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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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32 Comments

  1. Note : kernal is kernel it is misspelled.

  2. Kernal… I'm closing this

  3. 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.

  4. 02:24 grub configuration file /boot/grub/grub.conf 03:03 /etc/inittab

  5. Good explain

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

  7. 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

  8. do you have another video for linux architect?

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

  10. Hhhhhhh The sims loading screen theme

  11. kernAl LOL

  12. +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.

  13. Are run Levels still in use with systemd?

  14. nice explain

  15. Will surely help in exam

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

  17. thanks

  18. 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..

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

  20. great video

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

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

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

  24. Nice keep it up

  25. Great video for me – a newbie 😀 😀

  26. i love your video

  27. Good video along with nice content .. keep it up

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

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

  30. thanks

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

  32. Good Explanation with Good Content …added one like

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