System Users and Groups

This section provides a tutorial example on how to add and manage system users and groups on Linux computers.

On Linux systems, some special users and groups are created to run system and service programs. They are called system users and groups.

For example, "mysql" is a system user that runs the MySQL database service. On my Linux computer, user "mysql" has a user id of 27 and in a group called "mysql".

herong$ sudo grep mysql /etc/passwd
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash

herong$ sudo grep mysql /etc/group

System users and groups are separated from regular users and groups by their value ranges. Generally, system users and groups have lower id values than regular users and groups.

Most system users and groups are created automatically by software package installation scripts.

If you want to create a system user, you can use the "useradd -r -m" command. "-r" option indicates a system user, and "-m" option creates the "/home/{user}" directory. For example:

herong$ sudo useradd -r -m myservice

8 parameters are defined in the /etc/login.defs file to help "useradd/groupadd" command selecting correct id values:

herong$ sudo more /etc/login.defs

# Min/max values for automatic uid selection in useradd
UID_MIN                  1000
UID_MAX                 60000

# System accounts
SYS_UID_MIN               201
SYS_UID_MAX               999

# Min/max values for automatic gid selection in groupadd
GID_MIN                  1000
GID_MAX                 60000

# System accounts
SYS_GID_MIN               201
SYS_GID_MAX               999

You can use UID_MIN and UID_MAX values to extract regular users from the /etc/passwd file:

herong$ sudo awk -F: '($3>=1000) && ($3<=60000)' /etc/passwd

herong:x:1000:1000:Herong Yang:/home/herong:/bin/bash

By the way, system users are restricted by many applications. So don't use them to perform daily tasks.

Table of Contents

 About This Book

 Introduction to Linux Systems

 Cockpit - Web Portal for Administrator

 Process Management

 Files and Directories

Users and Groups

 Manage User Groups

 Group Access Permissions on Files

 "sudo" Command and Privilege

 "adduser/usermod/userdel" - Commands to Manage Users

System Users and Groups

 Add Users and Groups Manually

 File Systems

 Block Devices and Partitions

 LVM (Logical Volume Manager)

 Installing CentOS

 SELinux - Security-Enhanced Linux

 Network Connection on CentOS

 Internet Networking Tools

 SSH Protocol and ssh/scp Commands

 Software Package Manager on CentOS - DNF and YUM

 vsftpd - Very Secure FTP Daemon

 Postfix - Mail Transport Agent (MTA)

 Dovecot - IMAP and POP3 Server

 Email Client Tools - Mail User Agents (MUA)

 LDAP (Lightweight Directory Access Protocol)

 Administrative Tasks


 Full Version in PDF/EPUB