Linux Boot process, system initialization

System Initialization & Services

–> Boot sequence

i.          BIOS initialization

ii.         Boot Loader initialization

iii.        Kernel initailization

iv.        init initialization

Power on –> BIOS —-> BOOT LOADER(MBR) –> KERNEL–>init

Kernel- Monolithik kernel

-Boot Loader

i.          Grub   (Grand Unified Boot loader) (latest)

ii.         LILO (Linux Loader) (old)

Two types of Boot Loader:

i.First stage: It is very small, generally less than 446 bytes.

It resides either in MBR or first sector of boot partition.

If it resides into MBR, it acts as primary boot loader & if it resides in first sector of boot partition, it acts as secondary boot loader.

First stage is also called IPL (Initial Program Loader) & its main jobs is to locate & load second stage.

ii. Second stage: It always resides in boot partition. (/boot)

It drives the remaining process after being located & loaded by first stage.

#cd /boot/grub

#ls

While booting, the blue screen appears by this file splash.xpm.gz from the /boot/grub

. xpm extension is picture file

. grub.conf is main configuration file for grub

grub.conf resides also in /etc/grub.conf as soft link

# uname -r (displays kernel version)

# echo $BASH_VERSION (displays bash version)

When list the uname -r, it displays as this

a.b.c-d

a-> major kernel version

b-> minor ,,       ,,

.even no. for stable version

.odd no. for trial version

c-> Extraversion

d-> Patch level

(2.4.21-4.EL)

go to /boot

#ls

you can see this file

vmlinuxz-2.4.21-4.EL linux kernel file (contains device druivers of core h/w components)

(the main linux kernel file)

ii. initrd -2.4.21-4.EL.img—>           .initial RAM DISK image file

.It contains device drivers of ext3, lvm etc.

to see the partition lavel

#e2lavel

Grub CLI (command line interface)

three modes:

1. Append mode

press a

2. Edit mode

press e – after modifying press ‘b’ to boot into modified parameters

3. command mode

press c

>boot to boot into specified partameters

4. Press ESC to return to main grub menu

————————————————————————–

grub append> ro root=LABEL=/ 1 (direct run to root without password)

to configue the gui terminal for improper monitor select

#redhat-config-xfree86

to install grub

—————————

#grub-install /dec/heda (this command insta;;/load grub into MBR)

to make startup disk for linux (floppy)

————————————–

#mkbootdisk 2.4.21-4.EL (creates startup disk)

#uname -r (to see linux kernel verson)

to start from different version of startup disk

boot:linux root=/dev/hda3

to go to rescue mode (from bootable installation disk)

———————

boot: linux rescue

language selection

keyboard ,,

mouse   ,,

network ,,

after this

your will see following tabs

rescue mode

continue——-Readonly mode—-skip

select continue

#chroot /mnt/sysimage

#grub-install /dev/hda

#exit

#exit (reboot)