COMPUTER NETWORK

Another key element of information technology is computer network. You know the information technology is the merger of computer and communications so that easy and quick information may be provided to the different users for their day-to-day task. Now a network is a source of communicating data or information among a set of computers either using some sort of cable or wireless media. In case of cable media, we will be using the communication channels of fiber optic cables or twisted pair cables (both shielded and unshielded or coaxial cables (including both thin and thick coax) and fiber optic. On the other hand, if wireless media is used for communication, then Satellite transmission, radio waves, infra-red waves, microwaves, mobile communication or any other such medium will be used to provide its channels for the communication of information.

A network may be defined as "the environment in which two or more computers are connected together with the help of certain specific media, which can transmit data and information to each other." As there are differences between data and information, the network that is responsible for the communication of information among a set of computers is called computer network whereas the one with the help of which simple data is communicated among inter-networked computer is called data communication network. As information technology specifically belongs to the delivery of information to users, computer networks are highly associated to this technology. A good example of computer network is the Internet and World Wide Web.

This is belongs to techniques of communication of information across an information network so that users in each section of a business organisation may be benefited. These communication techniques, their fundamentals and uses are discussed in the topic below.

1. Technology of Workgroup Computing:
   Workgroup Computing is an important technique. It is used for communication of information within the boundaries of some business organisation. Using this technique, different departments of an organisation may have information in time. This technique is used to arrange a set of computers in a logical manner is such a way that this set is identified by a unique name. This unique name may be any name e.g. name of the organisation or some other proposed name. Due to workgroup computing, its members can access resources shared by other computers within the same workgroup.
   Each computer in a workgroup is responsible for its own security. An example of workgroup may be a network established among the different departments of an organisation e.g. a bank. One department can get benefit from information stored on workgroup member computer of another department of the same bank. Workgroup computing technology is very helpful in the communication of information through computer network. Thus sharing of information is easy for computer based information system. Some network operating system that are most appropriate for workgroup computing are Windows 95 and Windows 98.
2. The Benefits of Email;
   One of several benefits that workgroup provides to its user is Email, which means electronic mail. Using this facility, the user of a workgroup can send or receive their messages regarding business or other interests of their employer / organisation. It is considerably beneficial when the organisation spread over a large area and direct meetings with one another are time consuming. Special websites of the Internet / World Wide Web servers are Yahoo, Hotmail, Lycos and so on.
   Using email facility, the users can:
   *     Send or receive messages to / from other users of computer networks.
   *     Attache important document, pictures, audios and videos with their mails and send them to other people living anywhere in the wold through Internet.
   *     Get the latest information about any topic of interest e.g. jobs, admission, market, sports, stock exchange and what so ever by having membership of specific newsgroup.
   There are many more benefits of Email, however using Email services the most important advantage is that information we want to send or receive may be transmitted within seconds across thousands of miles even from one continent to another in the world. The Email facility can be achieved through subscription to an Email server and making an account in it. This subscription takes place through a form that is provided by Email server filling and submitting which grants the user his / her Email account and address.
   Email is also called offline communication. The receiving person doesn't need to be online or connected to Internet at the time of Emailing rather he / she can receive the message after days or months as long as his / her email account exists. The Email message remains stored on the specified email server computer. The receiving person gets it as soon as he / she connects the Internet and opens his / her email account. Chatting on the other hand is an example of online communication of information. Both sending and receiving persons must be online during the chatting. It is therefore considered as direct communication among the users of a computer network.
3. Internet:
   A good example of global computer is the Internet i.e. a network of almost all computer networks in the world. It is thus working as a huge computer network. It is considered as the backbone  of modern information technology. Now a days it contains World Wide Web (www) which is collection of several millions uploaded websites that exist in the world. These websites contain information about different fields of life. This information can be easily accessed if someone is connected to the Internet. The useful characteristics of Internet and World Wide Web are:
   *     Latest news about different matters of life may be obtained easily.
   *     Researchers are capable of reading the up[loaded research papers and other materials of scholars in a particular field to put forward their research work.
   *     Email and chatting may be used to send or receive instant messages to friends or relatives living far away in the world.
   *     Business companies are capable of advertising their products and services to the entire world.
   There are many more advantages of Internet but in short, Internet is a very cheap source of gaining the latest information about the world.
4. LAN & WAN:
   One of the important considerations of establishing a computer network is that of area in which the network will perform it functions. On the basis of this, there are two different types of computer networks i.e. LAN stands for (Local Area Network) and WAN stands for (Wide Area Network).
1. Local Area Network (LAN):
   A Local Area Network (abbreviated as LAN) is a collection of computers and peripherals that are usually connected together by cables and is confined or bounded to one building, site, campus or a city. A LAN offers a mean of pooling network resources and information between a number of users. A central mini computer or mainframe computer with a number of "dumb" terminals attached to it is one form of LAN, but the term is most frequently used to describe a number of interlinked Personal Computer (PCs), often with a more powerful computer controlling the network.
   
   A Local Area Network has several advantages over a collection of standalone minicomputers with respect to:
   *     Sharing of resources such as disk storage, printers and possibly a large powerful computer called server.
   *     Sharing of information held on disk drives that are accessible by all users of network.
   *     Sharing of installed software.
   *     Ability to communicate with other users on the network. This is not important when all computer are in the same room but can be very useful in case when the computers are distributed around a large building.
   
   Components of LAN:
   All LANs must have at least three basic components that are:
1. Workstation which are actually PCs or terminals
2. File Servers or Disk Servers, which are special PCs or larger computers. Shared software resources are stored on them including networking software for monitoring the entire network operations.
3. Cabling and connection hardware link the computers together. A special interface card or printed circuit board has to be inserted into each computer on the network. This card is used to give a unique identity and allow it is interact with other computers of the network. This card is called Network Interface Card (NIC) or LAN Card.
   
