Linux Boot Sequence

1) BIOS

2) Master Boot Record (MBR)

3) LILO or GRUB

4) Kernel

5) init

6) Run Levels

1) BIOS

Load boot sector from one of:

Floppy

CDROM

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.

2)(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

3) LILO

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:

image=/bzImage-2.4.14

label=14

read-only

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

run /sbin/lilo

reboot to test

GRUB

Understands file systems

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

4)Kernel

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 2.76 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.

initrd

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

Details in /usr/src/linux/Documentation/initrd.txt (part of the kernel source).

5) /sbin/init

reads /etc/inittab (see man inittab which specifies the scripts below for manpage click here)

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

6)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:

id:3:initdefault:

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

Always running in runlevels 2, 3, 4, or 5

Displays login on console (tty1)

2:234:respawn:/sbin/getty 9600 tty2

Always running in runlevels 2, 3, or 4

Displays login on console (tty2)

l3:3:wait:/etc/init.d/rc 3

Run once when switching to runlevel 3.

Uses scripts stored in /etc/rc3.d/

ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

Run when control-alt-d