Every file has two owners: a user and a group. The above case is a
little confusing because there’s a group called root in addition to
the root user. Groups are just collections of users who are
collectively permitted access to some part of the system. A good
example is a games group. Just to be mean, you might create a group
called games on your computer and then set up your system so that
only people in a games group are allowed to play games.
Here’s a more practical example. Consider a case in which you’re
setting up a computer for a school. You might want certain files to
be accessible only to teachers, not students, so you put all the
teachers in a single group. Then you can tell the system that
certain files belong to members of the group teachers, and that no
one else can access those files.
Let’s explore groups on the system. First, you can use the groups
command at the shell prompt. This will show you a list of the groups
to which you belong. Here’s an example:
$ groups
system-wide configuration!permissions!file
ownershipusername dialout cdrom floppy audio
It’s likely that you’re a member of only one group, which is
identical to your username. However, root can add you to other
groups. The above example shows a person that is a member of five
groups.
less /etc/group
This file lists the groups that exist on your system. Notice the
root group (the only member of this group is the root user), and the
group that corresponds to your username. There are also groups like
dialout (users who are allowed to dial out on the modem) and floppy
(users who can use the floppy drive). However, your system is
probably not configured to make use of these groups. It’s likely
that only root can use the floppy or the modem right now. For
details about this file, try reading man group.
ls -l /home
This command shows you that every user’s directory is owned by that
user and that user’s personal group.
Tip: If you just installed Debian, you may be the only user. You
can use the adduser command to add more users to the system.