   LAN may also have additional hardware such as printers and extra disk storage. Network printers are connected to computer designated as file server if files stored on it are shared by all other workstations. Each workstation on network may have its own processing power and disk storage (Floppy or Hard Disk) and printer but these are additional hardware.
2. Wide Area Network (WAN):
   A Wide Area Network (abbreviated as WAN) is a network of computers over the wide geographical area possible across several continents. The communication may be via microwave or a satellite link. It also takes place through ordinary telephone lines or a computers from sender to receiver.
   The use of global networks (including Internet) has increased enormously over the past few years due to:
   *     Changeover the telephone networks from old-style analogue to high-speed digital-technology.
   *     Reduction in cost of connecting to and using networks.
   *     Improved compression techniques for faster transmission of text and graphics.
   The implementation of information supper highway is also expected in future. This will use a combination of telephone lines, satellite links, computer networks and multimedia to bring interactive two-way communication to many homes. The distinction between a television, a microcomputer and a telephone may become blurred when we cand ialup and download videos or TV programs on request, send electronic mail and browse the World Wide Web all from the same piece of equipment.
   Such a network that is capable to sup[port voice, video and computer data communication is called an Integrated Services Digital Network (abbreviated as ISDN). An ISDN needs high-speed digital connections with a high band width to send video data for example for Video Conferencing or net-meetings in real time. A category of ISDN now a days is a DSL (stands for Digital Subscriber Line) which provides a mean of faster transmission of Audio, Video and Textual data or information. It must be noted that ISDN or DSL connections are provided by Tele-communication organisations of a country such as PTCL in Pakistan on payment of very high charges. Thus these lines are highly expensive.
5. Networking Concepts:
   Following are some of the important networking concepts that must be known for establishment of computer network:
   Client:
   A client may also be called terminal. It is an application installed on workstation computers which requests for services or resources from the network server. In some network operating system such as Windows NT and Windows 2000, these applications are called clients whereas in some others like UNIX or  LINUX, such applications are called terminals.
   Server:
   A server may also be called console. It is an application used to provide services or resources of computer network to client workstation on their requests. In Windows operating system, such computers on which servers are installed are called servers whereas from UNIX or LINUX point of view the are called consoles. As server computers are having various network resources and centeralised controls over the entire network, the entire client-server networks under the control of servers.
   Peers:
   Peers are servers as well as clients. The reason behind this is that in some operating systems, like Windows-95 or Windows 98, there is no central control due to absence of client and server applications. In such cases, the computers called peers share their resources in a cooperative manner. For example, if one peer has printer resource and the other one has scanner resource then these resources can be associatively shared between them.
1. Network Modals:
   In the networking concepts we have discussed the different roles computers may play in a network. A network operating system also plays its vital rol to make a computer client, server or peer. Now depending on the types of network operating system and their network applications i.e. clients, servers or peers following are the different networking models:
1. Client- Server Model:
   Different network operating system like Windows-NT, Windows 2000, UNIX and LINUX are used for client-server networks. Higher security measures and a centralised control of server on set of various network resources make the network strongly secure and safe for protection of information. However certain shortcomings also exist in client-server networks as such networks require the services of a network administrator for its proper look after. In case if server of the network fails due to any reason then entire network operations are badly suffered. Such networks are mostly used in a situation where security and privacy of data or information is a critical issue. Each user may have his / her rights of using the network. The network administrator gives these rights so that one user can't get the private data or information from account of another user without his / her permission.
2. Peer-to-Peer Modal:
   Some operating systems like Windows-95 and Window-98 are used for working with such networks. There is a central control due to absence of server and client software. Each computer (peer) is a server with respect to its own resources and a client with respect to resources of another computer. All of administration work is on part of the users. Thus expensive services of a network administrator are not required. However the major shortcoming of peer-to-peer networking is that there is less security and privacy of data or information than client-server architecture.
3. Hybrid Network Modal:
   The word hybrid means a mixture of two opposite qualities in one and the same thing. Now when both the client-server and peer-to-peer networking features exist in a network then such a network will be called hybrid network. The advantages of client-server network cancel the disadvantages of peer-to-peer may cancel the disadvantages of client-server whose services are still required due to the presence of client-server characteristics. Keep a network administrator may be expensive and that is a major drawback.
2. Networking Standards:
   Standards are actually documented agreements containing technical specifications or other precise criteria to be used consistently as rules, guidelines or definition of characteristics in order to ensure that materials, products, processes and services are fit for their purposes. For example the format of credit cards, phone cards and smart cards are derive from International Standard Organisation (ISO). According to the standards for example 0.76mm thickness of card means that these cards can be used worldwide with same thickness. These International standards make life simple; increase the reliability and effectiveness of services and goods that we are using.
   Different types of network standards may be used. For example.
   *     The number of switches in a hub or router.
   *     Licensing criteria for the number of computers to use a specific network operating system.
   *     Standards for cable media or bandwidth of communication channels etc.
   These standards make networking more successful, efficient and reliable. Two wide categories of standards are:
1. De Facto Standards:
   De fecto means "by fact" or "by convention". These standards are not approved by organised bodies however they have been adopted as standards through their widespread uses. De facto standards are often established originally by manufactures seeking to define the functionality of a new product or technology.
2. De Jure Standards:
   De jure means "by law" or "by regulation" and as its name refers, these standards have been properly legislated by an officially recognised body.
   Certain famous standards making bodies or organisation are listed one by one below:
• The International Standards Organisation (ISO) 
• The International Telecommunication Union-Telecommunication Standards Sector (ITU-T, formerly the CCITT).
• The American National Standards Institute (ANSI).
• The Institute of Electrical and Electronic Engineers (IEEE).
• The Electronic Industries Association (EIA) and
• Telcordia.
3. Network Typologies:
   The topology of a network may be defined as its physical layout i.e. the way in which computers and other equipment are connected. There are different basic layouts or typologies for networks. Following are the most commonly used typologies.


1. Star Topology:
   Each node (computer or other device in star topology is connected to a central minicomputer which controls the network. Network signals travel from server to the stations along each individual station's cable. There is a polling system involved in which the file server pools each station in turn to see if it has a signal to send. The server then handles the signals as they are received. Star topology may also use a central platform in the form of a hub where cables from different computers are plugged and thus providing a mean of signal transmission.
   
   These individual stations connected to the central platforms are also called node or devices. The topology has following advantages over its other counterparts:
1. If one cable fails then other stations are not affected by this failure.
2. The performance of network is consistent even in case when the network is heavily used.
3. There is no problem of collision among data signals as each station has its own individual cable to central platform i.e. server or hub.
4. It is very easy to add new station to star network with out disrupting the network.
   Few  drawbacks may be found in this topology are:
1. Entire network is badly suffered when central server fails.
2. Cable cost increases due to a large number of cables from individual stations to a central server or hub.
2. Bus Topology:
   A bus topology is that is which all of the devices or computers share a single cable. Information can be transmitted in either direction from any PC to any other. As several stations may want to transmit down the same line simultaneously, a popular scheme called "Ethernet" uses a collision system known as "Garrier Sense Multiple Access with Collision Detection" (abbreviated as CSMA-DC)to aviod collision among data signals. The mechanism of CSMA-CD is that before a station begins to transmit a signal of data, it conforms that the channel is not busy. In case when channel is busy, the station has to wait before transmission can begin. Once it begins transmission, it listens for other nodes which are beginning their transmission. IF the transmitted message collides with another transmitted message, both stations abort and wait for a random period of time before trying again. This all activity is controlled by the CSMA-CD protocol.
   The diagrammatic representation of a bus network is in figure below.
   In this diagram there are two terminators at the ends of cable connecting computers. These terminators are actually resistors and are used to absorb signal in case of ringing condition. Ringing condition arises when the signal send by a computer does not reach its target and thus creates disturbance in the network by moving here and there. The major advantages of this topology over other typologies are:
   
1. It is very easy and inexpensive to install as it needs and the least amount of cable and is simple too.
2. More stations or computer can easily be added without disrupting the network.
   The main shortcomings of this topology are given as:
• The whole network goes down if main cable fails at any point.
• Cable'e failure is difficult is isolate.
• The network performance degrades under heavy loads.
  
  Ethernet is a famous real world network scheme of this topology. An Ethernet may be physically star or bus logically it is a bus and uses CSMA-CD for its working. Using this scheme only a single user can use the same communication line at a time and if there is a chance of collision then user has to wait till the line becomes clear for his / her data to be communicated. An example of this may be dialing a telephone number. If the phone line is busy mean that required phone number is engaged in communication then user may be to wait until that telephone connection line becomes free. In case when telephone line is engaged in communication there is a chance of collision. This is avoided by specific tone indicating that the phone line is busy in communication with someone else.
3. Ring Topology:
   In ring network there is no central computer or hub. Each computer or station may communicate with any other in the ring with messages being specifically addressed to destination computer. A famous scheme called "token passing" is used in a token ring system. This scheme is used to pass a "message token" from one node to another one. A "message token" is actually a unique character sequence. Each node has a designated time a which it can removed token and adds message with address of the receiving node, sending node and some control bits. A receiving node i.e. computer or device acknowledges the receipt of a message by inverting a 1 bit field i.e. changes the bit from 1 to 0. The diagrammatic representation of ring network is given as under.
   
