The Linux Boot Process Explained
Posted by Admin on September 23rd, 2006
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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)
Kernel
init
Run Levels
Now we will have a look at each stage in detailed
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.
(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
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-2.4.27
label=27
read-only
man lilo.conf for details.Click here for man page 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
For more information check GRUB man page
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 2.83 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.
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
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:
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
