Linux Pocket Guide, 2nd Edition

Group Management

groups	Print the group membership of a user.
groupadd	Create a group.
groupdel	Delete a group.
groupmod	Modify a group.

A group is a set of accounts treated as a single entity. If you give permission for a group to take some action (such as modify a file), then all members of that group can take it. For example, you can give full permissions for the group friends to read, write, and execute the file /tmp/sample:

$ groups
users smith friends
$ chgrp friends /tmp/sample
$ chmod 770 /tmp/sample
$ ls -l /tmp/sample
-rwxrwx---  1 smith friends  2874 Oct 20 22:35 /tmp/sample

To add users to a group, edit /etc/group as root.^[18] To change the group ownership of a file, recall the chgrp commands from File Properties.

^[18] Different systems may store the group member list in other ways.