Cheatsheet: Difference between revisions

Source: [http://technochords.com/linux-booting-process-6-steps/ technochords.com]

The following are the 6 high level stages of a typical Linux boot process:

#BIOS

#MBR

#GRUB

#Kernel

#Init

#Runlevel programs

;BIOS(Basic Input/Output System) - loads and executes the MBR boot loader.

*Performs some system integrity checks (POST-Power On Self Test)

*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.

;MBR (Master Boot Record) - loads and executes the GRUB boot loader.

*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:

#primary boot loader info in 1st 446 bytes,

#partition table info in next 64 bytes(16,16,16,16) 4 partitions,

#magic numbers as mbr validation check in last 2 bytes.

*It contains information about GRUB (or LILO in old systems).

;GRUB (Grand Unified Bootloader) - loads and executes Kernel and initrd images.

*It is a Multiboot boot loader.

*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).

<pre>

#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

</pre>

*As you notice from the above info, it contains kernel and initrd image.

;Kernel

*Once the control is given to kernel which is the central part of all your OS and act as a mediator between hardware and software.

*Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown.

*Once the Kernel starts its operations the first thing it do is executing INIT process.

;Init (initialization)

*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

*Typically you would set the default run level to either 3 or 5.

;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.