Understanding Runlevels

Runlevels dictate the state of a Linux system, determining which services and processes are running. In this article, we’ll delve into the concept of runlevels, explore the files and libraries associated with them, and provide essential commands for managing runlevels and services in RHEL 9.

What are Runlevels? In Linux, runlevels represent different operating states of a system. Each runlevel is associated with a specific set of services and processes that are started or stopped when the system enters that runlevel. Traditionally, Linux systems have had seven runlevels, numbered from 0 to 6, with each serving a distinct purpose:

• Runlevel 0: Halt
• Runlevel 1: Single-user mode
• Runlevels 2-5: User-defined runlevels
• Runlevel 6: Reboot

Understanding Files and Libraries Associated with Runlevels: Several files and libraries play crucial roles in managing runlevels in RHEL 9:

1. /etc/inittab: Historically, this file was used to configure the default runlevel and define actions to be taken when entering or exiting a runlevel. However, modern versions of RHEL, including RHEL 9, have replaced it with systemd.
2. systemd: RHEL 9 utilizes systemd as its init system, superseding the traditional SysV init system. Systemd introduces the concept of targets, which are analogous to runlevels. The configuration files associated with systemd are located in /etc/systemd/system/.
3. systemctl start <service> systemctl stop <service>
4. /etc/rc.d/rc*.d/: These directories contain symbolic links to scripts that start or stop services at different runlevels. The naming convention for these scripts follows a pattern: Sxxservicename for start scripts and Kxxservicename for kill (stop) scripts, where “xx” represents the priority of the script execution.

Managing Runlevels and Services in RHEL 9: To effectively manage runlevels and services in RHEL 9, RHCSA candidates should familiarize themselves with the following commands:

1. Viewing Current Runlevel: systemctl list-units --type=target
2. Changing Default Runlevel: systemctl set-default <target>
3. Switching to a Specific Runlevel: systemctl isolate <target>
4. Checking Service Status: systemctl status <service>
5. Starting and Stopping Services: systemctl start <service> systemctl stop <service>
6. Setting Runlevels for Services: systemctl enable <service> – This command enables a service to start automatically at boot. systemctl disable <service> – This command disables a service from starting automatically at boot.

By mastering these concepts and commands, aspiring system administrators can effectively configure and troubleshoot runlevel-related issues and manage services in RHEL 9 environments.