6 Stages of Linux Boot Process

Press the power button on your system, and after few moments you see the Linux login prompt.

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

1. BIOS - "BASIC INPUT OUTPUT SYSTEM" EXECUTE MBR

2. MBR - "MASTER BOOT RECORD" EXECUTES GRUB

3. GRUB - "GRAND UNIFIED BOOT LOADER" EXECUTE KERNEL

4. KERNEL - EXECUTES /sbin/init

5. INIT - EXECUTE RUNLEVEL PROGRAM

6. RUN LEVEL - RUN LEVEL PROGRAMS EXECUTED FROM  /etc/rc.d/rc*.d/

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  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 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
• 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.
• 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/

Difference Between Linux & Windows

                                             Difference Between Linux & Windows

Windows:-

1. Windows is Multi-User & Multi-Tasking Operating System.
2. In windows if we End any Process of any user it will effect to all users.
3. Closed System.
4. Windows O/S is User Friendly.
5. It has GUI (Graphical User Interface).
6. Window File Systems are:- FAT16,FAT32,NTFS Etc.

Linux:-

1. Linux is also Multi-User & Multi-Tasking Operating System.
2. In linux if we kill any user Process it will not effect to other user.
3. Linux is an Open Source Operating System.
4. Linux is not User Friendly.
5. Linux has CUI (Character User Interface)
6. Linux File Systems are:- EXT,EXT2,EXT3,EXT4,VFAT  Etc.

Features of Linux

                                                                Features of Linux

1. Multi User

2. Multi Tasking

3. Open System

4. Security

5. Help Facility

1. Multi User :- More than one user can access same system resources (hardware,memory,printer,etc..) at the same time known as multi user.

2. Multi Tasking :- Execution of more than one task (or) application simultaneously known as multi tasking. The main concept of multi tasking is maximum utilizing CPU resources.

3. Open System :- Linux is open source O.S, means it's source code is free anyone can modify it's source code and can create own Operating System.

4 Security :- Linux has two level of security.

A. System Level Security:- Controlled By System Administrator.

B. File Level Security:- Controlled By owner of the file.

5. Help Facility :- In linux we can take help using "man" command.

                     Eg. #man date

                           #man useradd

                            

History of Linux

                                                              History of Linux

Who invented Linux? Linux is the first truly free Unix-like operating system. The underlying GNU Project was launched in 1983 by Richard Stallman originally to develop a Unix-compatible operating system called GNU intended to be entirely free software. Many programs and utilities were contributed by developers around the world, and by 1991 most of the components of the system were ready. Still missing was the kernel.

Linus Torvalds invented Linux itself. In 1991, Torvalds was a student at the University of Helsinki in Finland where he had been using Minix, a non-free Unix-like system, and began writing his own kernel. He started by developing device drivers and hard-drive access, and by September had a basic design that he called Version 0.01. This kernel, which is called Linux, was afterwards combined with the GNU system to produce a complete free operating system.