Boot Process of RHEL 6

To became a Redhat Certified System Administrator you should have to know about the boot process of RHEL 6. In this post I will explain you the boot process of the latest Redhat Enterprise Linux 6. RHEL uses GRUB (GRand Unified Boot Loader) as its boot loader. The step-by-step boot process is explained below.

• First we power on the machine, then BIOS (Basic Input Output System) takes over. BIOS is responsible to check RAM, Keyboard, Storage Devices, etc.
• Once the BIOS has setup the environment it finds the first bootable device (which may be CD-ROM, USB, IDE, SATA Hard Drive, etc.) 
• After BIOS finds first bootable media it searches for the Master Boot Record (MBR), which contains boot code, this stage is called STAGE 1 GRUB.
• After Stage 1GRUB, STAGE 1.5 GRUB is consulted, which presents a menu to the user. From that menu we select the OS that we would like to boot. (Grub files are stored in /boot/grub.).
• After that STAGE 2 GRUB is consulted. Stage 2 GRUB sets up the environment and then calls INIT process. The INIT process is located in /sbin/init. INIT reads a special file /etc/itittab, this is a table file which contains initialization entries like default runlevel and then sets up the appropriate runlevel environment. 
• All the scripts which are collectively stored in /etc/rc.d/init.d directory and rc.d contains runlevel directories like rc1.d, rc2.d,....rc6.d.

After all these process a runlevel environment is setup and you are able to do tasks on the Redhat EL 6.

Boot Process Summary:

Power On -> BIOS -> Searches for MBR in Bootable Media -> Finds Boot Code (Stage 1 GRUB) -> GRUB Menu (Stage 1.5 GRUB) -> Stage 2 Grub (Setup the environment calling INIT Process) -> Executes scripts of appropriate runlevel and provides the console. 

For any query regarding to GRUB or BOOT Process please comment .....

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