The usermod command will allow you to change a user's primary group, supplementary group or a number of other attributes. The -g switch controls the primary group.
For your other questions...
If you specify a group, groupname, that does not exist during the useradd stage, you will receive an error - useradd: unknown group groupname
The groupadd command creates new groups.
The group will remain if you remove all users contained within. You don't necessarily have to remove the empty group.
Create the hilbert group via groupadd hilbert. Then move David's primary group using usermod -g hilbert hilbert. (Please note that the first hilbert is the group name and the second hilbert is the username. This is important in cases, where you are moving a user to a group with a different name)
You may be complicating things a bit here, though. In many Linux distributions, a simple useradd hilbert will create the user hilbert and a group of the same name as the primary. I would add supplementary groups specified together using the -G switch.
The reason (the only reason, as far as I know) to put users in a group of their own is to make umask 002 or umask 007 a sensible default.
The umask is a mask for the default permissions of newly created files. The meaning of the digits are the same as in chmod; the first digit is for the user, the second for the group, and the third for others. If a bit is 1 in the umask, it is removed (masked) from the permissions of the newly created file. For example, if an application creates a non-executable file with no particular privacy requirement¹, it will pass 666 as the file permissions, and the application of the umask 002 will result in a file with permissions 664 (0666 & ~002 in C-like notation), i.e. a file which is readable by everyone and writable only by the user and group (rw-rw-r--).
With the umask 022, files are world-readable by default but only writable by their author. With the umask 002, files are additionally writable by the group that owns them. If users's primary group is one where they are the only user, and the umask is 002, then:
- By default, files are only writable by their author, because although the permissions are
rw-rw-r--, there is no other user in the group that has write permissions.
- To allow a file to be modified by members of a group, the author only needs to make it owned by that group with
chgrp. This can even happen automatically if the file is created in a directory with the setgid bit or an equivalent ACL.
The advantage over the 022 umask is that in that setting, in order to make a file editable by users, the author must do two things: set the group, and extend the permissions (chmod g+w). People tend to forget this second step (or sole step, in a setgid directory).
¹ Examples of files with particular privacy requirements: encryption keys; emails; any file in a public directory such as /tmp.
Best Answer
You can use grep:
This only lists supplementary group memberships, not the user who have this group as their primary group. And it only finds local groups, not groups from a network service such as LDAP.