[Dirvish] Question about UIDs and GIDs

Dave Howorth dhoworth at mrc-lmb.cam.ac.uk
Wed Aug 15 09:48:30 UTC 2012

Joe Aquilina wrote:
>> firstly, welcome Joe.


>> as you've noticed, the numerical UID and GID is preserved during 
>> backup. If the restore is done in the same fashion as the backup, 
>> the numerical UID and GID will also be preserved during restore.

The important take-home message is that the numerical IDs are what
count. The names are just 'sugar' for humans and don't really mean anything.

So when you copy anything, by pretty much any means, the numeric ids are
 preserved. If you copy something and the numeric id isn't preserved,
then start reading man pages until you find the option to make that
happen. If you do that, things will stay pretty organized, although they
may look weird in terms of user & group names as you copy things between

Because of the weirdness and the inreased likelihood of making mistakes
that follows from it, most people try to keep the same list of name <->
id mappings on all machines. You might want to see whether that is
possible at your site. You can copy the passwd (and shadow) files or use
some distributed service to manage them (NIS, LDAP etc).

> Hmmm ... I'm not sure that we are backing up the passwd file. I am 
> almost certain we don't, on any of the machines. The dirvish backups 
> just backup the /home directory tree as far as I can see, and there 
> are no other backups done that I am aware of. Sounds like a no there 
> for passwd which I will fix asap (and I guess group as well).
> It seems to me that a backup of the whole of the /etc tree is 
> probably a good idea? If so, I can just create another vault for that can't I?

Yes, backing up /etc is a good idea, and several other places as well.
Probably best to google for linux backup to find some ideas for what
backup strategy to use. And yes, you can just set up another vault. You
will come across a few gnarly details that you can fix up as you go along.

>> people doing restores often have to go through the process of 
>> chown/chmod after restoring.

As long as you're restoring to the same machine that the data originally
came from, you shouldn't need to chown anything. It should all
'magically' acquire the correct user name when it is restored, because
it is the numerical id that matters.

> I have been doing a test restore of a backup run yesterday, back onto 
> the original machine (into a temp directory as I thought this would 
> be best to not endanger any of the existing user files; it is my main 
> working machine - good or bad idea?)

Yes, test directories are a very good idea. Whether using your own
machine is a good idea is really down to your own self-confidence and
self-knowledge. If you KNOW that you're not going to accidentally type
rm -rf * or somesuch then it's fine. If you know that you sometimes do
do silly things, you might want to choose a test machine that you can
afford to trash.

> and the owners and groups appear 
> to have been changed for the restored files. For example, the whole 
> /home/central directory tree which used to be owned by user central, 
> group central is now showing on the restored files to be owned by 
> user joe, group joe. Has my doing the restore to a different location 
> affected the ownership?

If the passwd mapping on the machine you restored to (#3) is different
from that of the original source machine (#1), then the names will be
different. The important thing is to check that the numerical ids
haven't changed.

> I guess that means that I would have to run the loop to change them back?

If you want to use them on machine #3 then yes. And you would have to do
that with each restore. That's part of why it is usually a lot simpler
to have a single list of mappings from numerical ids to names for the
whole site.

> I also just realised that I haven't used the exact same command to 
> restore as was used to do the original backup. Should I have done 
> this with effectively the source and destination swapped around? Is 
> that what you meant earlier when you said "if the restore is done in 
> the same fashion as the backup"?

You can restore that way, but one of the beauties of dirvish is that you
can just restore files from the backup using cp (use the -p or -a
options to preserve ownership and file times).

HTH, Dave

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