Listing all users in a BOSS GNU/Linux system

All you have to do to find out the list of users for any given linux system is have a look at the /etc/passwd file.

On a terminal do $cat /etc/passwd or open it in your GUI editor of choice, do keep in mind that a regular user only has read permissions on this file.

For more on /etc/passwd have a look here or here

While you are at it have a look at the following links for information on /etc/shadow:

Link1

Link2

This file contains your encrypted passwords and related details like when it is set to expire and so on.