LINUX Booting process summary

LINUX BOOTING PROCESS
          6 STEPS PROCESS

  1. STEP1: BIOS (Basic Input Output) Process
    1. BIOS stands for Basic Input/Output System
    2. Performs some system integrity checks(POST-Power on Self Test)
    3. Searches, loads, and execute the boot loader program
    4. Once boot loader program is detected and loaded into the memory, BIOS gives the control to it
    5. SO in simple terms BIOS loads and executes the MBR boot loader
  2. STEP2: MBR ( Master Boot Record)
    1. MBR stands for Master Boot Record
    2. It is located in the 1st sector of the boot able disk. Typically /dev/hda or /dev/sda
    3. MBR loads and executes the GRUB boot loader
    4. MBR is less than 512 bytes in size. This has three components: 
      1. Primary boot loader: 446 bytes,
      2. partition table info - 64 bytes(16,16,16,16) 4 partitions
      3. magic number - 2 bytes
  3. STEP3: GRUB ( Grand Unified Bootloader)
    1. GRUB stands for Grand Unified Bootloader 
    2.  It is multi-boot boot loader which displays a splash screen, waits for few seconds, if you don't enter anything, it loads the defaults kernel image as specified in the grub configuration file - /etc/grub.conf
    3. It contains kernel and initrd image. Thus, GRUB just loads and execute kernel and initrd images.
  4. STEP4: Kernel
    1. Once control is given to kernel which is the central part of your operating system and act as mediator between hardware and software
    2. Kernel once loaded into to RAM  it always resides on RAM until the machine is shutdown
    3. Once kernel starts its operations the first thing it do is executing the INIT process
  5. STEPS5: Init ( initialization)
    1.  Looks at the /etc/inittab file to decide the Linux run level
      1. Following are the avaialbel run levels:
      2. 0---Halt
      3. 1---Single user mode
      4. 2---Multi-user mode-without NFS
      5. 3---Full Multi-user mode
      6. 4--unused
      7. 5--x11
      8. 6---reboot
    2. Typically we set 3 or 5 run levels
  6. STEP6: Run Level Programs
    1. When Linux system is booting up, you might see various services getting started. For example, it might say " starting send-mail ...OK" . Those are the run-level programs executed from the run level directory as defined by your run level
    2. Depending on your default init level setting, the system will execute the programs from on of the following directories.
      1. Run level 0- /etc/rc.d/rc0.d/
      2. Run level 1- /etc/rc.d/rc1.d
      3. Run level 2 -/etc/rc.d/rc2.d/
      4. Run level 3 - /etc/rc.d/rc3.d/
      5. Run level 4 - /etc/rc.d/rc4.d/
      6. Run level 5 - /ec/rc.d/rc5.d/
      7. Run level 6 - /etc/rc.d/rc6.d/

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