The following are the 6 high level stages of a typical Linux boot process-
1. BIOS
- BIOS
stands for Basic Input/Output System
- Performs
some system integrity checks
- Searches,
loads, and executes the boot loader program.
- It looks
for boot loader in floppy, cd-rom, or hard drive. You can press a key
(typically F12 of F2, but it depends on your system) during the BIOS
startup to change the boot sequence.
- Once the
boot loader program is detected and loaded into the memory, BIOS gives the
control to it.
- So, in
simple terms BIOS loads and executes the MBR boot loader.
2. MBR
- MBR stands
for Master Boot Record.
- It is
located in the 1st sector of the bootable disk. Typically /dev/hda, or
/dev/sda
- MBR is
less than 512 bytes in size. This has three components 1) primary boot
loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3)
mbr validation check in last 2 bytes.
- It
contains information about GRUB (or LILO in old systems).
- So, in
simple terms MBR loads and executes the GRUB boot loader.
3. GRUB
- GRUB
stands for Grand Unified Bootloader.
- If you
have multiple kernel images installed on your system, you can choose which
one to be executed.
- GRUB
displays a splash screen, waits for few seconds, if you don’t enter
anything, it loads the default kernel image as specified in the grub
configuration file.
- GRUB has
the knowledge of the filesystem (the older Linux loader LILO didn’t
understand filesystem).
- Grub
configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to
this). The following is sample grub.conf of CentOS.
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.el5PAE)
root (hd0,0)
kernel
/boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
initrd
/boot/initrd-2.6.18-194.el5PAE.img
- As you
notice from the above info, it contains kernel and initrd image.
- So, in
simple terms GRUB just loads and executes Kernel and initrd images.
4. Kernel
- Mounts the
root file system as specified in the “root=” in grub.conf
- Kernel
executes the /sbin/init program
- Since init
was the 1st program to be executed by Linux Kernel, it has the process id
(PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.
- initrd
stands for Initial RAM Disk.
- initrd is
used by kernel as temporary root file system until kernel is booted and
the real root file system is mounted. It also contains necessary drivers
compiled inside, which helps it to access the hard drive partitions, and
other hardware.
5. Init
- Looks at the
/etc/inittab file to decide the Linux run level.
- Following
are the available run levels
- 0 – halt
- 1 –
Single user mode
- 2 –
Multiuser, without NFS
- 3 – Full
multiuser mode
- 4 –
unused
- 5 – X11
- 6 –
reboot
- Init
identifies the default initlevel from /etc/inittab and uses that to load
all appropriate program.
- Execute
‘grep initdefault /etc/inittab’ on your system to identify the default run
level
- If you
want to get into trouble, you can set the default run level to 0 or 6.
Since you know what 0 and 6 means, probably you might not do that.
- Typically
you would set the default run level to either 3 or 5.
6. Run level programs
- When the
Linux system is booting up, you might see various services getting
started. For example, it might say “starting sendmail …. OK”. Those are
the runlevel programs, executed from the run level directory as defined by
your run level.
- Depending
on your default init level setting, the system will execute the programs
from one of the following directories.
- Run level
0 – /etc/rc.d/rc0.d/
- Run level
1 – /etc/rc.d/rc1.d/
- Run level
2 – /etc/rc.d/rc2.d/
- Run level
3 – /etc/rc.d/rc3.d/
- Run level
4 – /etc/rc.d/rc4.d/
- Run level
5 – /etc/rc.d/rc5.d/
- Run level
6 – /etc/rc.d/rc6.d/
- Please
note that there are also symbolic links available for these directory
under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
- 0 – halt
- 1 – Single user mode
- 2 – Multiuser, without NFS
- 3 – Full multiuser mode
- 4 – unused
- 5 – X11
- 6 – reboot
- Run level 0 – /etc/rc.d/rc0.d/
- Run level 1 – /etc/rc.d/rc1.d/
- Run level 2 – /etc/rc.d/rc2.d/
- Run level 3 – /etc/rc.d/rc3.d/
- Run level 4 – /etc/rc.d/rc4.d/
- Run level 5 – /etc/rc.d/rc5.d/
- Run level 6 – /etc/rc.d/rc6.d/