Linux Introduction

Linux Introduction & Installation Linux was developed in the early 1909’s by Linus Torvald, computer science student at the university of Helsinki. Linux performs well for most applications and is excellent as a network server, though it has yet prove its reliability in mission critical application like FreeBSD has. Linux is distributed freely under GNU. Since Linux became a buzzword many organizations have started offerin professional support and there are many sites on the NET where Linux questions are answered for free. Of course if you are good at coding you can tweak the source code directly. Linux is Multitasking and Multi user Operating system. Linux distribution can be downloaded for free.

ARCHITECTURE OF UNIX/LINUX

commands Application Program Kernel

Utilities

Hardware

Shell

Linux can be generally divided into four majaor components : KERNEL SHELL FILE STRUCTURE UTILITIES KERNEL The kernel is the core that runs programs and Hardware devices and mostly written in “C”. Allocates the system the system resources, scheduling of task and memory management. SHELL 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. FILE STRUCTURE 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. Types of File System  Ordinary files : No system imposed structure. Contain any desired information. Identified be “_” 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 : Contain the information about various files stored in the dorectory. Identified by “d” in the first column of the output of ls –l command. Each directory contain an entry for itself and its directory file. Entry for itself named as “.” And its directory file. Entry for itself named as “.” And its parent shown by “..”  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. Character special files are used for un-buffered I/O to /from the Device.

Block special files refer to disk drives, data is transferred in fixed blocks.

Directory Structure of Root File System. /(Root)

/etc

/home

/dev

/bin

/usr

/var

/boot

/mnt

/tmp

/

- Root directory that is base of the file system. It contains all other files and directories in a logical manner irrespective of their physical lication.]

/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 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.

UTILITIES Linux has various inbuilt software programs called utilities. The utilities are Specialized programs such as Editors, Compilers and communication programs. Linux Startup Process

When you startup a Linux system, series of events occur after you Power On and before receive a login prompt. This sequence is referred to as the boot process.

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 the 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 Run Levels Linux has different levels of operation , reffered to as run level. You can select the defaults run level by editing your /etc/inittab file. 0

- This run level is used to shutdown the system.

1

- Single User mode- Only one user may log on and 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 graphically X window system. Bypassing the console.

6

- This run level used to reboot the System.