Unix Tutorial

What is Unix ?

The UNIX operating system is a set of programs that act as a link between the computer and the user.

 

The computer programs that allocate the system resources and coordinate all the details of the computer's internals is called the operating system or kernel.

 

Users communicate with the kernel through a program known as the shell. The shell is a command line interpreter; it translates commands entered by the user and converts them into a language that is understood by the kernel.

 

Unix was originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna.

 

There are various Unix variants available in the market. Solaris Unix, AIX, UP Unix and BSD are few examples. Linux is also a flavour of Unix which is freely available.

 

Several people can use a UNIX computer at the same time; hence UNIX is called a multiuser system.

 

A user can also run multiple programs at the same time; hence UNIX is called multitasking.

 

Unix Architecture:

Here is a basic block diagram of a UNIX system:

The main concept that unites all versions of UNIX is the following four basics:

Kernel: The kernel is the heart of the operating system. It interacts with hardware and most of the tasks like memory management, tash scheduling and file management.

 

Shell: The shell is the utility that processes your requests. When you type in a command at your terminal, the shell interprets the command and calls the program that you want. The shell uses standard syntax for all commands. C Shell, Bourne Shell and Korn Shell are most famous shells which are available with most of the Unix variants.

 

Commands and Utilities: There are various command and utilities which you would use in your day to day activities. cp, mv, cat and grep etc. are few examples of commands and utilities. There are over 250 standard commands plus numerous others provided through 3rd party software. All the commands come along with various optional options.

Files and Directories: All data in UNIX is organized into files. All files are organized into directories.

 

Explain Karnel?

Kernel: the kernel is the master program that provide file related activities, process scheduling, memory management, and various other operating system functions through system calls. In other words we can say that it control the resources of the computer system, allocate them to different users and to different tasks.

The major portion of the kernel is written in C language. Therefore, it is easy to understand, debug, and enhance it. As it is written in C language, therefore, it is portable in nature.

As you can see in the diagram that it is written or placed between hardware and utility program (Like shells, editors, vi or sed) so it work between the two. Moreover, the kernel maintains various data structure to manage the processes. Each process  has its own priority. A higher priority process is execute  first than the lower priority process.

Kernel is divided into two parts:

1.    Process management

2.    File management

 

The primary task of the process management is to manage the memory management activities and process related activities at different states of the execution – creation/deletion of processes, scheduling of the processes and provision of mechanism for synchronization, communication and deadlock handling of the processes.

 

Where as the task of the file management is to manage the file related activities. Since Unix is such kind of operating system which treats the I/O devices as a file. Therefore each I/O devices has its own file, known as device drivers, to derive it. The file management pat of the kernel handles these device drivers and store these files in the directory “/dev” under root directory. If we attach any new I/O  device to the Unix than it is necessary to create a file for that device in “/dev” directory. Then we write down its characteristics; such as its type (character oriented or block oriented), address of the driver program, memory buffer reserved for the device and some other, in that specific file.

 

Explain Shells?

Shells: In Unix we cannot directly deals with kernels. It is the shell, one of the utility program, that starts up the kernel when the user logs in. the shell sends a prompt symbol. The shell prompt waits for input from users.

Now when   you type a command and press Enter key, the shell obtained your command, execute it if possible and display the prompt symbol again in order to receive your next command. That is why the shell is also called as the Unix system command interpreter.

 

Moreover, when we want to access the hardware, we will request for the shell, the shell will request to kernel, and finally the kernel will request to hardware. Basically the shell handles the user interaction with the system. Some built in commands are part of shell and the remaining commands are separated programs stored else where.

There are three types of shells that are widely used and are exist in the Unix OS:

 

Bourne shell: This shell was designed by Stephan Bourne of Bell’s Lab. It is most powerful an most widely used shell. The prompt symbol of Bourne shell is $ (Doller) sign.

 

The C Shell: the C shell was developed at University of California. It is designed by Bill Joy. C shell gets its name from its programming language, which resembles the C programming language in syntax. The prompt symbol of C shell is Percent (%) sign.

 

Korn Shell: Like Bourne shell, the Korn shell was also developed at Bell’s Lab of AT & T. This shell gets its name from its inventor David Korn of Bell’s Lab.

 

Moreover, the shells provide the meta characters like *, [ ], ? and so on. For better searching of files.

 

Explain programs?

