The Linux Boot Process Explained

The beginning of the boot process varies depending on the hardware platform being used. However, once the kernel is found and loaded by the boot loader, the default boot process is identical across all architectures.

The following are the main stages involved in Linux boot process.This process applied debian,ubuntu and most of linux Distributions

BIOS (basic input/output system)

Master Boot Record (MBR)

LILO (Linux Loader ) or GRUB (GRand Unified Bootloader)



Run Levels

Now we will have a look at each stage in detailed


Load boot sector from one of



Hard drive

The boot order can be changed from within the BIOS. BIOS setup can be entered by pressing a key during boot up. The exact key depends varies, but is often one of Del, F1, F2, or F10.

(DOS) Master Boot Record (MBR)

DOS in the context includes MS-DOS, Win95, and Win98.

BIOS loads and execute the first 512 bytes off the disk (/dev/hda)

Standard DOS MBR will

look for a primary partition (/dev/hda1-4) marked bootable

load and execute first 512 bytes of this partition

can be restored with fdisk /mbr from DOS


does not understand filesystems

code and kernel image to be loaded is stored as raw disk offsets

uses the BIOS routines to load

Loading sequence

load menu code, typically /boot/boot.b

prompt for (or timeout to default) partition or kernel

for “image=” (ie Linux) option load kernel image

for “other=” (ie DOS) option load first 512 bytes of the partition

Reconfiguring LILO

One minute guide to installing a new kernel

copy kernel image (bzImage) and modules to /boot and /lib/modules

edit /etc/lilo.conf

duplicate image= section, eg:




man lilo.conf for details.Click here for man page of lilo.conf

run /sbin/lilo

reboot to test


Understands file systems

config lives in /boot/grub/menu.lst or /boot/boot/menu.lst

For more information check GRUB man page


initialise devices

(optionally loads initrd, see below)

mounts root filesystem

specified by lilo or loadin with root= parameter

kernel prints: VFS: Mounted root (ext2 filesystem) readonly.

runs /sbin/init which is process number 1 (PID=1)

init prints: INIT: version 2.83 booting

can be changed with boot= parameter to lilo, eg boot=/bin/sh can be useful to rescue a system which is having trouble booting.


Allows setup to be performed before root FS is mounted

lilo or loadlin loads ram disk image

kernel runs /linuxrc

load modules

initialise devices

/linuxrc exits

“real” root is mounted

kernel runs /sbin/init


reads /etc/inittab (see man inittab which specifies the scripts below check man page )

Run boot scripts:

debian: run /etc/init.d/rcS which runs:

/etc/rcS.d/S* scripts

/etc/rc.boot/* (depreciated)

run programs specified in /etc/inittab

For more available options check init man page

Run Levels

0 halt

1 single user

2 Full Multi-User mode (default)

3-5 Same as 2

6 Reboot

Default is defined in /etc/inittab, eg:


The current runlevel can be changed by running /sbin/telinit # where # is the new runlevel, eg typing telinit 6 will reboot.

Run Level programs

Scripts in /etc/rc*.d/* are symlinks to /etc/init.d

Scripts prefixed with S will be started when the runlevel is entered, eg /etc/rc5.d/S99xdm

Scripts prefixed with K will be killed when the runlevel is entered, eg /etc/rc6.d/K20apache

X11 login screen is typically started by one of S99xdm, S99kdm, or S99gdm.

Run programs for specified run level

/etc/inittab lines

1:2345:respawn:/sbin/getty 9600 tty1

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