   The arrows show that flow of takes place in a single anti-clockwise direct in a ring. There is no collision among message tokens passing in a ring.|
   Certain advantages of ring network are:
1. There is no dependence on a central computer or file server and each node controls transmission to / from itself.
2. Transmission of messages around the ring is relative simple with messages traveling in a single anti-colckwise direction.
3. Very high transmission rates are possible in ring networks.
   
   Few shortcoming that exist in this networking topology are:
• The transmission between any two devices in network is disrupted with the breakdown of a single station. The reason is that each station regenerates the signal incase when it is not addressed for it.
• The entire network is disrupted in order to add new stations to the ring network.
  
  Some real world networks based on this topology are Token-Ring and FDDI i.d. Fiber Distributed Data Interface. FDDI is implemented as a dual ring
  
  
  
4. Mesh Topology:
   Mesh is that topology in which multiple links exist among network computers. Mesh networks may be found into one of the following types:


1. True Mesh:
   In true mesh the total number of links for "n" computers are "n(n-1)2" and every two computers in this network are directly connected to each other. When the total number of computer "n" is 6 then there will be total 15 links among them as demonstrated in the following diagram.
   
   In this diagram representation there is a two-way communication of data across each link.
2. Hybrid Mesh:
   In hybrid mesh total links are less then n(n-1)/2 for "n" computers. Each computer is not directly connected with each other computer in the network. If for example the total number of computers "n" is 6 and there are 9 links instead of 15 then such a network will be hybrid mesh.
   This topology is based on multiple links between any two computers in the network and it has the following major advantages over other typologies.
1. Mesh network has high fault tolerance due to multiple links.
2. Due to multiple links mesh network can work fine even under severe loads.
3. Troubleshooting of mesh network is easy as compared to other networks. If data is not communicated between any two computers then it means that there is some fault in direct links between them.
   Some shortcomings of this topology are given below:


• Cable cost is too high due to multiple links. This cable cost may even be more than a star network in case of true mesh network among a large number of computers.
• It is also very difficult to maintain a true mesh for a large number of computers.
  
  
  Even Mesh networks are rare in real world due to their shortcomings however a good example of Mesh network is the Internet in which every computer can be connected to every other computer directly (if we are connected with Internet).
6. Data Communication Protocol:
   Up till now, we have discussed about the different mechanisms that are used for data communication from one computer to another one. However there are also some software that take an active part in the communication of data. In order to allow equipments from different suppliers to be networked, a strict set of rules (protocols) has been devised covering standards of physical connections, cabling mode of transmission, speed, data format, error detection and correction etc. Any equipment that uses the same communication protocols can be connected together. In other words we can say that the data communication software used to control the transmission of data is called a protocol. Some common examples of protocols are TCP/IP (Transmission Control Protocol/Internet Protocol), FTP (File Transfer Protocol), HTTP (Hyper Text Transfer Protocol) and so on.
   Manufactures are gradually incorporating these standards in some of their products however it is unlikely that total standardisation will be achieved for sometime, if ever. Where two devices have different protocols they can sometimes communicate via  a protocol conversion computer.
   The OSI Reference Model:
   The working procedure of data communication protocols and other software can be fully described by OSI seven layers reference model. OSI seven layers reference model. OSI is a non-tangible model and is very useful in understanding how equipments from one manufacturer could be connected with totally different equipments from some other manufacturer. This model is under the process of development since 1977. OSI stands for Open System Interconnection and here the openness means that every detail about the working of the data communication software can be understood by studying this model. This model has devised a hierarchy of the following seven layers so that each layer is a service or procedure according to which the data to be communicated is processed and is forwarded to the lower level layers at the sending point or the higher level layer at the receiving point.
   The upper three layers of this model are called high-level layers as they are concerned with the software whereas the lower four layers are called low-level layers because they are concerned with the functions of different networking hardware. These layers are described one by one below in detail:
1. Application Layer:
   It is the first high-level layer and is closest to the users. It is used for providing support to the phenomena of the transfer of information between end-users, application programs and devices. Several types of protocols that exist in this layer are covering specific and general applications such as accounting, entry control and user identification.
2. Presentation Layer:
   This is the second high-level layer and accepts the processed data from application layer for further processing. The purpose of protocols in this layer is to ensure that different devices using data in different formats can communicate with each other for example, handling the conversion between ASCII and EBCDIC codes. It may also be used to carry out encryption to ensure the security of data during its transmission.
3. Session Layer:
   This the third and last high-level layer of the OSI reference model and is actually the user interface to the network. When a user requests for a particular service from the network, this layer handles the dialogue.
4. Transport Layer:
   This layer is the last low-level layer and fourth layer of the OSI reference model. This layer handles the data transmission between host computers by performing addressing and error controls to ensure a high quality data transmission service for the network.
5. Network Layer:
   This the third low-level layer and fifth layer in the OSI reference model which has the function of performing tasks like routing of information around the network, connecting adjacent nodes in the network and also carrying out accounting functions to enable the network's owners to charge the users.
6. Data Link Layer:
   This the second low-level and sixth layer of the OSI reference model. This layer is used to handle the data transmission errors caused by the corruption of data due to different types of interferences that affects physical data transmission media used in the networks. The techniques used for the receipt and acknowledgements of data by a receiver are handled through this layer.
7. Physical Layer:
   This the last layer of OSI reference model and is the first low-level layer as it is closest to the hardware. This layer is concerned with standards for mechanical, electrical and procedural aspects of the interface devices. An example of such standards may be the number of pins contained within a network's connector. This layer is especially concerned with how the binary data is transmitted along the communication channels.
   The diagrammatic representation of this layer-by-layer architecture of entirely different networking products and the way data is sent and received by them is given as follows:
1. OSI reference model gives a clear understanding about the working of communication software and equipments.
2. OSI reference model when followed make different networking software capable to transmit their data with each other.
3. This model provides a standard for the quality of communication between similar or different networking equipments.

BASIC OF INFORMATION TECHNOLOGY

Information Technology is the key to development s in the modern era of today's computerized world. It merges the computer and communications in order to make the easy availability of latest information to every one. Different business organizations may not work if they don't have the required data and information  for running their business activities. An example of this may be that an airline reservation company that may not be able to prepare the passengers list for its various flights if the required data about passengers is not available or a bank may never perform its banking functions without the data and information regarding its customers and other business perspectives.

As computer is a very important tool for quick and accurate production of our needed information, it is therefore the central topic of information technology, that how a computer performs its computation functions. The other element of IT is called communication, which actually means that how computers r receive the same from them.

In many business organizations, IT plays a significant role mostly in the form of computer based information system like Management Information System (MIS), Decision Support System (DSS), Executive Information System (EIS), Operational Information System (OIS) and so on. The detailed discussion about information system is out of the scope of this however the meanings of information technology may be clarified that is the technology based upon the use of computer based information system having the objective of easy and quick information production, distribution among its end-users, storage, retrieval and use. This information is obtained from the data collected from different data sources in many business organizations.

