Linux Boot Process: Brief & Detailed

By | August 10, 2013

boot-linuxbrainboxBoot Process Very Brief :1

As The Linux Machine is switched ON:
1. A boot loader finds the kernel image on the disk, loads
it into memory, and
starts it.
2. The kernel initializes the devices and its drivers.
3. The kernel mounts the root filesystem.
4. The kernel starts a program called init.
5. init sets the rest of the processes in motion.
6. The last processes that init starts as part of the boot
sequence allow you to log in.

Boot Process Very Brief :2

1)  BIOS is the lowest level interface between computer
and peripherals.
The BIOS performs integrity checks on memory and
seek instructions on the Master Boot Record (MBR) on the
2) The MBR points to the boot loader (GRUB or LILO: Linux
Boot Loader)
3) Boot loader (GRUB or LILO) will then ask for the OS label
which will identify which kernel to run and where it is
located (Hard drive and partitioned)
4) The first thing the kernel does is to execute init
program. init is the root/parent process to all processes in
the Linux.
5) The first processes that init starts is a script
6) Based on the appropriate run-level, scripts are executed
to start the various processes to run the system and make
it functional.

Boot Process Brief :3

BIOS (Basic Input/Output System) Loads from BIOS chip on motherboard

  • POST (Power On Self Test) Checks all connected devices
  • BIOS checks for Boot device availability
  • BIOS loads MBR (Master Boot Record) in Memory (which is first 512 bytes of primary disk)
  • MBR contains information about Boot Loader. And MBR loads default boot loader i.e. GRUB
  • Grub loads Kernel of Operating System, VMLinuz
  • Here onwards Kernel controls booting process
  • Kernel starts INITRD (Initial RAM DISK). InitRD contains preloaded drivers for hardware
  • After loading drivers from INITRD, partitions are mounted (ready only)
  • Init process is started, it becomes first process of system (PID = 1)
  • INIT will mount root and other partitions(read/write) and does FSCK
  • INIT sets up System Clock and Hostname, etc
  • Based on runlevel it will load the services and startup scripts
  • Finally, it will run rc.local script
  • Now the Login Prompt will appear


 Linux Boot Process: Detailed

1. BIOS  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.

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


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.


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 configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to

this). The following is sample grub.conf of CentOS.






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

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.                                                      Ignore ::::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.

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.

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