Every user in Linux must be a member of a primary group. Primary groups are also referred to as login groups. By default, whenever a new user is created, a group is also created with the same name as the user, and this group becomes the primary group of the new user.
On the other hand, a user may or may not be a member of a secondary group. Secondary groups are also sometimes referred to as supplementary groups. You can think of a secondary group as any group that a user is a member of aside from the user's primary group.
Do not worry if you don't understand the concept of primary and secondary groups just yet; it will become crystal clear by the end of this chapter.
Let's create a new user named dummy:
root@ubuntu-linux:~# useradd -m dummy
Now, if you look at the last line of the /etc/group file, you will see that a group named dummy is also created:
root@ubuntu-linux:~# tail -n 1 /etc/group
dummy:x:1004:
This dummy group is the primary...