The benefits of Information Technology are highly adequate, challenging and outstanding. IT now a days is playing its role in almost all aspects of our social, national and international life affairs and in a very true sense, latest information is highly crucial for the betterment of a nation.

1. Components of Computer System:
   A computer is not an ordinary machine but it is a complete functional system that exists due to the integration of two basic components i.e. hardware and software. Hardware is the physical component whereas software is the nonphysical component of the computer system. Here physical component means the one that can be seen and touched directly, where the adverse of this is the nonphysical i.e. the one which can not be seen and touched directly but can be found responsible for running the computer system. An example of this may be the living human body in which there is soul, wisdom, power and other characteristics that exist but can not be touched and seen physically. On the other hand, the different physical organs of the human body can seen and touched but they can't work without soul. In the similar way, computer system will be no more than collection of different equipments without the software. The basic definitions of these two components are given below:
• Hardware is the collection all those parts of computer system, which are actually working during the processing of data to get information.
• Software is a set of instructions (programs), which is responsible for monitoring the different tasks of the computer hardware during the processing of data into information.
  
  Command examples of hardware are processor. RAM, ROM, Hard Disk, Floppy Disk, Keyboard, Printer and so on. Whereas all those programs like operating system, device drivers, commands and computer programming languages etc, which are used to direct the computer hardware about the computational tasks that they perform, are examples of software. The types of hardware and software components will be studied in a great detail in the coming chapters of this book however ti will be better to discuss to main categories of computer hardware and software so that one could be able to understand the way in which the computer system is working.
1. Main Categories of Computer Hardware:
   Computer hardware falls into the following main categories:


• Processor
• Main Memory composed of RAM & ROM
• Secondary Memory composed of mass storage devices such as Hard Disk, Floppy Disk, Magnetic Disk, Compact Disk (CD), USB (Flash or Data Traveler)
• Input and Output devices like Keyboard, Mouse Printer, Scanner, Digital Camera, Microphone, Speaker and so on.
• Sources of interconnections among the above four components e.g. ports, buses, cables and connectors etc.
• Other networking hardware used to connect one computer with another one, such as MODEM, Bridge, Router, Hub, different types of cable media and so on.
  
  In these categories of computer hardware, the above 5 categories are highly necessary for a computer system in a working condition whereas the last category of hardware is required only in case of establishing a computer network. The first 3 categories of hardware are contained within the Central Processing Unit (abbreviated as CPU box) whereas the Input and Output devices are out of the CPU box and are connected to it through their cables. The main memory which is composed of RAM and ROM is directly connected with the computer processor and is thus called online to the processor whereas the mass/secondary storage and Input / Output devices are connected to the processor through main memory especially through RAM and are therefore called peripheral devices or offline to processor. 
  
  
  In the above figure, the bi-directional arrows mean the transfer of data in both directions whereas the unidirectional arrows mean the transfer of data in just single direction. The way in which the computer work may be described in the following steps:
1. Data is sent into RAM (Random Access Memory) from the input devices. 
2. For processing, the data present in RAM is given to the processor from where it is sent back to RAM after necessary processing.
3. Data can also be stored permanently in secondary memory (if required) from where it can be retrieved back into the RAM.
4. The output i.e. information obtained after the processing of data can be displayed through output devices.
2. Main Categories of Computer Software:
   There are two broad categories of computer software, which are described one by one below:
• System Software runs and monitors various hardware devices during their functions. The orientation of system software is towards the computer hardare and makes it available to the computer users for their computation tasks.
• Application Software or  packages are used for working in a particular area of application like Engineering, Graphics, Documentation, Accounting and so on. These software are applicable only in their specific field of application and can't be used everywhere.
3. Data and Information:
   The basic objective of hardware and software component of a working computer system is that they process the data accepted as input and convert it into output called information. If hardware performs actual computational work, software directs computer hardware about the way to perform.
   
   The difference between data and information is that data is just a collection raw facts and figures and it can't be used for decision making whereas information the processed form of data on the basis of which one can make decisions. Data is input to the computer where information is output from the computer system. An example of this may be the data obtained from admission forms of students. The college administration can't decide about which students should be admitted and which not on the basis of this data. However if this data is properly processed according to a set of procedure (software), the clerical staff and machinery equipment (hardware), it is converted into students merit list on the basis of which one can easily decide about student admission. This transformation of data into information is the basic goal of a computer based information system comprising necessary hardware and software components.
2. Input & Output Devices:
   As a computer system processes the data i.e. raw facts and figures into their information products, specific devices are used for input of data and display or output of information. These enable the computer system to interact with its users. These devices are also called input/output peripherals. Examples of input devices are keyboard, mouse, jay-stick, scanner, digital camera and microphone etc whereas common examples of output devices are monitor, printer, plotter and speaker etc. 
3. Operating System and Computer Programs:
   The data that is input to computer system may be processed and transformed into information products. These computational activities of data input, processing and information output are performed with the help of various hardware devices under the supervision of a set of computer programs i.e. operating system and other application or system software. An operating system is the most significant kind of system software that contains certain distinctive characteristics like:
• Managing the software and hardware resources of the computer system.
• Controlling the functionalities of all hardware and software resources in addition to their management.
• Providing user-friendly interfaces that enable users to operate the compute system.A system software that is equipped with all of such feathers will be an operating system however if any of these features is missing within a system software then it will be an ordinary system software or an application software have their important roles in the computational activities of transforming data into information.
4. Data Storage and Memory:
   Storage of data is important computational process that is carried out in order to use the data for future needs. Two types of data memories are associated with the data storage i.e. primary memory (RAM & ROM) and secondary memory (Hard Disk, Floppy Disk, and Magnetic Tape etc). The former i.e. primary memory also called internal memory is online to the processor and is composed of RAM and ROM. RAM is the only part of the primary memory that can be used for storing the users data temporarily whereas ROM can't be used for storage of user data as it is read only and we are unable to write or store our data into the ROM. On the other hand RAM (Random Access Memory) also called Read/Write Memory may be used to store the data temporarily as long as the computer is on. As soon as the computer system is shutdown, the data present into the RAM is washed away. RAM is the only way of users data to the processor because processor is directly connected to RAM and picks data from RAM to process it.
   The other type of computer memory is called secondary memory or external memory or auxiliary memory or mass memory. It stores the data permanently with the help of magnetic mechanisms however unlike primary memory, it is connected to the processor through RAM i.e. the data from the secondary memory is carried to RAM before it is processed. That is why secondary storage is also an offline or peripheral device of the computer system.
1. Basic Unit of Data Storage:
   The basic and smallest unit of computer memory is called bit that stands for binary  digits. As we know that computer internally represents its data in the form of binary zeros and ones, these bits are the primary sources of such representations. A bit may have two possible states i.e. 0 or 1. 0 is used for circuit off whereas 1 is used for circuit on. Thus a single bit may have two possible representation of data 0 and 1. Some other larger units of computer memory are given below:
• Nibble:
  One nibble is the combination of 4 bits. It can have 16 possible representations of data as 2⁴=16. An example is the Hexadecimal number system, which is composed of numbers from 0 i.e. 0000 to F i.e. 1111.
• Byte:
  A byte may be defined as "the memory required to store a single alphanumeric character of data." Here alphanumeric character may be any digit from 0,1,2,3,.......0 or an alphabet from a to z or A to Z or any other special symbol like , : / ? + - * etc. A byte is a combination of 8 bits or 2  nibbles. As a single bit can have 2¹=2 possible representation i.e. 0 and 1, therefore a Byte i.e. a combination of 8 bits can 2⁸ =256 possible representations of data in the form of 0s and 1s.
  Some commercial units of data storage that are composed of bytes are described in the table below:
  

  Name of Unit	Abbreviation	Numbers of Bytes
  Kilo Byte	KB	(2)10
  Mega Byte	MB	(2)20
  Gega Byte	GB	(2)40
  Tera Byte	TB	(2)80

  
  Example: Calculate the total number of Bytes and Kilo Bytes in 256 MB of RAM.
  