A program is refereed to as a process while the kernel is running it. The Kernel can run the same shell program(or any other program) simultaneously for many users on a Unix System, and each running copy of the program is a separate process. The Shell is also a program that the UNIX Kernel runs for the user.  As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile(which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt $ to the user, indicating that it is waiting for further commands.

UNIX File Protections

File protection (also known as access rights) enables the user to set up a file so that only specific people can read, write/delete, and execute it. Write and delete privilege are the same on a UNIX system since write privilege allows someone to overwrite a file with a different one.

 

 UNIX file permissions are defined according to ownership. The person who creates a file is its owner.

 

 In addition, there is a concept known as a group. Members of a group have special privileges to see each other's files. We generally create groups as the members of a single lab - the students, technicians, postdocs, visitors, etc who work for a given PI.

 

Only the owner of a file can change the protection on a file. To determine the owner and the current protections of a file, type the command ls -l filename (-l is for long) or just ls -l to get a listing of all files in the current directory.

 

$ ls -l

total 2514

-rw-r--r--   1 browns02 staff     366163 Jul 29 15:06 distrib.tar.Z

-rw-r--r--   1 browns02 staff      15721 Jul 29 15:06 input_file

drwxr-xr-x   3 browns02 staff        512 Jul 29 15:14 Phrap

-rw-r--r--   1 browns02 staff     878231 Jul 29 15:06 RepeatMasker050599.tar.Z

The username of the owner is shown in the third column. The owner of the files listed above is browns02.

 

The access rights for these files is shown in the first column. This column consists of 10 characters known as the attributes of the file. The possible attributes are: d, r, w, x, and -

 

r indicates read permission

 

w indicates write (and delete) permission

 

x indicates execute permission

 

- indicates no permission for that operation

 

The first charcter in the attribute string indicates if a file is a directory (d) or a regular file (-)

 

the next 3 characters give the file permissions for the owner of the file

 

the middle 3 characters give the permissions for members of the owner's group

 

the last 3 characters give the permissions for everyone else (the world)

 

 

 To change the protections on a file use the chmod (change mode) command. [Beware, this is a confusing command.]

 First you have to decide for whom you want to change the access permissions. Your choices are:

the file owner (u) (presumably yourself)

the members of your group (g)

others (o) (ie. the whole world)

or all 3 classes at once (a)

 Next you have to decide if you are adding (+), removing (-), or setting (=) some permissions.

 

 Taken all together, it looks like this:

 

        $ chmod u=rwx g+r o-x myfile.txt

This will set the owner to have read, write, and execute; add the permission for member of the group to read; and remove the permission for others to execute the file named myfile.txt.

 

 

The default file protection assigned to new files is -rw-r--r-- .

 

That is to say the owner can manipulate the files freely (read and write permission), those in the group can read them, but not write (or delete), and the world can also read them. If you feel you need to alter this default file protection then use this command to set the file privilages so no one else can see your files:

 

        chmod go-r *.*

(or set whatever privileges you want)

Directory Structure:

Unix uses a hierarchical file system structure, much like an upside-down tree, with root (/) at the base of the file system and all other directories spreading from there.

 

A UNIX filesystem is a collection of files and directories that has the following properties:

 

It has a root directory (/) that contains other files and directories.

 

Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.

 

By convention, the root directory has an inode number of 2 and the lost+found directory has an inode number of 3. Inode numbers 0 and 1 are not used. File inode numbers can be seen by specifying the -i option to ls command.

 

It is self contained. There are no dependencies between one filesystem and any other.

 

The directories have specific purposes and generally hold the same types of information for easily locating files. Following are the directories that exist on the major versions of Unix:

 

Directory  Description

/               This is the root directory which should contain only the directories needed at the top level of the file structure.

/bin  This is where the executable files are located. They are available to all user.

/dev These are device drivers.

/etc  Supervisor directory commands, configuration files, disk configuration files, valid user lists, groups, ethernet, hosts, where to send critical messages.

/lib   Contains shared library files and sometimes other kernel-related files.

/boot        Contains files for booting the system.

/home      Contains the home directory for users and other accounts.

/mnt Used to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively

/proc        Contains all processes marked as a file by process number or other information that is dynamic to the system.

