Step by step Linux Boot Process

Linux System Administrator

Linux Boot Process


BIOS means Basic Input/Output System.Bios make some system integrity checks, loads, and executes the boot loader program.It looks for the bootloader floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2,del) during the BIOS startup to change the boot sequence.Once the bootloader program is detected and loaded into the memory, BIOS gives the control to it.only word, BIOS loads and executes the MBR boot loader.


MBR means Master Boot Record.It is fixed in the 1st sector of the bootable disk. like /dev/hda, /dev/sda.MBR size less than 512 bytes. there are 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 accommodates news about GRUB (or LILO in old systems).in general terms, MBR loads and executes the GRUB boot loader.


GRUB means Grand Unified Bootloader.If you have multiple kernel images installed on your system, you can select 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.Grub configuration file is /boot/grub/grub.conf in general terms. GRUB just loads and executes Kernel and initrd images.


Mounts the root file system in grub.conf,Kernel executes the /sbin/init program.Since init was the 1st program to be executed by Linux Kernel.its process id is 1.initrd stands for Initial RAM Disk.


init decide the Linux run level. here is the following run level:

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.xecute ‘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 general, you would set the default run level to either 3 or 5.

Runlevel programs

When the Linux system is booting up, you might see various services getting started.default init level setting, the system will execute the programs from one of the following directories.

un 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/



Leave a Reply

Your email address will not be published. Required fields are marked *