  Solution:
  This problem can be solved as:
  
  Number of Bytes in 1 MB = 2²⁰ Bytes
  So total number of Bytes in 256 MB of RAM 256 x 2²⁰ Bytes = 268435456 Bytes
  Similarly, the number of Kilo Bytes within 1 MB =2²⁰ KB or 1024 KB
  So the total number of Kilo Bytes within 256 MB RAM = 256 x 1024 = 262144 KB
• Word:
  A word is a combination of 2 Bytes. Some machine instructions need computer words comprising 16 bits. However the larger units that are composed of words are described in the table below:
  

  Name of Unit	Abbreviation	Number of words	Number of Bytes
  Double Word	DW	2	4
  Quad Word	QW	4	8
  Double Quad Word	DQ	8	16

  Example: Calculate the total number of Words, Double Words and Quad Words in a hard disk having a storage capacity of 40GB.
  Solution:
                 In order to solve this problem, we will have to find:
                Total number of Bytes in 1 GB =  2⁴⁰ Bytes
                Total number of Bytes in 40 GB = 40 x 2⁴⁰ Bytes
                Number of Words contained in 2 Bytes = 1 W
                Number of Words contained in 40 GB = 40 / 2 x 2⁴⁰ Words= 20 x 2⁴⁰ DW
  Similarly
                  Number of Double Words contained in 4 Bytes = 1 DW
                  Number of Double Words contained in 40 GB=4x2⁴⁰DW=10x2⁴⁰DW
  and in this way
                  Number of Quad words contained in 8 Bytes=1QW
                  Total number of QW in 40 GB=40/8x2⁴⁰QW=5x2⁴⁰QW
  Example: Calculate the total number of characters possibly represented by a single Byte.
  Solution:
  It is to be noted that one Byte can represent just a single alphanumeric character at a single time. As a character is a combination of 0s and 1s in binary form, this problem can be solved as:
                            Total possible states of a single bite =2 i.e. 0 and 1
  This means that a single bit may represent 2¹ possible states. Since a Byte is a combination of 8 bits in usual conditions, the total number of possible representation of 0s and 1s by a single Byte are 2¹ representations. Thus 2¹ or 256 possible representations characters may be represented by a single Byte, which in other words means that 256 characters may possibly be represented by a single Byte.
5. System Development:
   After discussing the fundamental concepts about the computer system and its working. Lety us study computer based information system briefly. A system in a wide sense may be define as "an integration of components or parts linked together according to some specific scheme to achieve certain common objective". According to this definition, some systems are natural, some are man-made and some are computer based. Here emphasis is the computer based information system i.e. the system that uses computers and communication equipments for the delivery of information to the different levels of end-users in a business organisation. These computer based information system play and vital role in information technology and their potential uses are so many that will be out of the scope of this discussion. However a brief overview of computer based information system development is useful to know how the software development process for an information system is performed.
   
   Developing an information system is not an easy task rather it may be troublesome in some respects as it involves several distinct phases each of which must be completed before a subsequent task can begin. For this purpose, the system development life cycle is adopted so that a successful information system may be achieved. A system development life cycle is a repetitive process in which a system may be created, upgraded, matured, in its operations and may die  with the introduction of new hardware and software technologies, environmental changes (like government policies or customer demands) and so the introduction of a new information system before. The system development life cycle is composed of the following phases:


1. Problem Definition: 
   It is also called problem identification and is somewhat similar to the famous saying that necessity is the mother of invention and it tend to find out the problem and its reasons that motivate the huge projection of information system development. This is the first and the most crucial step in system development. The whole system depends upon this step because the actual problem due to which the system development is initiated is identified in this step and all of the future work will be correct if the definition of the problem is correct or otherwise all of the future efforts, money, time and other assets used for development will be wasted. The reason behind the dependence of system development will be wasted. The reason behind the dependence of system development on correct problem definition is that if a problem is correctly identified, it means that it has been solved almost about 50% and the rest of the solution will depend upon the practical work in correct direction whereas if a problem is identified with inaccuracy in its definition then it means that the direction of solution is wrong from its very beginning and the process of system development will miss its target if continued this way.
   The reasons behind problem definition and new system development may be:
   
   *     Current system is difficult to use.
   
   *     Current system doesn't work according to the expectation of its users or it is producing inaccurate result.
   *     The current system needs to be improved for making it more efficient.
   
   Once any of such things happens and the problem is realised, the system analyst is requested for change in the current system improve its outcome. The system analyst begins a preliminary investigation to find out the problem and its sources accurately. The definition of the problem results into the submission of report that is put before the management of the business organisation and about problem definition may contain.
   
   *     The correct definition of the problem.
   *     The rough cost estimation for solution to the problem.
   *     The amount of time taken by this development project or project duration.
   *     Staff required e.g. programmers, project managers, operators and so on.
   *     The hardware and software technologies needed.
   
   On the basis of this report, management of the organisation may decide whether to approve the development project or not.
2. Analysis:
   When the authorities or management of the business organisation take a decision to proceed with the solution of the problem, the next step is to thoroughly investigate the entire current information system. The system analyst has to work very close to people directly facing the problem and finds out how it can be solved. In this phase, the present system is studied in depth to collect all the available information about the current system. The most commonly used techniques are:
• Review of Existing Records:
  This means that all the available written or computer-stored material about current system may be collected and studied.
• Interviews:
  An interview is the face-to-face communication between two persons. It can be used to arrange meetings with the users of current system. It may be very useful as it produces upto-date information that can not be documented officially. The system analyst may also be able to judge the truth of answers from expressions of the interviewee.
• Questionnaire:
  In this technique, a set of proposed questions with a proper space among them is arranged for the collection of information about the current system. It is usually used in case when the business organisation is spreaded over a wide geographical area and interviews take a lot of time and cost.
• Observation:
  As its name indicates, it is that technique with the help of which it is observed that what people at a place e.g. in the different sections of business organisation are doing and what actions are taking place over there. This means that we have to observe the people at work. This is very useful in checking the truth of information collected through interviews and questionnaires.
  After all the information is collected about the system, the system analyst has to analyse the information and purpose a set of solutions to the problem. A typical solution may include:
  *     What should be the output given by the system?
  *     What should be input into the system?
  *     What should be the hardware and software of the system?
  *     What tools are needed for the system?
  Throughout analysis the focus remains on "What the system must do?" and "How the system is to be developed?" After all the solutions are defined, these are given to organisation for selecting the best possible solution. These solutions are identified on the basis of constraints, budgets, resources and schedule of the organisation.
3. Design:
   In this phase the project team develops software for information system. The best solution selected from the set of alternative solution or candidates found in analysis phase and is programmed. Here emphasis is the practical or physical design of solution to the problem. This design include input design, process design, file design and output design i.e. the different component of the information system are designed in this phase. There are two types of system design.
• Logical Design:
  In which different algorithms and system flowcharts are designed. One has a clear understanding about how proposed information system will perform its functions and how it will be free of the problems existing in the current system. This design is very important because it provides a road map for the proposed information system to work and it also has its impacts on the later coming physical design so great care must be taken in logical design.
• Physical Design:
  Physical design or coding in which software for proposed information system and its components is prepared in the light of algorithms and flowcharts obtained from logical design process. The outcome of physical is the information system software in its crude form. The actual work of information system development takes place in this phase to create a newly proposed system software solution in its practical form.
4. Testing & Debugging:
   The newly created information system may contain serious bugs in it that need t be properly refined so that it may be able to smoothly handle the business activities of organisation. For this purpose, a detailed software testing criteria is adopted to check its quality, performance, speed and working in real world situations. The software is tested with dummy data and if it works correctly, it is recommended for other tests. If all the test are satisfactorily cleared then it is finally tested for acceptance. Successful clarification of this test will bring it into its practical implementation. Any weakness or bugs found in testing process are attempted to be removed and the system is properly refined to make it capable of working with original data of business organsiation.
5. Implementation:
   In this phase of system development life cycle the system analyst and his/her project team puts the developed system into practice after thorough testing and debugging. Implementation may consist of several steps like purchase of necessary hardware and other materials required for system, installation of new hardware and software, conversion of data files to use them in the current system and so on.
   
