1. Linux can be generally divided into four major components: 1. KERNEL – OS, ultimate boss The kernel is the core program that runs programs and manages Hardware devices and mostly written in “C”. Allocates the system resources, scheduling of task and memory management. 2. SHELL – interface between user and kernel The Shell provides an interface for the user. It receives commands from the user and sends those commands to the Kernel for execution. Linux shell is BASH shell. It can be described as an interpreter between the USER and the Machine. 3. FILE STRUCTURE – file organization File Structure organizes the way; the files are stored to or retrieved from a storage device. Files are organized into directories, each directory may contain many number of sub directories Structure of file system is hierarchical. It provides a logical method of organizing and managing information.

2. Types of File System Ordinary files : No system imposed structure. Contains any desired information. Identified by “_” in the first column of the output of ls –l command. There may be two types of regular files Executable-Executable files are program that can be run Non-Executable-Non-executable files are text or data files. Directory Files Contains the information about various files stored in the directory. Identified by “d” in the first column of the output of ls –l command. Each directory contains an entry for itself and its directory file. Entry for itself named as “.” And its parent shown by “..”

3. Types of File System Special Files Represents a physical device e.g. Terminal, Disk Drives etc. Reads and writes to special files in the same way it writes to ordinary files. There are two types of device files : Block and Character. 1. Character special files are used for un-buffered I/O to/from the Device. 2. Block special files refer to disk drives, data is transferred in fixed blocks. Directory Structure of Root File System.

4. /- Root directory that is base of the file system. It contains all other files and directories in a logical manner irrespective of their physical location. /bin- Contains the executable programs of Linux operating system such as cat, cp, ls, more, etc. /dev- Contains special files that represent devices attached to the system, such as floppy disk. Hard disk, printers etc. /etc- System configuration files are kept here. /boot- Linux kernel and other files needed by boot loader are kept here. These files may be kept at any other location also, but are normally kept at /boot. /home - Home directories of all users are kept here. /mnt- This directory contains temporarily mounted file system such as CDROM and floppy drive. /usr- contains sub directories of X window system. /var- contains various system files, such as log files. /tmp- contains temporary files created by system, are normally deleted as the system boots up.

5. 4. UTILITIES – built-in Linux has various inbuilt software programs called utilities. The utilities are specialized programs such as Editors, Compilers and communication programs.

6. Linux Startup process When you startup a Linux system, a series of events occur after you Power On and before you receive a login prompt. This sequence is referred to as the boot process. The kernel is the core program that runs programs and manages Hardware devices and mostly written in “C”.

7. 1.BIOS-Starts checks for hardware devices. The BIOS performs its checks and then looks to the MBR, which contains the first boot loader, such as LILO or GRUB after finding LILO or GRUB, the BIOS initiates LILO or GRUB. 2. LILO or GRUB -Then reads in the partition table as looks for the second stage boot loader on the partition listed in the /etc/lilo.conf or /etc/grub.conf file 3. The second stage boot loader (/boot/boot.b) finds the KERNEL image and runs it. 4.The kernel starts the init process by running /sbin/init. Then init process starts getty process for the console prompt $ or #. Shutdown process [root@server]# init 0 [root@server]# halt [root@server]# Shutdown –y –g0 –i0

8. Run levels Linux has different levels of operation, referred to as run level. You can select the default run level by editing your /etc/inittab file. - Functionality 0- This run level is used to shutdown the system 1- Single User mode - Only one user may log on and that user will be operating as the super user. This mode is mainly for maintenance and repair. 2 - Multi user mode - Users are allowed to log on, but the network has not yet been initialized. 3.- Full multi user mode-All services and network have been initialized. 5- This run level will cause the System to boot directly into the graphical X window system. Bypassing the console. 6.- This run level used to reboot the System.

9. Boot Process STRUCTURE OF GRUB.CONF (/boot/grub/grub.conf) Installing LILO or GRUB In order to boot your Red Hat Linux system, you usually need to install LILO (the LInux LOader)or GRUB (Grand Unified Boot loader).You may install LILO/GRUB in one of the two places :

10. (1)The master boot record (MBR) The recommended place to install boot Loader, unless the MBR already starts another operating system loader, such as System Commander or OS/2's Boot Manager. The MBR is a special area on your hard drive that is automatically loaded by your computer's BIOS, and is the earliest point at which Boot Loader can take control of the boot process. If you install Boot Loader in the MBR, when your machine boots, Boot Loader will present a prompt. You can then boot Red Hat Linux or any other operating system that you have configured Boot Loader to boot. (2)The first sector of your root partition Recommended if you are already using another boot loader on your system (such as OS/2's Boot Manager). In this case, your other boot loader will take control first. You can then configure that boot loader to start LILO/GRUB (which will then boot Red Hat Linux). If you choose to install LILO, please select where you would like LILO to be installed on your system.If your system will use only Red Hat Linux, you should choose the MBR. For systems with Windows 95/98, you should install LILO to the MBR so that LILO can boot both operating systems.