Linux 5 Booting Process

                         Linux 5 Booting Process:

1.When the computer starts the power good signal is generated and the system performs POST(power on self test).

2.Here the processor looks at the end of the system memory for BIOS and program and runs it.

3.The BIOS program is the read-only permanent memory and is available for use .It controls first step of boot process.

4.BIOS tests the system and peripherals and then looks for drive(floppy,CD-ROM) and the hard drive as bootable media.Also BIOS settings controls the order of drives used for booting.

5.Now whenever the system boots you require to install a Boot Loader.It is the 1^st s/w program that runs when a computer starts and loads and transfers control to the OS kernel s/w.

6.The task of BL is to display the list of OS in case of systems where there is more than one OS installed.So that the user can select the OS from which to boot.

7.There are 2 varieties of BL:

 

 a)GRUB(Grant unified BL) which contains mechanisms for loading unsupported OS by loading another BL like DOS or windows.

 b)LILO(Linux Loader) This does not depend on specific file system and can boot Linux Kernel images from HD and can even boot other OS.                                        

8.While installing Linux we get the option as Advanced BL configuration:

This is used to determine where you want the BL to be installed.There are 2 ways:

1.MBR-This is loaded by the computer’s BIOS and this is where the BL can take control of the boot process and if it is installed in MBR the BL either GRUB/LILO will display the boot prompt and then we can select the OS you have configured the BL to boot.

2.1^st sector of boot partition-this is used if another BL is on your system. Other BL takes control 1^st and then you configure BL to start GRUB which then boots linux.

9.When Linux is installed in HD the BIOS program looks for MBR that starts at 1^st sector of HD and passes control to it.

10.MBR contains instructions to load the BL(GRUB,LILO) & then the BL takes control of the booting process.BL uses settings in MBR to display the boot options and allows the user to choose the OS to start with.

11.If LILO is booting linux,then it loads kernel images into memory located in /boot directory and then kernel passes control to init.

12.When kernel is loaded in memory it becomes operational but the user can’t provide any meaningful i/p to the system.This issue is resolved by init that brings various services which allows the system to perform its role.

INIT

1. It is the parent or 1^st process that starts automatically.
2. Functions of init:

a)    /etc/rc.d/rc.sysinit script -> it enables and checks the file system and takes care of everything that your system needs. System requires clock so on them rc.sysinit uses /etc/sysconfig/clock file to initialize clock. Also on rc.sysinit you can also run rc.serial if there are serial ports processes that needs to be initialized.

b)    /etc/inittab -> this script is used to setup default runlevel.

Runlevel 0 - > shutdown

Runlevel 1 -> single user mode

Runlevel 2 -> muti-user mode without NFS n/wing

Runlevel 3 -> multi-user mode with NFS n/wing

Runlevel 4 -> reserved

Runlevel 5 -> Graphical mode[default runlevel for linux]

Runlevel 6 -> Reboot

            

 c)   /etc/rc.d/init.d script -> This is used to start & stop various  services like web server.dns et....