Runlevels and the Boot Process in Linux

Detailed explanation of the startup process and operation level of Linux system

Linux, this powerful and versatile operating system, has an extremely wide range of applications, from micro embedded devices to large servers. Whether you are an experienced system administrator or a Linux-curious enthusiast, understanding the startup process and running level of Linux is crucial. This article aims to clarify these concepts, reveal the path of Linux from boot to full operation, and explain the complexity of its run-level system.

Linux startup process

BIOS/UEFI stage

The journey of Linux systems begins with a basic input/output system (BIOS) or a unified extensible firmware interface (UEFI). These firmware interfaces are responsible for performing initial hardware checks and configurations. BIOS is a traditional firmware used in many older systems, while UEFI is its modern counterpart that provides enhanced features such as secure boot and support for larger hard drives.

Bootloader stage

After the initial hardware setup is completed, control is passed to the bootloader, which is the software responsible for loading the operating system. GRUB (Grand Unified Bootloader) is a common example, known for its flexibility in managing multiple operating systems. This phase involves selecting the kernel to boot and specifying any necessary parameters or options.

Kernel initialization

Once selected, the kernel (the core of the Linux operating system) will be loaded into memory. This stage is critical because the kernel sets up all the necessary drivers and subsystems required for basic operation of the system. At this stage, the drivers and modules required to start the system can be temporarily saved using the initial RAM disk (initrd or initramfs).

System initialization

After kernel initialization, the system's initial initialization process begins. This phase is managed by an init system such as SysVinit or systemd, which starts basic services, mounts the file system, and ensures that everything required for a fully running system is up and running.

Linux running level

Definition and use

The run level in Linux is a predefined state that the system can be in, each characterized by specific services and processes that are running or stopped. Understanding the run-level is essential for managing Linux systems, especially when customizing their behavior for different scenarios.

Types of run-level

Linux usually has seven run levels, numbered from 0 to 6:

• 0: Shutdown status.
• 1 : Single-user mode, mainly used for managing tasks.
• 2-5 : Multi-user mode, with varying degrees of network and user interface.
• 6 : Restart state.

Manage operation level

To manage run levels, you can use the runlevel command to check the current status, or use the telinit command to switch to a different run level. These commands are critical for system maintenance and troubleshooting.

Transition to systemd and target

systemd overview

systemd is a modern init system that has basically replaced SysVinit in many Linux distributions. It provides faster startup times and relies on a concept called a target, which is similar to the run level, but offers more functionality and flexibility.

Systemd target and run level

The systemd target is consistent with the traditional run level, which allows users familiar with the old system to make transitions smoother. For example, multi-user.target in systemd corresponds to run level 3 in SysVinit, and graphical.target is similar to run level 5.

Troubleshooting common startup problems

Identify startup issues

Identifying where the startup process fails is the first step in troubleshooting. Whether it is a failure in the bootloader phase or a service in the initialization phase is not started, understanding each step can help isolate the problem.

Rescue and Recovery

Linux provides various recovery methods such as single-user mode or emergency targets in systemd. These modes allow users to start with minimal service, making it easier to diagnose and resolve problems.

in conclusion

Understanding the startup process and run level in Linux is not just academic; it is a practical skill that enhances the ability to manage, troubleshoot and optimize Linux systems. This knowledge reveals what is happening behind the scenes, providing a solid foundation for everyday use and complex system management tasks.

The above is the detailed content of Runlevels and the Boot Process in Linux. For more information, please follow other related articles on the PHP Chinese website!