/tmp Holds temporary files used between system boots

/usr  Used for miscellaneous purposes, or can be used by many users. Includes administrative commands, shared files, library files, and others

/var  Typically contains variable-length files such as log and print files and any other type of file that may contain a variable amount of data

/sbin Contains binary (executable) files, usually for system administration. For example fdisk and ifconfig utlities.

/kernel     Contains kernel files.

 

In Unix, how do I change the permissions for a file?

You can change file permissions with the chmod command. In Unix, file permissions, which establish who may have different types of access to a file, are specified by both access classes and access types. Access classes are groups of users, and each may be assigned specific access types. The access classes are "user", "group", "other", and "all". These refer, respectively, to the user who owns the file, a specific group of users, the other remaining users who are not in the group, and all three sets of users. Access types (read, write, and execute) determine what may be done with the file by each access class.

 

There are two basic ways of using chmod to change file permissions:

 

Symbolic method

The first and probably easiest way is the relative (or symbolic) method, which lets you specify access classes and types with single letter abbreviations. A chmod command with this form of syntax consists of at least three parts from the following lists:

 

Access Class  Operator  Access Type

u (user)    + (add access) r (read)

g (group)  - (remove access)  w (write)

o (other)   = (set exact access)        x (execute)

a (all: u, g, and o)

For example, to add permission for everyone to read a file in the current directory named myfile, at the Unix prompt, you would enter:

 

  chmod a+r myfile

The  a  stands for "all", the  +  for "add", and the  r  for "read".

Synchronizing UNIX files?

Using cp, tar, and rsync

File synchronization is the process of adding, changing, or deleting a file in one location, and having the same file added to, changed, or deleted at another location.

This article covers three utilties, cp, tar, and rsync, that can aid with synchronization of UNIX files. While cp and tar commands have limited synchronization abilities,

rsync provides you with the full range of options; however, all three have their place.

 

The Boot Procedure?

Bootstrapping is the process of starting up a computer from a halted or powered-down condition. When the computer is switched on,

it activates the memory-resident code which resides on the CPU board. The normal facilities of the operating system are not available

 at this stage and the computer must 'pull itself up by its own boot-straps' so to speak. This procedure therefore is often referred to

as bootstrapping, also known as cold boot. Although the bootstrap procedure is very hardware dependent, it typically consists of the following steps:

 

The memory-resident code

Runs self-test.

Probes bus for the boot device

Reads the boot program from the boot device.

Boot program reads in the kernel and passes control to it.

Kernel identifies and configures the devices.

Initializes the system and starts the system processes.

Brings up the system in single-user mode (if necessary).

Runs the appropriate startup scripts.

Brings up the system for multi-user operation.

What is inode in UNIX OS?

All UNIX files have its description stored in a structure called inode. The inode contains info about the file-size,

 its location, time of last access, time of last modification, permission and so on. Directories are also represented

as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the

data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks

(this further aggregates for larger files). A block is typically 8k.

Inode consists of the following fields:

File owner identifier

File type

File access permissions

File access times

Number of links

File size

Location of the file data

Describe the terms spooling and buffering. [HSEB 2060]?

Spooling: It is referred as Simultaneous Peripheral Operation Online. It means to put jobs in a buffer, a special area in memory

or on a disk where a device can access them when it is ready. Spooling is useful because devices access data at different rates.

The spooling technique is used in multiprogramming environment to offer first chance to the program of higher priority and reduce processor

idle time. Each application output files are spooled to separate disk file called spool files and spooling system make queue for output

process. The most common spooling application is print spooling.

Buffering: Buffering is the name given to the technique of transferring data into temporary storage prior to processing or output,

thus enabling the simultaneous operation of devices.

Swapping: The swapping is a memory management technique applied by Operating systems. Swapping is a technique in which high priority

process is swap in main memory and low priority process from main memory to swap out. When high priority process is completed, low priority

process is swap in again.

What is hit ratio ?

The performence of memory is frequently mesuread in terms

of quntity is called hit ratio.when the cpu needs to find

the word in the cache,if the word is found in the cache

then its proudes a hit.if the word is not found in the

cache,it is in main memory as counted miss.the ratio of

number of hits is divided by the total cpu refrence of

memory is called hit ratio.

 Types of Operating Systems?

An operating system is the single most important software when you run a computer, it is what takes care of pretty much everything on a computer

system, while the majority of computers we see happen to be using one ‘type’ of operating system performing the same functions,

operating systems can be branched into several different types as well.

