Unveiling the Linux Boot Process: From Power‑On to Init

1. Power‑On Self Test (POST)

After power is applied, the motherboard runs the POST program to check all internal devices. If a fatal error is detected, the system halts; for non‑fatal errors, it emits a beep or visual alert for the user to address.

2. BIOS Boot Stage

Once POST passes, the BIOS searches for bootable devices in order. The first device containing a boot program is selected. Users can enter the BIOS (typically via F2 or Del) to adjust the boot order. The BIOS then reads the MBR of the selected device and loads its bootloader.

3. GRUB Boot Stage

GRUB loads the selected Linux distribution (e.g., CentOS) automatically, though manual boot is also possible.

3.1 Manual boot from the GRUB command line

grub> root (hd#,#)

grub> kernel /vmlinuz-VERSION-RELEASE ro root=/dev/DEVICE

grub> initrd /initramfs-VERSION-RELEASE.img

grub> boot

3.2 GRUB command line interface

e – edit mode for menu entries

c – command mode (interactive)

help – list of commands

help KEYWORD – detailed help

find (hd#,#)/PATH/TO/SOMEFILE – locate a file

root (hd#,#) – set GRUB root device

kernel /PATH/TO/KERNEL_FILE – specify kernel file and optional cmdline parameters (e.g., init=/path/to/init, selinux=0)

initrd /PATH/TO/INITRAMFS_FILE – provide an initramfs image

boot – start the selected kernel

3.3 GRUB configuration file (/boot/grub/grub.conf)

Key entries include:

default=# – default menu entry (starting from 0)

timeout=# – wait time for user selection

splashimage=(hd#,#)/PATH/TO/XPM_PIC_FILE – background image

hiddenmenu – hide the menu

password [--md5] STRING – menu authentication

title TITLE – define a menu entry title

root (hd#,#) – device containing stage2 and kernel

kernel /PATH/TO/VMLINUZ_FILE [PARAMETERS] – kernel to boot

initrd /PATH/TO/INITRAMFS_FILE – associated initramfs

4. Kernel Loading Stage

The kernel initializes itself, detects all hardware, loads drivers (potentially via ramdisk), mounts the root filesystem read‑only, and then starts the first user‑space program: /sbin/init.

5. Init Initialization Stage

After the kernel finishes loading, it runs the init program, which becomes the parent of all processes. init reads /etc/inittab and performs several tasks:

Executes system initialization scripts ( /etc/rc.d/rc.sysinit) to configure the system and mount filesystems read‑write.

Determines the target run level.

Runs /etc/rc.d/rc, which starts services in order (K scripts first, then S scripts). Service status for each run level is stored in /etc/rc.d/rcn.d (n=0~6) and linked to /etc/init.d scripts.

Handles key sequence settings, UPS scripts, virtual terminal startup ( /sbin/mingetty), and X server launch at run level 5.

Configuration file: /etc/inittab

Each line defines an action and its associated process in the format id:runlevels:action:process:

id – task identifier

runlevels – levels on which the task runs (empty means all)

action – condition for execution (e.g., wait, respawn, initdefault, sysinit)

process – the script or program to run

Common actions

wait

– run once when entering the specified level respawn – restart the task if it terminates initdefault – set the default run level (process omitted) sysinit – system initialization, usually

/etc/rc.d/rc.sysinit

Service start/stop prefixes

K*

– stop services (lower numbers stop earlier) S* – start services (lower numbers start earlier)

chkconfig command

Manages the enable/disable state of /etc/init.d scripts across run levels: chkconfig --list [name] – view status chkconfig --add name – add script chkconfig --del name – remove script chkconfig [--level LEVELS] name on|off|reset – set run‑level control (default 2,3,4,5)

init command

init #

– change run level who -r – display current run level

Note

At the normal run level, the last service S99local is linked to /etc/rc.d/rc.local rather than an /etc/init.d script, allowing custom programs to run at boot without creating a service script.