The stages involved in Linux Booting Process are:
BIOS
Boot Loader
- MBR
- GRUB
BIOS
Boot Loader
- MBR
- GRUB
Kernel
Init
Runlevel scripts
Init
Runlevel scripts
BIOS
·
This is the first thing which loads once you power on your
machine.
·
When you press the power button of the machine, CPU looks out into
ROM for further instruction.
·
The ROM contains JUMP function in the form of instruction which
tells the CPU to bring up the BIOS
·
BIOS determines all the list of bootable devices available in the
system.
·
Prompts to select bootable device which can be Hard Disk,
CD/DVD-ROM, Floppy Drive, USB Flash Memory Stick etc (optional)
·
Operating System tries to boot from Hard Disk where the MBR
contains primary boot loader.
Boot Loader
To be very brief this phase includes loading of the boot loader
(MBR and GRUB/LILO) into memory to bring up the kernel.
MBR (Master Boot Record)
MBR (Master Boot Record)
·
It is the first sector of the Hard Disk with a size of 512 bytes.
·
The first 434 - 446 bytes are
the primary boot loader, 64 bytes for
partition table and 6 bytes for MBR validation timestamp.
NOTE: Now MBR directly cannot load the kernel as it is unaware of
the filesystem concept and requires a boot loader with file system driver for
each supported file systems, so that they can be understood and accessed by the
boot loader itself.
To overcome this situation GRUB is used with the details of the filesystem in /boot/grub.conf and file system drivers
GRUB (GRand Unified Boot loader)
This loads the kernel in 3 stages
GRUB stage 1:
To overcome this situation GRUB is used with the details of the filesystem in /boot/grub.conf and file system drivers
GRUB (GRand Unified Boot loader)
This loads the kernel in 3 stages
GRUB stage 1:
·
The primary boot loader takes up less than 512 bytes of disk space
in the MBR - too small a space to contain the instructions necessary to load a
complex operating system.
·
Instead the primary boot loader performs the function of loading
either the stage 1.5 or stage 2 boot loader.
GRUB Stage 1.5:
·
Stage 1 can load the stage 2 directly, but it is normally set up
to load the stage 1.5.
·
This can happen when the /boot partition is situated beyond the
1024 cylinder head of the hard drive.
·
GRUB Stage 1.5 is located in the first 30 KB of Hard Disk
immediately after MBR and before the first partition.
·
This space is utilized to store file system drivers and modules.
·
This enabled stage 1.5 to load stage 2 to load from any known
location on the file system i.e./boot/grub
GRUB Stage 2:
·
This is responsible for loading kernel from /boot/grub/grub.conf and any other modules
needed
·
Loads a GUI interface i.e. splash image located at /grub/splash.xpm.gz with list of
available kernels where you can manually select the kernel or else after the default
timeout value the selected kernel will boot
The original file is /etc/grub.conf of which you can observe a
symlink file at/boot/grub/grub.conf
Sample /boot/grub/grub.conf
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux Server (2.6.18-194.26.1.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.26.1.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.26.1.el5.img
title Red Hat Enterprise Linux Server (2.6.18-194.11.4.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.11.4.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.11.4.el5.img
title Red Hat Enterprise Linux Server (2.6.18-194.11.3.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.11.3.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.11.3.el5.img
Sample /boot/grub/grub.conf
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux Server (2.6.18-194.26.1.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.26.1.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.26.1.el5.img
title Red Hat Enterprise Linux Server (2.6.18-194.11.4.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.11.4.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.11.4.el5.img
title Red Hat Enterprise Linux Server (2.6.18-194.11.3.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.11.3.el5 ro root=/dev/VolGroup00/root clocksource=acpi_pm divisor=10
initrd /initrd-2.6.18-194.11.3.el5.img
Kernel
This can be considered the heart of operating system responsible
for handling all system processes.
Kernel is loaded in the following stages:
Kernel is loaded in the following stages:
1.
Kernel as soon as it is loaded configures hardware and memory
allocated to the system.
2.
Next it uncompresses the initrd image (compressed
using zlib into zImage or bzImage formats) and mounts it
and loads all the necessary drivers.
3.
Loading and unloading of kernel modules is done with the help of
programs like insmod, and rmmod present in the initrd image.
4.
Looks out for hard disk types be it a LVM or RAID.
5.
Unmounts initrd
image and frees up all the memory occupied by the disk image.
6.
Then kernel mounts the root partition as
specified in grub.conf as read-only.
7.
Next it runs the init process
Init Process
·
Executes the system to boot into the run level as specified in /etc/inittab
Sample output defining the default
boot runlevel inside /etc/inittab
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
As per above O/P system will boot into runlevel 5
You can check current runlevel details of your system using below command on the terminal
# who -r
run-level 3 Jan 28 23:29 last=S
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
As per above O/P system will boot into runlevel 5
You can check current runlevel details of your system using below command on the terminal
# who -r
run-level 3 Jan 28 23:29 last=S
·
Next as per the fstab entry file system's
integrity is checked and root partition is re-mounted as
read-write (earlier it was mounted as read-only).
Runlevel scripts
8.
A no. of runlevel scripts are defined inside /etc/rc.d/rcx.d
Runlevel Directory
0 /etc/rc.d/rc0.d
1 /etc/rc.d/rc1.d
2 /etc/rc.d/rc2.d
3 /etc/rc.d/rc3.d
4 /etc/rc.d/rc4.d
5 /etc/rc.d/rc5.d
6 /etc/rc.d/rc6.d
Runlevel Directory
0 /etc/rc.d/rc0.d
1 /etc/rc.d/rc1.d
2 /etc/rc.d/rc2.d
3 /etc/rc.d/rc3.d
4 /etc/rc.d/rc4.d
5 /etc/rc.d/rc5.d
6 /etc/rc.d/rc6.d
·
Based on the selected runlevel, the init process then executes
startup scripts located in subdirectories of the /etc/rc.d directory.
·
Scripts used for runlevels 0 to 6 are located in subdirectories /etc/rc.d/rc0.d through/etc/rc.d/rc6.d, respectively.
·
Lastly, init runs whatever it finds in /etc/rc.d/rc.local (regardless of run
level). rc.localis rather
special in that it is executed every time that you change run levels.
NOTE: rc.local is
not used in all the distros as for example Debian.
Next if everything goes fine you should be able to see the Login
Screen on your system.