   After installation conversion takes place, which is the process of moving from the old system to newly developed system. These conversion phenomena may be accomplished according to the following four methods:
1. Direct Conversion:
   Direct Conversion or Abrupt Cutover is that method of conversion in which users immediately stop using the old system and begin using the new system. This method is fast but is highly risky and in case when some problem occurs in working of the new system, entire business of organisation will badly suffer and there will be no way to go back to use the old system for recovery.
2. Parallel Conversion:
   Parallel Conversion is that method in which both new and old system are used for some time. Outputs of both of these systems are compared and in case if they match together, then we switch from the old system to the newly created information system. This method is safe but it is highly time consuming and expensive as the organisation has to run two systems at same time.
3. Phased Changeover:
   Phased Changeover is the method of conversion into new system in a component-by-component fashion so that first component of the newly developed system is installed first and if it works accurately then the other components will be installed in sequence.
4. Pilot Conversion:
   This method is adopted in case when organisation is spread over multiple locations. The newly developed system is tested in single pilot phase and if it works correctly then the entire organisation switches from old system to newly developed information system in all of its locations.
   
   It must be notified that proper training and personnel support play important role during the conversion or changover phenomena. These people deliver lectures, hands on training and computer training to the end-users of the newly developed system.
6. Maintenance: 
   This phase is also called post-implementation as after the implementation of the new system, the Information System Department (i.e. the developers of new system continue to provide their support to the new system in order to ensure that it works properly. It involves:
   *     Corrections of errors that occur in the newly implemented system.
   *     Changes due to changes in the organisation.
   *     Changes or upgrades or improvements in the system.
   Post-implementation or maintenance guarantees the long life of the system in the business organisation and it ensures that the newly implemented system may work fine for longer times. A detailed documentation is also accomplished starting from problem definition to the post-implementation phases of system development life cycle so that it may provide detailed information about the system if at any time problems arise in the newly implemented system. This may be specifically helpful for other system analysts or IT professional (who will join the Information Department later on) to understand the newly implemented system and maybe capable of handling any problem in it.
   The system development life cycle is diagrammatically represented in figure below:
   
   
   From this diagrammatic representation of system development life cycle, it is clear that as soon as the newly implemented information system becomes old after a fixed period of its life, problems will arise in its operations. Consequently development of another information system becomes crucial. This process will again take a start from Problem Definition phase of the system development life cycle and will end at Post-Implementation or Maintenance phase. That is why system development is continuously repetitive or cyclic process that takes place over and over with the introduction of new hardware and software of computer system certain other factors.

LINUX COMMANDS

There are some commands which are used in LINUX operating system which are as follow:

1. Clear Command:
   The external clear command clears your terminal screen if possible.
   $ clear and press (Enter)
   The terminal screen is cleared and the shell prompt $ appears at the top of the screen.
2. Pwd Command:
   The internal cd command changes your current working directory. cd lets you do the following:
   -          change working directory.
   -          change to last previous working directory.
   -          use a sting substitute in the current path to change to a similar directory path.
   The general format of the command is:
   $ chdir/cd-
   $ cd [directory]
   $ cd.. The present working directory is changed to previous working directory.
3. Mkdir Command:
   The external mkdir command makes a new directory. You can pass multiple arguments to mkdir. Each argument is used by mkdir as the name of a new directory to create.
   The general format of mkdir command is:
   $ mkdir [-m mode] [-p] directory_lisht
   Options:
   -m mode:
                     Mode to use for new directories. This allow you to specify what mode all of new directories will have when they are created
   -p:
                     Parent directory creator. If you specify a pathname to create a new directory and the parent directories do not exist, mkdir will create them as needed.
   Note:
                     You have must have write permission fr the directory when you are creating new sub-directories. If no write permission is available, you will not be able to create a new directories.
   Example:
1. To create a sub-directory called misc which has another sub-directory named misc1.
   $ mkdir misc misc/misc1
4. Rmdir Command:
   The external rmdir command remove a directory. The directory must be empty.
   rmdir is a safe way to perform directory removal than using rm -r command. It checks the directory for existence of files and sub-directories. If any exists, rmdir complaints and exits without removing the directory.
   The general format of rmdir is:
   $ rmdir [-ps] directory_list
   Options: 
   -p
                Removes empty directories specified in the directory path. Name of each directory is displayed on standard output as it is removed. Directories that are not removed are displayed on standard error.
   -s:
                 Suppress any messages produced. Both standard output and standard error are suppressed.
   Examples:
   
   1. $ rmdir misc/misc1 misc
      to check whether the directories misc and misc/misc1 are removed use the following command to list the directory.
      $ Is -l
5. Cat Command:
   The external command reads each file in the argument list and displays the data to the standard output. It only writes to the standard output. By using shell re-standard input. It only writes to the standard output. By using shell redirection capabilities cat can be used to combine multiple files into one large file that is why it is called concatenate. Some of the uses of cat command are as under:
   -              Display contents of ASCII files.
   -              Make a copy of a file.
   -              Combine multiple file into one file.
   -              Copy input from your keyboard to a file.
   -              Send output to a pipe used for multiple file input.
   -              Combine input from screen and a file into one output stream.
   General format of the cat command is
                  cat [-] [-su] [-v] file_list.
   Option:
   -:                      Read from standard input, treated as one input file.
   -s:                    Error  messages for non-existent files are suppressed.
   -v:                   Prints non-printing characters is visible form. Control characters. e.g., Control-X is printed as ^X.
   -e:                   Print as $ sign to represent new-line character.
   -t:                   Print a ^I (Control-I) in place of tabs.
   file_list:         One or more files in sequential order and write to standard output. If no argument is specified cat reads from standard input.
   
