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RHCSA - Understand and Use Essential Tools: Log in & Switch Users in Multiuser Targets

This guide covers essential Linux system administration topics for the RHCSA certification. It focuses on multi-user targets in systemd, different methods for user login (graphical, text-based, SSH), and user switching techniques using su and sudo commands. These topics are essential for understanding and managing multi-user environments in Linux.

Multi-User Targets

Linux systems utilizing systemd, incorporate a system state and management structure known as 'targets'. A target in the context of systemd is essentially a group of systemd units, which can include services, sockets, device mounts, and other components. These units are collectively organized to define and achieve specific system states for various purposes.

One of the most significant and commonly employed targets within the Linux environment is the multi-user target. This target is tailored for scenarios where a system needs to support multiple users, typically in settings that do not require a graphical user interface (GUI) but do require network capabilities.

The multi-user target is distinguished by its non-graphical nature. It primarily provides a command-line interface (CLI), which is an ideal setup for servers or headless machines where a GUI might be unnecessary or even a hindrance due to resource constraints. Users interact with the system in this target through terminal sessions, which can be accessed either directly on the machine or remotely via network protocols like SSH.

A key characteristic of the multi-user target is its readiness for network interactions. This target ensures that the system is prepared to handle various network services. These services can range from SSH for remote access to web servers for hosting websites, and even include file sharing services. The network-enabled state of the multi-user target makes it an excellent choice for running server applications that necessitate constant network connectivity.

As the name implies, the multi-user target accommodates simultaneous use by several users. This support is not limited to local access; remote logins are facilitated, assuming necessary services such as SSH are enabled and properly configured. This functionality is important in shared server environments, educational settings, and businesses where multiple individuals need to access the system concurrently.

In the multi-user target, common system services are active and operational. These services encompass system logging (like syslog or journald), cron services for scheduling tasks, as well as print and email services. The selection of active services aims to provide a comprehensive environment suitable for a variety of tasks typical in a multi-user context.

The multi-user target finds extensive use in server environments where a GUI is unnecessary. It's also prevalent in workstations dedicated to development, scripting, or remote management. The target's versatility and its non-reliance on a GUI make it a preferred choice for backend systems, data servers, and application servers.

Log in as a User

There are 3 mains ways for logging in as a user in a multiuser environment, which are:

  • Using the Graphical environment
  • Using the text based terminal console
  • Connecting remotely through SSH

You would have already logged in using the graphical environment and remotely via SSH when following along with this course. Therefore the text based console will be demonstrated here.

On server1 you can switch to a text based terminal console by using the chvt command or by using Ctrl+Left Alt+F3.

Info

The Ctrl is optional depending on what hypervisor or bare metal setup you are running on. Additionally, F3 can be replaced up to F12, however for both the chvt command and the key shortcuts, most installations will have 3-6 available by default.

The chvt command stands for "change virtual terminal", and is a simple yet effective tool used to switch between different virtual terminals. These virtual terminals are text-based consoles that you can access independently of your graphical desktop environment. By executing chvt followed by a terminal number, like chvt 2, you can switch to the second virtual terminal, allowing you to manage multiple sessions from a single physical machine. It's particularly useful in situations where you need to perform tasks outside of the graphical interface or when automating processes across different terminals.

Exercise: Using the chvt command to change to terminal /dev/tty3

sudo chvt 3

Enter your password to elevate your privileges.

[sudo] password for user1:

You are now presented with the text based terminal console where you first enter the username you wish to login as, and then enter that users password.

Red Hat Enterprise Linux 9.2 (Plow)
Kernel 5.14.0-284.18.1.el9_2.x86_64 on an x86_64

Activate the web console with: systemctl enable --now cockpit.socket

server1 login: root
Password:
Last login: Sun Jun 25 14:43:42 on tty3

To return to the graphical environment, run:

sudo chvt 2

Switching Users

The su command can be used to switch users. When run with the -, -l or --login flags, the shell will start as a login shell (similar to a real login). When run without those arguments, you will retain the majority of the environment settings of the current user.

In this exercise you will switch to the root user with a login environment:

Exercise: Switch to root user with a login environment:

When switching to root you can omit the username, as by default, su - will perform su - root.

su - root

pwd

Enter the root password when prompted and ensure you have simulated a login environment by running pwd. The output should show /root.

In this exercise you will switch to the root user with a non-login environment:

Exercise: Switch to root user with a non-login environment:

su root

pwd

Enter the root password when prompted and ensure you have a non-login environment by running pwd. The output should show that you did not enter the /root directory, for example /home/user1 would return if that was where you ran the su command from.

Unlike the su command where you can switch to any user, you can use the sudo command with the -i or --login flags to login as the root user, which you will try out in the below exercise:

Exercise: Login as the root user using sudo.

When using sudo it will prompt for the password of the current user, in this example, user1:

sudo -i

Or:

sudo --login

Conclusion

This guide provides a thorough understanding of managing multi-user environments in Linux, focusing on systemd's multi-user targets. It covers various user login methods, including graphical interfaces & text-based consoles. Additionally, it delves into user-switching techniques using su and sudo commands.

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