linux boot process step by step.

• When power on a system, the BIOS conduct Power on self test for system integrity checks, called as POST Test.

• It checks all h/w components are properly working or not.

• After completed POST test successfully the BIOS looks for a valid-MBR on available hard drive.

Master Boot Record- MBR which is available in first sector of bootable disk with 512 bytes of size.

1. 1st 446 bytes contains the primary boot loader information.

2. Next 64 bytes contains the partition table information.

3. The last 2 bytes contains the MBR validation check information.

• Then MBR checks for boot loader of Linux and pass control to it.

• GRUB is Grand unified boot loader of Linux, It reads /boot/grub/menu.lst file and get loads the kernel and initrd (initial ram disk).

The Kernel loads the actual root file system by the reference of /etc/fstab.

Initrd initially loads the need device modules which are needed to load root file system, such as Ethernet devices, SCSI, LVM and RAID and so on.

• After root file system loaded the first program execute that is /sbin/init, which checks the configuration in /etc/rc.sysinit and checks the fsck is required and so on.

• Then read /etc/inittab file and get load with specified run level.

• There after it runs all the boot scripts from (/etc/rcN.d/*) to get load the programs on each run level.

• Finally it runs /etc/rc.local file and operating system will get load.