Booting Sequence


Booting  Sequence
A boot sequence is the initial set of operations that the computer performs when it is switched on. Every computer has a boot sequence. The average computer doesn’t understand the boot sequence but is important to know for customizing and troubleshooting your computer. Alternatively referred to as boot options or boot order, the boot sequence defines which devices a computer should check for the operating system’s boot files. It also specifies in which order those devices should be checked. Booting a Linux installation involves multiple stages and software components, including firmware initialization, execution of a boot loader, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. For each of these stages and components there are different variations and approaches; for example, GRUB, LILO, SYSLINUX or Loadlin can be used as boot loaders, while the startup scripts can be either traditional init-style, or the system configuration can be performed through more modern alternatives such as systemd or Upstart.

boot

Bios
MBR
Grub
Kernel
Init
Runlevel

BIOS
A BIOS (basic input output system) is a small program that controls a personal computer’s hardware from the time the computer is started until the main operating system (e.g., Linux, Mac OS X or MS-DOS) takes over. 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.

MBR
MBR (Master Boot Record) simply use the dd command. dd command works under all Linux distros and other UNIX like operating systems too. A master boot record (MBR) is the 512-byte boot sector that is the first sector of a partitioned data storage device of a hard disk.It contains information about GRUB (or LILO in old systems).

Grub
GNU GRUB (Grand Unified Bootloader) was developed from a package called the Grand Unified Bootloader (a play on Grand Unified Theory). It is predominantly used for Unix-like systems. The GNU operating system uses GNU GRUB as its boot loader, as do most Linux distributions. Grub configuration file is /boot/grub/grub.conf

Kernel
The kernel is the essential center of a computer operating system, the core that provides basic services for all other parts of the operating system. A synonym is nucleus. A kernel can be contrasted with a shell, the outermost part of an operating system that interacts with user commands.Kernel executes the /sbin/init program.

Init
In Unix-based computer operating systems, init (short for initialization) is the first process started during booting of the computer system. Init is a daemon process that continues running until the system is shut down.

Init Levels
0
Halt
1
Single User Mode
2
Multi User Mode
3
Multi User Mode with Networking
4
User Definable
5
Start the system normally with appropriate display manager (with GUI)
6
Reboot

Runlevel
A runlevel is a preset operating state on a Unix-like operating system. A system can be booted into any of several runlevels, each of which is represented by a single digit integer.

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