usermod

[Alexey Fadyushin]

I think that there is no difference between
steve:x:590:
and
steve:x:590:steve
as far as the kernel access control mechanisms are concerned. Every user 
in Linux/Unix is always a member of its primary group (as defined in 
/etc/passwd) even if the list of users in that group in /etc/group is 
empty. Therefore, it is not important what to use: 'usermod -G "" steve' 
or 'usermod -G steve steve' - the records in /etc/group will be 
different, but their effect on the access control checks will be the same.

Alexey Fadyushin
Brainbench MVP for Linux.
http://www.brainbench.com

Steve Buehler wrote:

> At 08:50 AM 3/30/2005, McDougall, Marshall (FSH) wrote:
> 
>>> >>I know you can use "usermod" on RedHat Linux to add a user to another
>>> group
>>> >>by typing:
>>> >>usermod -G sam,frank steve
>>> >>This would effectively add steve to the "frank" and "sam" group.  
>>> How do
>>> >>you remove them from those groups without editing the /etc/group file
>>> >>manually?  I can do:
>>> >>usermod -G steve steve
>>> >>That will remove him from "sam" and "frank" groups but would add 
>>> steve to
>>> >>his own group so the /etc/group file would look like:
>>> >>steve:x:590:steve
>>> >>instead of just:
>>> >>steve:x:590:
>>> >>For the life of me, I can't figure this one out.  Does anybody know?
>>>
>>> >If you "usermod -G sam,frank steve" and realize that you shouldn't 
>>> have put
>>> >steve in the frank group, just "usermod -G sam steve" and by 
>>> omission, will
>>> >remove the user from the group.  HTH
>>>
>>> Yes, but what if I want to remove him from both groups.  You can't type
>>> "usermod -G steve".  Basically, I am wanting to remove him from ALL 
>>> groups
>>> except for his initial group.  And "usermod -g steve steve" will not 
>>> do it
>>> either.  The "-G" must have atleast one group as an
>>> option..................................I just tried something that 
>>> seems
>>> to work, but not sure if it is suppose to work this way.  I typed:
>>> usermod -G "" steve
>>> and that removed steve from all extra groups.  So either my problem is
>>> solved, or there is another "proper" way of doing it.
>>
>>
>> "usermod -Gsteve steve" should remove him from all groups except 
>> steve.  If
>> the group is in the list, then the user is added to the group.  If the 
>> group
>> is not in the list, then the user is removed from any groups not in the
>> list.  This is the way I go about it.  We all know that with *nix, 
>> there is
>> always more than 1 way to accomplish it.
> 
> 
> I know it shouldn't hurt to do that, but if you do, it actually adds 
> steve to his own group again.  So instead of the line in the /etc/group 
> file looking like the following which is how it should look if they are 
> not in any other group:
> steve:x:590:
> it would then look like the following if you run "usermod -Gsteve steve":
> steve:x:590:steve
> 
> Thanks
> Steve
> 
> 
>