List users who are currently logged in

users

View users on a linux system

getent passwd

Display a list of all users registerd in the system

cat /etc/passwd

Scroll through the entire /etc/passd file

less /etc/passwd

List only usernames

awk -F: '{print $1}' /etc/passwd

List logged in users

who

Display list of groups and associated users

cat /etc/group

List groups a user is a member of

groups <username>