BOOT-PROCESS

6 Stages of Linux Boot Process (Startup Sequence)

Press the power button on your system, and after few moments you see the Linux login prompt.

Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?

The following are the 6 high level stages of a typical Linux boot process.
BIOS
1. BIOS

  • BIOS stands for Basic Input/Output System
  • Performs some system integrity checks
  • Searches, loads, and executes the boot loader program.
  • It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
  • Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
  • So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

  • MBR stands for Master Boot Record.
  • It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
  • MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.
  • It contains information about GRUB (or LILO in old systems).
  • So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

  • GRUB stands for Grand Unified Bootloader.
  • If you have multiple kernel images installed on your system, you can choose which one to be executed.
  • GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
  • GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
  • Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

#boot=/dev/sda

default=0

timeout=5

splashimage=(hd0,0)/boot/grub/splash.xpm.gz

hiddenmenu

title CentOS (2.6.18-194.el5PAE)

root (hd0,0)

kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/

initrd /boot/initrd-2.6.18-194.el5PAE.img

  • As you notice from the above info, it contains kernel and initrd image.
  • So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

  • Mounts the root file system as specified in the “root=” in grub.conf
  • Kernel executes the /sbin/init program
  • Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.
  • initrd stands for Initial RAM Disk.
  • initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

  • Looks at the /etc/inittab file to decide the Linux run level.
  • Following are the available run levels
    • 0 – halt
    • 1 – Single user mode
    • 2 – Multiuser, without NFS
    • 3 – Full multiuser mode
    • 4 – unused
    • 5 – X11
    • 6 – reboot
  • Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
  • Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
  • If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.
  • Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

  • When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.
  • Depending on your default init level setting, the system will execute the programs from one of the following directories.
    • Run level 0 – /etc/rc.d/rc0.d/
    • Run level 1 – /etc/rc.d/rc1.d/
    • Run level 2 – /etc/rc.d/rc2.d/
    • Run level 3 – /etc/rc.d/rc3.d/
    • Run level 4 – /etc/rc.d/rc4.d/
    • Run level 5 – /etc/rc.d/rc5.d/
    • Run level 6 – /etc/rc.d/rc6.d/
  • Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
  • Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.
  • Programs starts with S are used during startup. S for startup.
  • Programs starts with K are used during shutdown. K for kill.
  • There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.
  • For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

There you have it. That is what happens during the Linux boot process.
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Step by step explanation of Linux boot sequence
In this topic we will discuss indepth of Linux Boot Sequence.How a linux system boots?This will help administrators in
troubleshooting some bootup problem.Before discussing about I will notedown the major component we need to know
who are responsible for the booting process.
1.BIOS(Basic Input/Output System)
2.MBR(Master Boot Record)
3.LILO or GRUB
LILO:-LInux LOader
GRUB:-GRand Unified Bootloader
4.Kernel
5.init
6.Run Levels
1.BIOS:
i.When we power on BIOS performs a Power-On Self-Test (POST) for all of the different hardware components in the system to make sure everything is working properly
ii.Also it checks for whether the computer is being started from an off position (cold boot) or from a restart (warm boot) is
stored at this location.
iii.Retrieves information from CMOS (Complementary Metal-Oxide Semiconductor) a battery operated memory chip on the motherboard that stores time, date, and critical system information.
iv.Once BIOS sees everything is fine it will begin searching for an operating system Boot Sector on a valid master boot sector
on all available drives like hard disks,CD-ROM drive etc.
v.Once BIOS finds a valid MBR it will give the instructions to boot and executes the first 512-byte boot sector that is the first
sector (“Sector 0″) of a partitioned data storage device such as hard disk or CD-ROM etc .
2.MBR
i. Normally we use multi-level boot loader.Here MBR means I am referencing to DOS MBR.
ii.Afer BIOS executes a valid DOS MBR,the DOS MBR will search for a valid primary partition marked as bootable on the hard disk.
iii.If MBR finds a valid bootable primary partition then it executes the first 512-bytes of that partition which is second level MBR.
iv. In linux we have two types of the above mentioned second level MBR known as LILO and GRUB
3.LILO
i.LILO is a linux boot loader which is too big to fit into single sector of 512-bytes.
ii.So it is divided into two parts :an installer and a runtime module.
iii.The installer module places the runtime module on MBR.The runtime module has the info about all operating systems installed.
iv.When the runtime module is executed it selects the operating system to load and transfers the control to kernel.
v.LILO does not understand filesystems and boot images to be loaded and treats them as raw disk offsets
GRUB
i.GRUB MBR consists of 446 bytes of primary bootloader code and 64 bytes of the partition table.
ii.GRUB locates all the operating systems installed and gives a GUI to select the operating system need to be loaded.
iii.Once user selects the operating system GRUB will pass control to the karnel of that operating system.
see below what is the difference between LILO and GRUB
4.Kernel
i.Once GRUB or LILO transfers the control to Kernel,the Kernels does the following tasks
• Intitialises devices and loads initrd module
• mounts root filesystem
5.Init
i.The kernel, once it is loaded, finds init in sbin(/sbin/init) and executes it.
ii.Hence the first process which is started in linux is init process.
iii.This init process reads /etc/inittab file and sets the path, starts swapping, checks the file systems, and so on.
iv.It runs all the boot scripts(/etc/rc.d/*,/etc/rc.boot/*)
v.starts the system on specified run level in the file /etc/inittab
6.Runlevel
i.There are 7 run levels in which the linux OS runs and different run levels serves for different purpose.The descriptions are
given below.
• 0 – halt
• 1 – Single user mode
• 2 – Multiuser, without NFS (The same as 3, if you don’t have networking)
• 3 – Full multiuser mode
• 4 – unused
• 5 – X11
• 6 – Reboot
ii.We can set in which runlevel we want to run our operating system by defining it on /etc/inittab file.
Now as per our setting in /etc/inittab the Operating System the operating system boots up and finishes the bootup process.
Below are given some few important differences about LILO and GRUB
LILO GRUB
LILO has no interactive command interface GRUB has interactive command interface
LILO does not support booting from a network GRUB does support booting from a network
If you change your LILO config file, you have to rewrite the LILO stage one boot loader to the MBR GRUB automatically detects any change in config file and auto loads the OS
LILO supports only linux operating system GRUB supports large number of OS
To know more about the booting process you can follow the link below

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