.  "tar -xp" specifies "extract, preserving attributes"; "v" indicates
verbosity (each filename will be printed to the console (well, stderr)
as it's extracted); "f -" signifies that you want the file to be decoded
to come from standard input.  You can leave off the v if you don't care
to see the progress of the command.

That being said, "cp -a" *should* work correctly.  With either way of
doing it (tar or cp), you can check to see if anything is different with
a command like so:

diff <(ls -lAR /mnt/home |sed -e 's|/mnt/home||') <(ls -lAR /mnt/backup
|sed -e 's|/mnt/backup||') |less

Explanation: You probably know what diff does, so I don't belabor that
here.  What's really interesting is the "<()" construction.  That's
known as "process substitution".  What it does is it gives a filehandle
to the output of a process so that commands (like diff) which only work
on files can be made to work on nearly any arbitrary output.  The sed
command replaces the specified string (/mnt/home or /mnt/backup, as
appropriate) with nothing, to reduce false positives.  The rest should
be pretty self-explanatory.  One thing to note: I've noticed that
neither "cp -a" nor the tar method preserve the modification times of
symlinks, so you might get many false positives.  If you don't want to
see the symlinks listed, you could add something like " |grep -v '\->'"
or " |grep -v '^l'" to each command, just before the ).

Hope that helps.

-- 
Bill Jonas    *    bill@billjonas.com    *    http://www.billjonas.com/

Developer/SysAdmin for hire!   See http://www.billjonas.com/resume.html

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