How to add a user to a group

This small how-to is to show you how to add users to existing groups, either new or existing users may be added to one or more groups at once.

In all examples I will use:

user: go2linux groups: admin,printers

There are two possible scenarios

1. The user is new

In this scenario we have to use useradd, and also have two different scenarios.

1.1 Adding user to supplementary group

useradd -G admin go2linux

To add the user to more than one group

useradd -G admin,printers go2linux

1.2 Adding user to primary group

useradd -g admin go2linux

2. The user already exists

In this scenario we have to use usermod

2.1 Add the user to supplementary groups

usermod -a -G admin go2linux

2.2 Add the user to a primary group, in fact change the primary group of that user

usermod -g admin go2linux