   Example:
   1. $ cat my-file.                                                          To display a file.
   2. $ cat my-list your-list his-list                                 To display multiple files.
   3. $ cat original-file > his-list                                     To copy a file.
   4. $ cat file1 file2 file3 > file 123                               Combining multiple files.
   5. $ cat > new-file                                                       Creating a file from keyboard.
      { here you type whatever you like and at the end press Ctrl-D}
6.  Is Command:
   Is command is used to display files and directories names. The format of Is command is:
              $ Is [option] [file-names or wild cards]
   Some of the essential option are:
   
   • a:
     Lists all files. Including hidden files starting from "."
   • -d:
     Displays data about directories.
   • -l:
     Long list files and directories.
   • -t:         
     List file in order of last modification time.
   • -R:       
     Recursively lists file in all sub-directories.
   • -r:        
     Displays files in reverse alphabetical order of filenames.
   • -U:      
     Sort entries by the last access time.
   • -x:       
     Sort files by file extension in alphabetical order.
7. My Command:
   The external my command moves a file from an existing location to a new location. It is also used as the rename command to change the name of a file. It has three formats:
   -           Rename a file with a new directory name.
   -           Move one or more files to reside under a different directory.
   -           Rename a directory with a new directory name.
   General format of the my command is:
                $ my [-if] file-name new_file-name.
                $ my [-if] file-name directory.
                $ my [-if] old_directory new_directory
   Option:-f:         Force the move to occur. Response "Yes" is assumed and move is performed.
   -i:         Commands runs in interactive mode. If destination file already exist, my prompts you with the file-name followed by a question mark (?). If you respond with "Y" or "Yes", move is performed. Any other response will cause my to skip to next move.
   
   Example:
   1. Display the list of files in the current directory
      $ Is -x
      caller file1 file2 db letters
   Now use my command to rename the file file2 as stuff
                $ my file2 stuff
                Now display the list of files
                $ Is -x
                caller file file1 db letters stuff
8. CP Command:
   The external CP command copies a file. It reads the contents of a file and creates a new file or overwrites an existing file. There are two basic formats of CP that allow you to:
   -          copy one file to another file.
   -          copy multiple files to a directory.
   -          copy input from your keyboard to a file.
   -          copy a file to your terminal.
   General format of the cp command is:
        $ cp [-ip] source_file dest_file
   $ cp [-ipr] source_file_list dest
         directory
               $ cp [-ip] source_directory
                      dest_directory
   First format copies one ordinary file to a new file. Second format will copy one or more files to a specific directory. Third format allow you to copy an entire directory structure to a new directory.
   Option:
   
   • i:
     Interactive confirmation is required. You are prompted if an existing file is overwritten. If "Y" or "Yes" is given in response, copy is performed otherwise not.
   • -p:
     Preserve the characteristics of the source file. Copy the contents modification times and permission modes of the source file to the destination file.
   • -r:
     Recursively copy and source directories. If a directory is given as the source file, then all of its files and sub-directories are copied. The destination must be a directory.
   Example:
   
   1. $ cp letter letter.bak                                       make a backup copy of file letter
   2. $ cp letter ../temp                                           copy letter to temp working directory
   3. $ cp /dev/tty note                                           copy input from keyboard to file note
   4. $ cp file1                                                        copy file1 to file stuff
   5. $ cp calendar letters                                       copy file calendar directory
9. RM Command:
   The external RM command removes (delete) files or directories.
   The general format of the RM command is
                       $ rm [fi] file_list
                       $ rm [-fir] directory_list [file_list]
   Option:
   
   • -f:
     Forces the removal of all files listed. Does not check whether file is write-protected or not. Does not prompt for confirmation. If directory is write-protected, files are never removed.
   • -i:
     Interactive file removal. Response is awaited.
   • -r:
     Recursively removes files and directories. All files are removed from each directory. Each directory is then remove. If a file is write-protected, then rm prompts you for confirmation to remove the file.
   Example:
   
   1. # rm stuff                            to remove a file named stuff
   2. $ rm-ri letters
      letters calendar ?y
      letter ?
      press Return not to delete letter directory
10. More/Page Command:
    The external more command allows you to view a file on your terminal.. It lets you view one screen of text at a time. The page command performs the same function as more but with a different screen control. The screen is cleared before each full screen of text is displayed.
    The general format of the move command is:
                                       $ more [-option] [+linenum] [file_list]
    OR                             $ page [-option] [+linenum] [file_list]
    Option:
    
    • -f:
      Do not long lines. Court logical lines instead of screen lines. Normally more truncates lines longer than the screen width.
    • -s:
      Squeeze out adjacent blank lines. Two or more blank lines are grouped together and are reduced to one blank line. This increase amount of text displayed on the screen.
    • -w:
      Prompts and waits for any key to be pressed before exiting. By default more / page will exit without waiting when end-of-file reaches.
    • -rows:
      Specifies the No. of rows (lines) to display on your terminal screen.
    • +linenum:
      Begin display at line number linenum of the input.
    • file_list:
      The list of files to be read by more.
    Example:
    
    1. Type more/etc/passwd /etc/group /u/ts/my/login/file1 and press Return. First screen of /etc/passwd file appear.
    2. Press Speacebar, next screen of data is displayed. (If the data in the file has been finished, next file is started)
    3. Type 2:n to skip to /etc/group file. The output will like
       .... skipping
       .... skipping to file /ul/ts/mylongin/file1
                       ---more---(Next file:/ul/ts/mylogin/file1)
    4. To return to second file, type :p to skip backward to /etc/group file.
       ... skipping
       ... skipping of file /etc/group
       ---more---(Nextfile:/etc/group)
    5. Press Spacebar to move forward on screen.
    6. Press Spacebar to move until (Next file: /ul/ts/mylogin/file1) appears
       At the end, type 'q' to exit from more.
11. Grep Command:
    Search files for lines matching a specified pattern and display the lines:
    Syntax                 Grep [option] [pattern] files
    Option:
    
    • -c:
      Instead of displaying matching lines simply output a count of total number matching the expression.
    • -i:
      Ignore case in both the pattern and file.
    • -L:
      Instead of displaying each line simply display the names of each file that contains no matches for the pattern.
    • -n:
      Prefixes each out put line its line number in the file.
    • -w:
      Display only those lines with matches for the patterns that complete word.
    • -x:
      Display only those lines with matches for the patterns that complete line.
     
12. Zip Command:
    Create a Zip archive from one or more files and directories.
    Syntax            zip zipfilename file1 file2 file3
    Option:
    
    • -f:
      Recursively work with directories adding all files in the sub-directories to the archive.
    • -m:
      Move files into the archive deleting them form original location.
    • -e:
      Encrpt the archive after prompting for a password this password will be use in extracting.
     
