Press
the power button on your system, and after few moments you see the Linux login
prompt.
Have
you ever wondered what happens behind the scenes from the time you press the
power button until the Linux login prompt appears?
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).
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. Runlevel 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.
§ Under
the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and
K.
§ Programs
starts with S are used during startup. S for startup.
§ Programs
starts with K are used during shutdown. K for kill.
§ There
are numbers right next to S and K in the program names. Those are the sequence
number in which the programs should be started or killed.
§ For
example, S12syslog is to start the syslog deamon, which has the sequence number
of 12. S80sendmail is to start the sendmail daemon, which has the sequence
number of 80. So, syslog program will be started before sendmail.
There
you have it. That is what happens during the Linux boot process
About /etc/fstab:
1.
The /sbin/init reads /etc/inittab file
2.
Set default runlevel ( the telinit command
allows administrators to tell the init process to
change its current runlevel)
3.
Calls /etc/rc.d/rc.sysinit and /etc/rc.d/rc x (where ‘x‘ is a runlevel)
4.
In /etc/rc.d/rc5.d directory files starting with letter K –> kill scripts and files
starting with letter S –> Startup scripts.
5.
Start up the tty processes
and xdm ( X display manager)
6.
Starts User’s login screen
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