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Boot process in Linux

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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 MUSIC CREDITS :ON MY WAY KEVIN MACLEOD-IN COMPETECH.COM https://creativecommons.org/licenses/by/3.0/ ISRC: USUAN1600062 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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Text Comments (37)
Linux Query (2 years ago)
Note : kernal is kernel it is misspelled.
nice explain
pratik Deshmukh (2 months ago)
Will surely help in exam
Hitesh Panjwani (2 months ago)
Thanks for video..Fully exam oriented and for last moment revision
nini (2 months ago)
thanks
iForgot My Name (4 months 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..
Shailen Sobhee (5 months ago)
There's no talking here. Watch at 2x speed. Thank me later.
Omar Mukhatar (6 months ago)
great video
Nicholas Johnson (6 months ago)
Missing almost everything about the boot process. What about ACPI, CRW, PCI bus enumeration, MMU, firmware initialisation, driver loading, interrupts, etc?
Bubiribabiri (6 months ago)
Thank you for the video. In which program are you doing the animation??
Kristein Salmath (9 months ago)
Thanks, your explanation was clear, but the unique mistake was Kernal!, no problem.
Swapnil Shinde (11 months ago)
Nice keep it up
Linux Query (11 months ago)
Thank you please subscribe for more videos
Ngọc Quân Trần (1 year ago)
Great video for me - a newbie :D :D
Linux Query (1 year ago)
Thank you
jon alo (1 year ago)
i love your video
Linux Query (1 year ago)
Thank you
Technical Techs (1 year ago)
Good video along with nice content .. keep it up
nenny sundri (1 year ago)
Hacker menemukan bug INTEL, AMD & ARM https://youtu.be/4S_wLchtNAI
Vemuri Pavan (1 year ago)
So from 1:40 boot loader is present inside MBR? but it should be along with bios in ROM chip...?
nifty penguin (9 months ago)
bios executes code on the mbr (bootloader). so control goes from bios to the bootloader present in the mbr.
maheshwar gurav (1 year ago)
thanks
staj (1 year ago)
Thanks for the video. p.s. this videos is for the sysvinit systems & not systemd which have different run level specifications
Linux Query (1 year ago)
yes this video is for SysVinit systems For Example RHEL 6 uses SysVinit RHEL 7 uses systemd
Pramod Tumma (1 year ago)
Good Explanation with Good Content ...added one like
ElectrodeYT (1 year ago)
Why did you mention GRUB
se Nisha (1 year ago)
very nice!! :)
nghnino (1 year ago)
Wonderful presentation
Linux Query (1 year ago)
Thank You...
Vaibhav Deshmukh (1 year ago)
really useful video, thanks
Linux Query (1 year ago)
Thank You..plz subscribe If you didn't
Linux Query (2 years ago)
New Video Watch It https://youtu.be/uxuETukQJ8o
magdum faizul (2 years ago)
it is really nice video about linux booting sequence what ever i have seen i do not know like this much clear explanation
Linux Query (2 years ago)
Thank you... Please Subscribe for more videos
This was very well presented.
JATOTH PRAVEENLAL (2 years ago)
Finally i was clear about MBR....
DynoHippo (2 years ago)
Nice Explanation

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