Linux – How to get the primary group of a user

grouplinux

The following command will list all of the groups of someUser (the primary group and the supplementary groups):

groups someUser

But is there a way to only get the primary group?

Best Answer

See the FreeBSD handbook (information also valid for Linux):

Group ID (GID)

The Group ID (GID) is a number used to uniquely identify the primary group that the user belongs to. Groups are a mechanism for controlling access to resources based on a user's GID rather than their UID. This can significantly reduce the size of some configuration files and allows users to be members of more than one group. It is recommended to use a GID of 65535 or lower as higher GIDs may break some software.

If so, running id <username> will show gid=<primary group>:

id <username>
uid=1000(<username>) gid=1000(<username>) groups=1000(<username>),4(adm),24(cdrom),27(sudo)

If you want the command to return just the primary group name, see man id:

   -g, --group
          print only the effective group ID

   -G, --groups
          print all group IDs

   -n, --name
          print a name instead of a number, for -ugG

so, id -gn <username> should give you what you want.

Related Solutions

Group IDs – Understanding GID, Primary, Supplementary, Effective, and Real Group IDs

You mix two different distinctions here:

Between real and effective group ids
Between primary and supplementary users' groups

The first distinction refers to how processes are being run. Normally, when you run a command/program, it is run with the privileges of your user. It has the real group id same as your user's primary group. This can be changed by a process in order to perform some tasks as a member of another special group. To do that, programs use the setgid function that changes their effective group id.

The second distinction refers to users. Each user has his/her primary group. There is only one per user and is referred to as gid in the output of the id command. Apart from that, each user can belong to a number of supplementary groups - and these are listed at the end of id output.

[Edit] :

I agree that the manpage for id is somewhat misleading here. It is probably because it is a stripped-down version of the description provided by the info document. To see it more clearly, run info coreutils "id invocation" (as suggested at the end of the id manual).

Delete non-unique group with group ID as primary group of a user

The error you're getting is a limitation of the way groupdel was written and the fact that the system is designed around numbers (IDs) and not names. As you can see in source code of groupdel, it only checks if there's a user having the GID you want to delete, as its primary group. It doesn't matter if there's another group having the same ID, but named differently.

/* [ Note: I changed the style of the source code for brevity purposes. ]
 * group_busy - check if this is any user's primary group
 *
 *      group_busy verifies that this group is not the primary group
 *      for any user.  You must remove all users before you remove
 *      the group.
 */
static void group_busy (gid_t gid)
{
        struct passwd *pwd;

        /* Nice slow linear search. */
        setpwent ();
        while ( ((pwd = getpwent ()) != NULL) && (pwd->pw_gid != gid) )
                ;
        endpwent ();

        /* If pwd isn't NULL, it stopped because the gid's matched. */
        if (pwd == (struct passwd *) 0)
                return;

        /* Can't remove the group. */
        fprintf (stderr,
                 _("%s: cannot remove the primary group of user '%s'\n"),
                 Prog, pwd->pw_name);
        exit (E_GROUP_BUSY);
}

So you either you mess with the configuration files using other tools like Perl in mtm's answer or you temporarily change the GID of that user so that group_busy doesn't fail anymore.

Best Answer

Related Solutions

Group IDs – Understanding GID, Primary, Supplementary, Effective, and Real Group IDs

Delete non-unique group with group ID as primary group of a user

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