CentOS: List groups/users

How To List Users and Groups on Linux – devconnected 

cat /etc/passwd

cat /etc/group

Using the groups command

The most memorable command to list all groups a user is a member of is the groups command. When executed without an argument the command will print a list of all groups the currently logged in user belongs to:

groups
The first group is the primary group.
john adm cdrom sudo dip plugdev lpadmin sambashare
To get a list of all groups a specific user belongs to, provide the username to the groups command as an argument:
groups linuxize