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The way this works in Linux is that every file or directory has both a user and a group 
that takes ownership of it. This is contrary to platforms such as Windows, which can 
have multiple groups assigned to a single resource. With Linux, it's just one-to-one 
ownership: just one user and just one group assigned to each file or directory. If you 
list the contents of a directory on a Linux system, you can see this for yourself:
ls -l 
The following is a sample line of output from a directory on one of my servers:
-rw-r--r-- 1 root bind  490 2020-04-15 22:05 named.conf 
In this case, we can see that 
root
 owns the file and that the group 
bind
 is also 
assigned to it. Ignore the other fields for now; I'll explain them later when we get to 
the section of this chapter dedicated to permissions. For now, just keep in mind that 
one user and one group are assigned to each file or directory.
While each file or directory can only have one group assignment, any user account 
can be a member of any number of groups. Entering the 
groups
 command by itself 
with no options will tell you what groups your logged-in user is currently a member 
of. If you add a username to the 
groups
 command, you'll see which groups that user 
is a member of. Go ahead and give the 
groups
 command a try with and without 
providing a username to get the idea.
On the Ubuntu Server platform, you'll likely see that each of your user accounts is a 
member of a group that's named the same as your username. As I mentioned earlier, 
when you create a user account, you're also creating a group with the same name as 
the user. On some Linux distributions, though, a user's primary group will default to 
a group called 
users
 instead. If you were to execute the 
groups
 command as a user 
on the Ubuntu desktop platform, you would likely see additional groups. This is due 
to the fact that distributions of Linux that cater to being a server platform are often 
more stripped down and users on desktop platforms need access to more things such 
as printers, audio cards, and so on. Some packages that can be installed also add 
additional system users to the server.
If you were curious as to which groups exist on your server, all you would need 
to do is 
cat
 the contents of the 
/etc/group
 file. Similar to the 
/etc/passwd
 file we 
covered earlier, the 
/etc/group
 file contains information regarding the groups that 
have been created on your system. Go 
ahead and take a look at this file on your 
system:
cat /etc/group 

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Managing Users and Permissions