User & Group Management

User ID
UID 0: Root User
UIDs 1-99: Special System Users
UID 100-999: Dynamic allocated system users
UID 1000-9999: Used for User accounts
UID 10000-65534: Remote Users
UID 65534: nobody (Special user with no privileges)

Add/ Delete User

adduser <username>
deluser <username>
 
usermod -l <new-user-name> <old-user-name> # Change User Login Name
usermod -d <username> <home-directory> # Create Home Directory
usermod -s <username> <new-shell> # Change user shell

The files that are going to be added into the home directory for a newly created user is decided by the /etc/skel directory

NOTE

• adduser is an high level binary that makes the process of adding user of the system simpler
• On systems where adduser is not present useradd can be used
• This command requires as to manually all the compenents that need to be created for the user

Add/ Delete Group

groupadd <group-name>
groupdel <group-name>
usermod -aG <group-name> <username> # Add User to Group
usermod -g <group-name> <username> # Change Group
gpasswd -d <username> <groupname> # Remove user from a group
 
cat /etc/groups # File with Group Information

User Password

passwd <username>
chage -l <username> # View Password Information

Sudoers File

Specifies the rules that need to be satisfied by the user when using sudo command
Open Sudoers File: sudo visudo (Best Practice)

Syntax
User Host = (Runas) Command
Read this as “User may run Command as the Runas user on Host”

%adm ALL=(ALL) NOPASSWD: ALL
This means “any user in the adm group on any host may run any command as any user without a password”
If (ALL : ALL) is used it means the command can be run as any user with further ALL group level privilege as well

Group Names Prefixed : %
UIDs Prefixed : #
GIDs Prefixed : %#

Understanding sudoers(5) syntax

What is the proper sudoers syntax to add a user? - Unix & Linux Stack Exchange

How to Create Users in Linux (useradd Command) | Linuxize