useradd and the default group

[Aaron Konstam]

On Tue, 2009-04-14 at 14:42 +0000, Carl D. Roth wrote:
> On Tue, 14 Apr 2009 04:29:41 +1000, Cameron Simpson wrote:
> 
> > On 13Apr2009 16:28, Carl D. Roth <roth at ursus.net> wrote: | Can some one
> > explain the following weird behavior with useradd? |   # useradd -g mock
> > -r -m -d /var/lib/mockuser mockuser |   --> create a new 'mockuser' user
> > that can be used to run /usr/bin/mock |   # id mockuser
> > |   uid=494(mockuser) gid=491(mock) groups=491(mock) |   # grep mock
> > /etc/group
> > |   mock:x:491:roth
> > | Hm, that's interesting, 'mockuser' is not in the 'mock' group.  This
> > can | be verified using 'getgrent()'.
> > 
> > If you look at /etc/passwd you will see the gid field there is "mock"
> > (494). Eg:
> > 
> >   $ grep cameron /etc/passwd
> >   cameron:x:1000:1000::/home/cameron:/bin/zsh
> > 
> > The -g option to useradd specifies the primary group, which is recorded
> > in the passwd file, not the group file. A UNIX user has a primary group
> > which comes from the passwd file and secondary groups which come from
> > the group file. Absent the setgid bit on a directory, new files and
> > directories a process makes get their group ownership from the primary
> > group. _Access_ (open, cd, etc) is governed by uid and all the groups.
> 
> So from a UNIX programming perspective, then, a test for group membership 
> is then:
> 
>   1. is the user listed in the group membership list
> 
>   OR
> 
>   2. is the user's primary group equal to the target gid
> 
> That seems strange; it means that the group file is not canonical for 
> establishing group permissions.
> 
> C
> 
> 
Not strange at all. In this case from the view of the group file some
othere user had that group rpeviosuly as its primary group so thew group
file just lists those users in the group. In this case the user entry
just added will appear second in the group file entry.. Seems ok to me.
--
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Aaron Konstam telephone: (210) 656-0355 e-mail: akonstam at sbcglobal.net