Linux Boot Process

Press the power button on your system, and after few moments you see the Linux login prompt. 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).

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.

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

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.