13. Unzip Command:
    Manipulate and extract ZIP archive
    Syntax           unzip [option] ziped filename
    Option:
    
    • -f:
      Extract only those files that are newer than already-existing version of the file.
    • -I:
      Displays the contents of the archive without extraction.
14. Chmod Command:
    Change the access permission of one or more files or directory
    Syntax                chmod [option] mode files
    Option:
    
    • -r:
      Recursive Recursively changes the permissions of all files in all sub-directories.
    • -v:
      Verbose Displays the results of all permission changes.
    • -f:
      Silent Suppress display of error messages when files permissions cannot be changed
      Mode it is in the form [ugoa][+-=] [rwx]
      u            =              user who owned the file
      g            =              all members of the group that owns the file
      o            =              anyone who is not the owner or in the owner group
      a            =              all user
      +            =             specified modes should be add to already specified permission
      -             =             specified modes should be remove from already specified permission
      =            =             specified modes will replace existing permission
      r             =             read permission
      w           =             write permission
      x            =             execute permission
      Example
      $ Chmod u+x a.bat                      will give execute permission to user on a.bat file
      $ Chmod u-x a.bat                       will remove execute permission to user
      $ Chmod a+x a.bat                       will give execute permission to all user
      $ Chmod a=rx a.bat                      will give read and execute permission to all user and
                                                           remove existing permission
     
15. At Command:
    Schedules commands to be executed at a specific time. User is prompted for the commands.
    Syntax          at [option] time
    
    • -f filename:
      Read commands from the specified file rather than prompting for the commands.
      example                    $at 2.30
                        rm*.Temp                ctrl-d
      specifying times
              now                                  =              specifies the current time
              today                                =              specifies the today
              tomorrow                         =              specifies the next day
                   +                                 =               specifies offsets in minutes hours days of weeks
               now + 2 hours                =               3 days from today
      Job Control
      Using Job Control, it is possible to use a single shell to execute and control multiple programs running simultaneously.
      Normally, when you execute a command, it runs in the foreground. That is, the shell executes the command and prompt and the prompt doesn't return until the command is finished.
      I.t. executing a command
                     $ find / -name'*.tmp' -print > findfile
      This command searches the entire structure of Linux system for files with the .tmp extension and store the result in the findfile file. While executing this command you won't be able to run other commands while find is running.
      To place a job in the in background is to add an ampersand(&) to the end of the command when you run it.
                    $ find / -name'*.tmp' -print&
      Once you press enter to execute this command, you are immediately presented with a new command prompt; at the same time and you will be able to execute the next command same time
      Thus you can execute multiple task an the same time
16. Job Command:
    This command will list all jobs running
    Syntax               $ jobs
    
    1. Running                                        $ find / -name '*.tmp' -print > findfile &
    2. Running                                        $ Is -IR / '*bmp' >dirlist &
17. Fg Command:
    This command bring the background job to foreground
    Syntax               fg Jobs no
    Example:
                 $ fg 1
    This will bring $ find / -name '*tmp' -print >findfile from background to foreground ie on screen.
18. Bg Command:
    This command start the stopped job in the background
    Syntax              bg Jobs no
    The foreground job first stopped by pressing Ctrl+Z and command prompt will be available. Where you can run the bg command to run the stopped command in the background.
    Example:
    Suppose you enter another command.
             $ IS -IR / '*.bmp' >dirlist
    This will Execute in the foreground then press Ctrl+Z. This will temporary stopped the Is job.
    And give command prompt to see the all current job.
              $ jobs
    
    1. Running                    find / -name '*.tmp' -print>findfile &
    2. Stopped                     $ Is -IR / '*.bmp' >dirlist &
    Now to start the stopped job ie [2] enter the command and prompt.
           $ bg 2
    To see the result
           $ jobs
    
    1. Running                    $ find / -name '*.tmp' -print >findfile &
    2. Running                    $ Is -IR / '*.bmp' >dirlist &
19. Cal Command:
    The external call command generates a simple calender and write it to the standard output. The default output for cal is current month. Using specific argument, a calendar for specific year or year and month can be created. Valid years are 1 to 9999. Valid months are 1 to 12.
    The general format of the cal command is
    Syntax              $ cal [[month]year]
    year:               Produces calendar for given year
    month:            Produces calendar for specified month of given year
    Example:
    To create a calendar for September 2010, the command is given as:
               $ cal 9 2010
20. Echo Command:
    Displays a line of text to your terminal. Some of the feature and use of echo are
    
    • Display string of text
    • Display variables
    • Display menu screen
    Syntax              echo "string"
    Example:
                $echo "my home directory is $home"
    Result will be display a line of text
                 May home directory is alihomedir
    Sending or eceiving messages from or to another uiser.
21. Write Command:
    The external write command sends message you type on your terminal to another user's terminal. Write reads the standard input and writes to the specified user's terminal tty. The write commands used to communicate with the fellow users on an interactive basis. It is useful to send a brief message to someone immediately or to carry on conversations using terminals. The general format of the write command is:
                          $ write user_name [tty]
    Option:
    
    • \user_name:
      The name of a user who is currently logged in to the system. Use the who command to list who is on the system.
    • tty:
      Specify which terminal tty to connect to if the same user is logged on more than once
    Example:
    
    1. Create a file and type some message in it, then use the following command:
       $ write alpha < tempmesg
       Where alpha is the name of the user to whom the message is to send while tempmesg is file containing the text of the message.
    2. Type the following command:
       $ echo "How are you? I want to meet you in the library at 09:00" ! write alpha. The echo will send the message to write via a pipe. The message will appear on the screen of username alpha all at one time as shown below:
       message from mylogin tty11 [date & time]
       How are you? I want to meet you in the library at 09:00
22. Wall Command:
    The external wall command performs a write to all users currently logged on to the system.
    The general format of the wall command is
            $ wall
    The wall message precedes its message text with the following line:
            Broadcast message from use_name
    It is usually used by super user to warn all users of immediate problems. The most command warning is that the system is being shutdown within 60 seconds.
    The super user can write to any user's terminal regardless of the permissions set by that user using mesg command. The wall command is commonly used by the super user. It can also be used by any user but may users do not need to send message to every one on the system.
    Common formats are:
                  $ wall < mesgfile
    OR        $ cat mesgfile ! wall
    Example:
    
    1.    $ wall
       This is a broadcast message of the Linux Beginners Broadcasting Foundation. Please disregard this message and get back to your work immediately. Ctrl-D.
    The message will also be sent to your tty since you are also logged on to the system. This provides a verification that the message was sent as expected.
23. Man Command:
    The external man command display help about any command to your terminal screen.
    The general syntax format of the man command is
    $ man command
    Example:
             $ man cp
    Will display help page of cp command.
24. Head Command:
    The External  head command will display the first 23 lines of the file on the screen.
    The general format of the head command is
               $ head file_name
    Example:
               $ head abc
    Will display first 23 lines of abc file.
25. Tail Command:
    The external tail command will display the last 23 lines of a file on screen.
    The general format of the tail command is
                 $ tail file_list
    Example:
                 $ tail abc
    will display last 23 lines of abc file.
26. Mail Command:
    The external mail command is an electronic post office. You can send, receive and store messages using mail command. There is a long list of options associated with mail command which can be used according to the requirement. The general format of mail command is
                 $ mail options
    To read mail, use the following command
                 $ mail
    All the messages stored in the mailbox can be displayed.
    To send mail to another user of the system, the following command is used:
               $ mail user_name
    Example:
    
    1. To send mail to a user named alpha
       $ mail alpha and press Return. The following screen appears
       Subject --------------- (type the subject of the message) (type the message) press Ctrl-d to end the message
    2. To read mail, type
       $ mail
       The display will show          mail version................ 01/29/99 type? for help
       Type ? to display the commands to view, edit, delete or store the messages to file. aq
27. Shutdown Command:
    the external Shutdown command will shutdown the system. The root user are allowed to use this command on server, specifying time, will broadcast the message to all connected users. This command be used according to the requirement. The general format of shutdown command is
    Syntax             $ shutdown option time [message]
    Option:
    
    • -h:
      Will halt after shutdown.
    • -r:
      System will reboot after the shutdown or restart the system.
    • Time:
      Time specified in minutes
     Example:
    $ shutdown -h 3 "please save your work, the system is shutdown in 3 minutes.