 

Batch Processing Operating System

In a batch processing operating system interaction between the user and processor is limited or there is no interaction at all during the execution of work. Data and programs that need to be processed are bundled and collected as a ‘batch’ and executed together. ExampleIBM's OS360 

Batch processing operating systems are ideal in situations where:

- There are large amounts of data to be processed.

- Similar data needs to be processed.

- Similar processing is involved when executing the data.

The system is capable of identifying times when the processor is idle at which time ‘batches’ maybe processed. Processing is all performed automatically without any user intervention.

 

Real-time Operating System

A real-time operating system processes inputs simultaneously, fast enough to affect the next input or process. Real-time systems are usually used to control complex systems that require a lot of processing like machinery and industrial systems.

Example  Windows CE,  RTLinux 

 

Single User Operating System

A single user OS as the name suggests is designed for one user to effectively use a computer at a time. Example MS Dos

 

Multi-Tasking Operating System

In this type of OS several applications maybe simultaneously loaded and used in the memory. While the processor handles only one application

at a particular time it is capable of switching between the applications effectively to apparently simultaneously execute each application. This type of operating system is seen everywhere today and is the most common type of OS, the Windows operating system would be an example.

 

Multi-User Operating System

This type of OS allows multiple users to simultaneously use the system, while here as well, the processor splits its resources and handles one user at a time, the speed and efficiency at which it does this makes it apparent that users are simultaneously using the system, some network systems utilize this kind of operating system.

Example of multi-user OS include Unix server & thin client.

Distributed Operating System

In a distributed system, software and data maybe distributed around the system, programs and files maybe stored on different storage devices

which are located in different geographical locations and maybe accessed from different computer terminals.

While we are mostly accustomed to seeing multi-tasking and multi-user operating systems, the other operating systems are usually used in

companies and firms to power special systems.

 

What is an Operating System? Explain all the four types of OS?

An operating system (commonly abbreviated OS and O/S) is the software component of a computer system that is responsible for the management and coordination of activities and the sharing of the limited resources of the computer.

 

There are 4 types of operating system which is explained below:-

 

i. Batch Operating System:-A batch system is one in which jobs are bundled together with the instructions necessary to allow them to be processed without intervention. The basic physical layout of the memory of a batch job computer is shown below:

 

Monitor (permanently resident)

User Space

(compilers, programs, data, etc.)

 

The monitor is system software that is responsible for interpreting and carrying out the instructions in the batch jobs. When the monitor starts a job, the entire computer is dedicated to the job, which then controlls the computer until it finishes.

 

1. Advantages of batch systems  move much of the work of the operator to the computer

increased performance since it was possible for job to start as soon as the previous job finished

2. Disadvantages    turn-around time can be large from user standpoint

more difficult to debug program

due to lack of protection scheme, one batch job can affect pending jobs (read too many cards, etc)

a job could corrupt the monitor, thus affecting pending jobs

a job could enter an infinite loop

 

ii. Multiprogramming Operating System:- As machines with more and more memory became available, it was possible to extend the idea of multiprogramming (or multiprocessing) as used in spooling batch systems to create systems that would load several jobs into memory at once and cycle through them in some order, working on each one for a specified period of time. There are different type of Multiprogramming Operating System, some main stream are discussed below:-

 

a. Multitasking Operating System:- A running state of a program is called a process or a task.multitasking allows the computer system to more reliably guarantee each process a regular "slice" of operating time. It also allows the system to rapidly deal with important external events like incoming data, which might require the immediate attention of one or another process. So, multitasking operating system is a type of multiprogramming operating system which can perform several process simultaneously.The earliest multitasking OS available to home users was the AmigaOS. All current major operating system support this feature.

 

b. Multi-user Operating System:- A multi-user operating system allows for multiple users to use the same computer at the same time and/or different times. Linux,Unix,Windows OS are some example of multitasking operating system.

 

c. Multiprocessing Operating System:- An operating system capable of supporting and utilizing more than one computer processor.Linux,Unix,Windows OS are some example of multitasking operating system.

d. Real Time Operating System:- It is an OS where there are a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system was coupled with process prioritization to ensure that key activities were given a greater share of available process time.

 

iii. Network Operating System:- A network operating system (NOS) is software that controls a network and its message (e.g. packet) traffic and queues, controls access by multiple users to network resources such as files, and provides for certain administrative functions, including security.

