Linux Boot Process

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When we press the power button of computer which has Linux Operating system, and after few minutes Linux Login prompt comes.
We wonder that how it happens behind the screen. Let’s simplify Linux OS Booting process.
1)    BIOS – Basic Input Output System
*      Checks system integrity
*      Looks for boot loader program at cdrom / flopy / harddisk
*      Have a facility to change booting parameter via F12/Del/F2 as per the system
*      Load the boot loader and transfers control to it
Summary:  It goes with first boot options and provides control to MBR (Master Boot Record)
2)    MBR – Master Boot Record
*      Situated into first sector of bootable disk . example - /dev/sda ,  /dev/sdb,  /dev/hda       
*      It has information about boot loader ( GRUB / LILO )
*      Its size is less than 512 bytes and having 3 components
o   Primary Boot Loader Information
o   Mbr validation check
o   OS partition table information
 Summary: It executes GRUB (GRand Unified Boot loader).
3)    GRUB – GRand Unified Boot Loader
*      You can decide which kernel should be booted from multiple installed kernel versions.
*      Run splash image for specified interval time as per config file
*      It has understanding of file system
*      Grub.conf


4)    Kernel
*      Responsible for mounting the root file system
*      Executes INIT program
*      INIT is the first program to be executed by kernel and its PID is 1.
*      Initrd stands for initial RAM disk
*      Initrd has necessary driver information and other information which is required to mount file system until kernel booted , it stands as temporary root file system
5)     Init
*      Run configured run level as per inittab file
*      We can also change run level at boot time by define specific run level
*      Available Different Run Levels
0        HALT
1        Single User Mode
2        Multiuse without networking
3        Console Based Multiuser mode – but no GUI.
4        Not defined
5        GUI based Multiuser mode
6        Reboot
6)     Run Level
*      While OS is booting up, we can see various services coming up and getting status.
*      It happens due to configuration of those services in that specific run level
*      Various programs are in run level directory which has initial word S or K.
*      S stands for startup and K stands for Kill.

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