Ok-Guru: Linux Booting Procedure

Linux Booting Procedure

How Linux boot?

System Startup

Booting is a bootstrapping process that starts operating systems when the user turns on a computer system
A boot sequence is the set of operations the computer performs when it is switched on that load an operating system

Booting sequence

Turn on
CPU jump to address of BIOS (0xFFFF0)
BIOS runs POST (Power-On Self Test)
Find bootable devices
Loads and execute boot sector form MBR
Load OS

BIOS (Basic Input/Output System)

BIOS refers to the software code run by a computer when first powered on
The primary function of BIOS is code program embedded on a chip that recognizes and controls various devices that make up the computer.

Boot loader

MBR (Master Boot Record)

OS is booted from a hard disk, where the Master Boot Record (MBR) contains the primary boot loader
The MBR is a 512-byte sector, located in the first sector on the disk (sector 1 of cylinder 0, head 0)
After the MBR is loaded into RAM, the BIOS yields control to it.

The first 446 bytes are the primary boot loader, which contains both executable code and error message text
The next sixty-four bytes are the partition table, which contains a record for each of four partitions
The MBR ends with two bytes that are defined as the magic number (0xAA55). The magic number serves as a validation check of the MBR

Extracting the MBR

To see the contents of MBR, use this command:
# dd if=/dev/hda of=mbr.bin bs=512 count=1
# od -xa mbr.bin

**The dd command, which needs to be run from root, reads the first 512 bytes from /dev/hda (the first Integrated Drive Electronics, or IDE drive) and writes them to the mbr.bin file.

**The od command prints the binary file in hex and ASCII formats.

Boot loader could be more aptly called the kernel loader. The task at this stage is to load the Linux kernel
Optional, initial RAM disk
GRUB and LILO are the most popular Linux boot loader.

Other boot loader (Several OS)

bootman

GRUB

LILO

NTLDR

XOSL

BootX

loadlin

Gujin

Boot Camp

Syslinux

GAG

GRUB: GRand Unified Bootloader

GRUB is an operating system independant boot loader
A multiboot software packet from GNU
Flexible command line interface
File system access
Support multiple executable format
Support diskless system
Download OS from network

GRUB boot process

The BIOS finds a bootable device (hard disk) and transfers control to the master boot record
The MBR contains GRUB stage 1. Given the small size of the MBR, Stage 1 just load the next stage of GRUB
GRUB Stage 1.5 is located in the first 30 kilobytes of hard disk immediately following the MBR. Stage 1.5 loads Stage 2.
GRUB Stage 2 receives control, and displays to the user the GRUB boot menu (where the user can manually specify the boot parameters).
GRUB loads the user-selected (or default) kernel into memory and passes control on to the kernel.

LILO: LInux LOader

Not depend on a specific file system
Can boot from harddisk and floppy
Up to 16 different images
Must change LILO when kernel image file or config file is changed

Kernel image

The kernel is the central part in most computer operating systems because of its task, which is the management of the system's resources and the communication between hardware and software components
Kernel is always store on memory until computer is tern off
Kernel image is not an executable kernel, but a compress kernel image
zImage size less than 512 KB
bzImage size greater than 512 KB

Task of kernel

Process management
Memory management
Device management
System call

Init process

The first thing the kernel does is to execute init program
Init is the root/parent of all processes executing on Linux
The first processes that init starts is a script /etc/rc.d/rc.sysinit
Based on the appropriate run-level, scripts are executed to start various processes to run the system and make it functional

The Linux Init Processes

The init process is identified by process id "1“
Init is responsible for starting system processes as defined in the /etc/inittab file
Init typically will start multiple instances of "getty" which waits for console logins which spawn one's user shell process
Upon shutdown, init controls the sequence and processes for shutdown

System processes

Process ID	Description
0	The Scheduler
1	The init process
2	kflushd
3	kupdate
4	kpiod
5	kswapd
6	Mdrecoveryd

Inittab file

The inittab file describes which processes are started at bootup and during normal operation /etc/init.d/boot and /etc/init.d/rc
The computer will be booted to the runlevel as defined by the initdefault directive in the /etc/inittab file

id:5:initdefault:

Runlevels

A runlevel is a software configuration of the system which allows only a selected group of processes to exist
The processes spawned by init for each of these runlevels are defined in the /etc/inittab file
Init can be in one of eight runlevels: 0-6

Runlevel	Scripts Directory (Red Hat/Fedora Core)	State
0	/etc/rc.d/rc0.d/	shutdown/halt system
1	/etc/rc.d/rc1.d/	Single user mode
2	/etc/rc.d/rc2.d/	Multiuser with no network services exported
3	/etc/rc.d/rc3.d/	Default text/console only start. Full multiuser
4	/etc/rc.d/rc4.d/	Reserved for local use. Also X-windows (Slackware/BSD)
5	/etc/rc.d/rc5.d/	XDM X-windows GUI mode (Redhat/System V)
6	/etc/rc.d/rc6.d/	Reboot
s or S		Single user/Maintenance mode (Slackware)
M		Multiuser mode (Slackware)

rc#.d files

rc#.d files are the scripts for a given run level that run during boot and shutdown
The scripts are found in the directory /etc/rc.d/rc#.d/ where the symbol # represents the run level

init.d

Deamon is a background process
init.d is a directory that admin can start/stop individual demons by changing on it

/etc/rc.d/init.d/ (Red Hat/Fedora )

/etc/init.d/ (S.u.s.e.)

/etc/init.d/ (Debian)

Start/stop deamon

Admin can issuing the command and either the start, stop, status, restart or reload option
i.e. to stop the web server:

cd /etc/rc.d/init.d/
(or /etc/init.d/ for S.u.s.e. and Debian)
httpd stop

Linux files structure

All directories are grouped under the root entry "/"
root - The home directory for the root user
home - Contains the user's home directories along with directories for services

ftp
HTTP
samba

bin - Commands needed during booting up that might be needed by normal users
sbin - Like bin but commands are not intended for normal users. Commands run by LINUX.
proc - This filesystem is not on a disk. It is a virtual filesystem that exists in the kernels imagination which is memory

1 - A directory with info about process number 1. Each process has a directory below proc.

usr - Contains all commands, libraries, man pages, games and static files for normal operation.

bin - Almost all user commands. some commands are in /bin or /usr/local/bin.
sbin - System admin commands not needed on the root filesystem. e.g., most server programs.
include - Header files for the C programming language. Should be below /user/lib for consistency.
lib - Unchanging data files for programs and subsystems
local - The place for locally installed software and other files.
man - Manual pages
info - Info documents
doc - Documentation
tmp
X11R6 - The X windows system files. There is a directory similar to usr below this directory.
X386 - Like X11R6 but for X11 release 5

boot - Files used by the bootstrap loader, LILO. Kernel images are often kept here.
lib - Shared libraries needed by the programs on the root filesystem
modules - Loadable kernel modules, especially those needed to boot the system after disasters.
dev - Device files
etc - Configuration files specific to the machine.
skel - When a home directory is created it is initialized with files from this directory
sysconfig - Files that configure the linux system for devices.
var - Contains files that change for mail, news, printers log files, man pages, temp files

file
lib - Files that change while the system is running normally
local - Variable data for programs installed in /usr/local.
lock - Lock files. Used by a program to indicate it is using a particular device or file
log - Log files from programs such as login and syslog which logs all logins and logouts.
run - Files that contain information about the system that is valid until the system is next booted
spool - Directories for mail, printer spools, news and other spooled work.
tmp - Temporary files that are large or need to exist for longer than they should in /tmp.
catman - A cache for man pages that are formatted on demand