[Dirvish] Question about UIDs and GIDs

wes dirvish at the-wes.com
Wed Aug 15 04:25:13 UTC 2012

hurray! a question I can help with!

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. It's up to the machine the
file is being accessed from (as opposed to the machine the file is being
stored on) to determine what names those numbers correspond to.

this is a common problem with any backup software. unfortunately, often a
restore is needed after a total crash of a machine, thus having the name/ID
matchup either disappear (you DO backup your passwd file, right?) or become
out of sync or otherwise fubar. If you're backing up the entire system,
this should not be an issue. If you're only backing up user data, this may
well be a problem.

people doing restores often have to go through the process of chown/chmod
after restoring. when I restore many users' data (ok, I only actually did
this once) I ran a loop in bash to chown the files to the name of the
directory under /home. in my case, the directories had the same names as
the users. I don't recall the exact incantation, and I'm sure I tried
several times before I got it right. something along the lines of:

for a in ls;do chown -R $a:$a $a;done

so for the following contents of /home:

central joe vince wes

we should see a loop 4 times of this:

chown -R central:central central
chown -R joe:joe joe
chown -R vince:vince vince
chown -R wes:wes wes

obviously, this gets more complicated if you're restoring more than just
user data, or if your users' primary groups are not their own groups (and
they are different; if they're the same, just specify that instead of the

hopefully some of this makes sense. please feel free to respond back with
any further questions you may have :)


On Tue, Aug 14, 2012 at 8:21 PM, Joe Aquilina <joe_linux at westnet.com.au>wrote:

> Hello all.
> I am relatively new to Linux and even more so to Dirvish. I have
> inherited the admin of a system where some of our machines are backed
> up nightly via dirvish to a central server. The current system has
> been in place for some years and I am just beginning to learn about
> how it all works. We haven't yet had to restore anything yet which is
> good because I am not yet sure of exactly how to do that.
> Experimenting with that on another computer as I type here...
> My question relates to UIDs and GIDs of user accounts on the systems
> being backed up. On these systems, there a number of users, some of
> which are duplicated across the various machines, usually with
> different UIDs and/or GIDs on the various machines. These are also
> different to the IDs used on the corresponding accounts on the backup
> server.
> When I look at the backups on the server after they complete, the
> directories &files have the UIDs and GIDS for the accounts as they
> exist on the backup server. These correspond to the numeric IDS from
> the machines that have been backed up.
> For example, there may be an account called joe, in group joe on a
> machine to be backed up with a UID of 1002 and a GID of 1003. When
> the backup completes, and I check it, everything has an owner for
> example of central and a group of vince. These correspond to accounts
> on the backup server of central with a UID of 1002 and a GID of 1003.
> Hope this makes sense.
> Does this matter? Is there going to be a problem if I restore the
> files back to the original machine? Will the UID & GID revert back to
> joe when the restore is done? Do I need to change anything on the
> system(s) for future backup runs to avoid this?
> Do you need any more information (e.g. contents of master.conf,
> default.conf, rsync command for the backup or anything else) to be
> able to assist with this query?
> I am hoping I am worrying about nothing but I don't fully understand
> dirvish yet so am a bit apprehensive about changing anything (and
> shudder at the thought of possibly having to change all the UIDs and
> GIDS on the various machines and then changing all the files!) I have
> other dirvish questions but will follow up with those if I can't work
> out an answer for myself. Thanks in advance.
> Joe Aquilina
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> Dirvish at dirvish.org
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