Understanding Linux Kernel and CentOS Boot Process: A Complete Guide

1. Linux Kernel Components

The Linux system consists of the kernel and the root file system. The kernel handles process management, memory management, network protocol stack, file systems, and device drivers. IPC (Inter‑Process Communication) includes Unix IPC, System V IPC, socket‑based IPC, and POSIX IPC.

Running environments are divided into kernel space (kernel code and system calls) and user space (applications, processes, or threads). Kernel design styles include monolithic kernels (e.g., Linux) and microkernels (e.g., Windows). Linux kernels support modularity via .ko files and allow dynamic loading/unloading of modules.

Key kernel files include /boot/vmlinuz‑VERSION‑release and initramfs images such as /boot/initrd‑VERSION‑release.img (CentOS 5) or /boot/initramfs‑VERSION‑release.img (CentOS 6/7).

2. CentOS System Boot Process

Overall boot order: POST → BIOS (Boot Sequence) → MBR (Boot Loader) → Kernel (ramdisk) → rootfs → switch_root → /sbin/init → set runlevel → system init scripts → start/stop services → launch terminal.

Detailed steps:

POST (Power‑On Self Test) : BIOS on the motherboard checks hardware and initializes devices.

Boot Sequence : BIOS loads the first sector (MBR) of the boot disk.

MBR : Contains 446 bytes of bootloader code and a partition table; presents a menu for selecting OS or kernel.

GRUB (Grand Unified Bootloader) : A multi‑stage bootloader. Stage 1 resides in the MBR, stage 1.5 provides a minimal filesystem driver, and stage 2 (in /boot/grub/) offers the menu and loads the selected kernel and initramfs.

Kernel initialization : The kernel mounts the ramdisk, detects hardware, loads drivers, mounts the root filesystem read‑only, then starts /sbin/init.

/sbin/init : Reads /etc/inittab (CentOS 5) or systemd units (CentOS 7) to set the default runlevel, execute initialization scripts, and launch the appropriate services and terminals.

Runlevels : Levels 0–6 define system states (shutdown, single‑user, multi‑user text, multi‑user with GUI, reboot, etc.).

System initialization script ( /etc/rc.d/rc.sysinit): Sets hostname, welcome message, activates udev/SELinux, mounts filesystems from /etc/fstab, checks and remounts the root filesystem, sets the clock, applies kernel parameters, activates LVM, swap, loads extra drivers, and performs cleanup.

Service start/stop : Scripts in /etc/rc.d/rc read /etc/rc$runlevel.d/ directories; K* scripts stop services (lower numbers stop first), S* scripts start services (lower numbers start first).

Entering single‑user mode involves editing the GRUB entry, adding single (or 1) to the kernel line, and booting.

3. Boot Process Summary

Kernel level: POST → BIOS → MBR/GRUB (three stages) → Kernel (detect hardware, load drivers, mount root read‑only, hand over to /sbin/init). Modern CentOS 6/7 use initramfs instead of initrd to avoid double buffering.

User space level: /sbin/init reads configuration, sets the default runlevel, runs /etc/rc.d/rc.sysinit, stops/starts services according to the runlevel, and finally launches the login terminal (text or graphical).