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Boot process in Linux

admin by admin
July 23, 2020
in Công Nghệ
32
Boot process in Linux



Linux Booting Process Steps:
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

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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.

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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Comments 32

  1. Linux Query says:
    2 years ago

    Note : kernal is kernel it is misspelled.

    Reply
  2. Дмитрий Нагиев says:
    2 years ago

    Kernal… I'm closing this

    Reply
  3. Joe Milosch says:
    2 years ago

    You will never get me to use grub….. lilo is better. ->https://www.youtube.com/watch?v=sVB2QKk55uM Linux: Stop Using GRUB! How to Try LILO (bootloader) Grub is bootloader bloatware.

    Reply
  4. Patrice Kenmoé says:
    2 years ago

    02:24 grub configuration file /boot/grub/grub.conf 03:03 /etc/inittab

    Reply
  5. Ahmed Abdrabo says:
    2 years ago

    Good explain

    Reply
  6. Chris Mahoney says:
    2 years ago

    May have some good info, but the graphics are VERY annoying! Cannot watch.

    Reply
  7. Alfie C says:
    2 years ago

    So the added value of this video vs. powerpoint slides is the animation of a weird hand that writes computer text, all while the music makes me feel like I'm waiting on the phone for customer service

    Reply
  8. gilsyst_ ArchLover says:
    2 years ago

    do you have another video for linux architect?

    Reply
  9. iqueen_x says:
    2 years ago

    Yas I needed to find a girl like this for my test on Saturday

    Reply
  10. Hamza Bjitro says:
    2 years ago

    Hhhhhhh The sims loading screen theme

    Reply
  11. Russ Russ says:
    2 years ago

    kernAl LOL

    Reply
  12. ansh sachdeva says:
    2 years ago

    +Linux Query amazing content. Loved your video!
    Hey can you tell which software did you used for making this video? i want to try my hands in making youtube tutorials too.

    Reply
  13. C Czar says:
    2 years ago

    Are run Levels still in use with systemd?

    Reply
  14. Bishoy AbdelMassieh Mikchaiel says:
    2 years ago

    nice explain

    Reply
  15. pratik Deshmukh says:
    2 years ago

    Will surely help in exam

    Reply
  16. Hitesh Panjwani says:
    2 years ago

    Thanks for video..Fully exam oriented and for last moment revision

    Reply
  17. nini says:
    2 years ago

    thanks

    Reply
  18. χ̶я̶ι̶м̶ѕ̶σ̶η̶ says:
    2 years ago

    I'd like to know how GRUB works in UEFI PC or how to manage the boot entries of multiple OS installed in the PC, by editing GRUB or making GRUB default? etc..

    Reply
  19. Shailen Sobhee says:
    2 years ago

    There's no talking here. Watch at 2x speed. Thank me later.

    Reply
  20. Omar Mukhatar says:
    2 years ago

    great video

    Reply
  21. Nicholas Johnson says:
    2 years ago

    Missing almost everything about the boot process. What about ACPI, CRW, PCI bus enumeration, MMU, firmware initialisation, driver loading, interrupts, etc?

    Reply
  22. Bubiribabiri says:
    2 years ago

    Thank you for the video. In which program are you doing the animation??

    Reply
  23. Kristein Salmath says:
    2 years ago

    Thanks, your explanation was clear, but the unique mistake was Kernal!, no problem.

    Reply
  24. Swapnil Shinde says:
    2 years ago

    Nice keep it up

    Reply
  25. Ngọc Quân Trần says:
    2 years ago

    Great video for me – a newbie 😀 😀

    Reply
  26. jon alo says:
    2 years ago

    i love your video

    Reply
  27. Technical Techs says:
    2 years ago

    Good video along with nice content .. keep it up

    Reply
  28. nenny sundri says:
    2 years ago

    Hacker menemukan bug INTEL, AMD & ARM https://youtu.be/4S_wLchtNAI

    Reply
  29. Vemuri Pavan says:
    2 years ago

    So from 1:40 boot loader is present inside MBR?
    but it should be along with bios in ROM chip…?

    Reply
  30. maheshwar gurav says:
    2 years ago

    thanks

    Reply
  31. staj says:
    2 years ago

    Thanks for the video. p.s. this videos is for the sysvinit systems & not systemd which have different run level specifications

    Reply
  32. Pramod Tumma says:
    2 years ago

    Good Explanation with Good Content …added one like

    Reply

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