Re: usermod

From: Steve Buehler (steve_at_ibapp.com)
Date: 03/30/05

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    Date: Wed, 30 Mar 2005 10:12:00 -0600
To: General Red Hat Linux discussion list <redhat-list@redhat.com>

    At 09:46 AM 3/30/2005, you 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

    <snip>

    >>>>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
    >
    >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.

    I am sure you are right. But if someone who didn't know looked into the
    /etc/group file, it could confuse them. It would just be more clear to not
    have the "steve:x:590:steve" in there and have just "steve:x:590:". That
    way, it doesn't start a discussion like this. :)

    Thanks for all your help and knowledge
    Steve 

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