RHEL Booting Process

   Boot Proceess Of Linux 

 1.Phase BIOS:-

 

When we turn on the system it run a programe called Basic input output system. After the BIOS load it perform some diagnostics on the hardware, Check the installed components to be sure they are functioning and check the system RAM, The BIOS tries to find out a system drive from which it can load the boot programe to begin the boot process of starting the operating system. Usually the first sector of the drive has an area called the Master Boot Record (MBR), Which hold the programe used to begin the actual loading of OS. As soon as the BIOS find the MBR it gives up control of the boot process and load boot loader in to memory. Usually Grand Unified Boot Loader (GRUB)

 

2. Boot Loader Phase

 

Linux has two step processes to begin loading the operating system. These two steps are typically referred to as stage1 and stage2.  In stage one a programe in MBR is used to find the second stage program that will begin the process of loading the operating system into memory, GRUB uses a configuration file called /boot/grub/grub.conf to provide information to the secound stage loader

 

3. Kernel Phase 

 

After the stage2 passes successfully. It load kernel into memory. And kernel will uncompressed the initrd  in RAM .  mount it as ram disk and then run linuxrc in the The

Ramdisk. After that kernel detect all hardware and configure it properly. And mount the root file system. After the kernel has configured all the system devices and mount the root file system drives, kernel main functions at boot time

a)      Device detection

b)      Device Driver initializations

c)      Mount root filesystem read only

d)     Load  initial process

 it run the /sbin/init command.

 

4. The /sbin/init Phase

 

/sbin/init programe is the first system process that runs after the kernel has configured the system devices and mounted the system. It just likes a project mamager of the system because it manages the remaining step of booting the system and it is the parent of all process. init have its configuration file /etc/inittab. And it consists of many scripts. After reading the /etc/inittab file, init turns over control to the rc.sysinit programe which reads the /etc/rc.d/init.d/functions file to determine the procedure to use to set the default system path, start and stop programe, sysinit also check

a)      set kernel parameter in /etc/sysctl.conf

b)      set system clock

c)      load keymap

d)     Enable swap partiotion

e)      Set hostname

f)       Add raid devices

g)      Enable disk quota

h)      Check and mount other filesystem

 

 The next script to run is /etc/rc.d/rc, which is responsible for starting and stoping services when the runlevel changes and determine the new runlevel. /etc/rc.d directory are additional directory rc0.d, rc1.d, rc2.d, rc3.d. The number in the directory name correspond to the runlevel, Each of these directory contain scripts that are used to stop and start services for the runlevel,All the scripts in the rc5.d directory are symbolic link to the actual scripts that are located in the /etc/rc.d/init.d/ directory 

 

After that init run the gettys specified in the /etc/initab file, These provide six terminal you can use tologin on your server, The last thing the init programe does it run the /etc/rc.d/rc.loacl scripts

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