 

The upper 5 layers of the OSI Reference Model provide the foundation upon which many network operating systems are based. 6WINDGate,Cisco IOS,BSD are some of the example of NOS.

 

iv. Distributed Operating System:- Distributed systems are very much like traditional operating systems. First, they act as resource managers for the underlying hardware, allowing multiple users and applications to share resources such as CPUs, memories, peripheral devices, the network, and data of all kinds. Second, and perhaps more important, is that distributed systems attempt to hide the intricacies and heterogeneous nature of the underlying hardware

by providing a virtual machine on which applications can be easily executed.

Features of unix operating system?

The UNIX Operating System is available on machines with a wide range of computing power, from microcomputers to mainframes, and on different

manufacture's machines. No other operating system can make this claim. We see the reasons of popularity and success of UNIX.

 

The reasons are

Portability:

The system is written in high-level language making it easier to read, understand, change and, therefore move to other machines. The code

can be changed and complied on a new machine. Customers can then choose from a wide variety of hardware vendors without being locked in with

a particular vendor.

 

Machine-independence:

The System hides the machine architecture from the user, making it easier to write applications that can run on micros, mins and mainframes.

 

Multi-User Operations:

UNIX is a multi-user system designed to support a group of users simultaneously. The system allows for the sharing of processing power and

peripheral resources, white at the same time providing excellent security features.

 

Hierarchical File System:

UNIX uses a hierarchile file structure to store information. This structure has the maximum flexibility in grouping information in a way

that reflects its natural state. It allows for easy maintenance and efficient implementation.

 

UNIX shell:

UNIX has a simple user interface called the shell that has the power to provide the services that the user wants. It protects the user

from having to know the intricate hardware details.

 

Pipes and Filters:

UNIX has facilities called Pipes and Filters which permit the user to create complex programs from simple programs.

 

Utilities:

UNIX has over 200 utility programs for various functions. New utilities can be built effortlessly by combining existing utilities.

 

Software Development Tools:

UNIX offers an excellent variety of tools for software development for all phases, from program editing to maintenance of software,

What are features of the operating system in a computer?

1. Program Execution

2. Handling Input/Output Operations

3. Manipulation Of File System

4. Error Detection And Handling

5. Resource Allocation

6. Accounting

7. Information And Resource Protection

 

What Are The Functions Of An Operating System?

There are many functions of an operating system.  There are two main things that it does however, and these are: Managing the software and hardware, and providing an interface, which the installed applications can work with, without having to be programmed to know how the hardware works.

Both computers and mobile phones use operating systems in order to work.  They work in different ways as they have different features involved.

The hardware and software on a desktop computer include memory, the processor, disk space and more, so it has quite a lot to deal with.  On a mobile phone, this can be the screen, key or touch pad, address book, dialler, network connection and the battery.

 

The operating system is the main aspect of a device, and is very important to its overall function.  Without it, the computer or phone would not be able to work.

There are many other functions that an operating system has.  It also takes care of task management, and basically multitasks, by running many programs at once, without slowing it down.

It also looks after the data within the device and through the use of discs and memory sticks or cards.  It needs to know where they are stored within the system and how to read and write them.

 

It also has to control the devices plugged into the system.  These are sometimes known as peripherals and can include printers, a mouse, and a keyboard.

It provides a user interface for the user to interact with the device easily. These are all graphics based now, so the user can understand what each button will do, which once clicked upon, is then translated into codes for the computer to make sense of.

An operating system is able to deal with security aspects and use passwords to protect it.

Context Switches?

The scheduler maintains a queue of executable threads for each priority level. These are known as ready threads. When a processor becomes available, the system performs a context switch. The steps in a context switch are:

Save the context of the thread that just finished executing.

Place the thread that just finished executing at the end of the queue for its priority.

Find the highest priority queue that contains ready threads.

Remove the thread at the head of the queue, load its context, and execute it.

 

The following classes of threads are not ready threads.

Threads created with the CREATE_SUSPENDED flag

Threads halted during execution with the SuspendThread or SwitchToThread function

Threads waiting for a synchronization object or input.

Until threads that are suspended or blocked become ready to run, the scheduler does not allocate any processor time to them, regardless of their priority.

The most common reasons for a context switch are:

The time slice has elapsed.

A thread with a higher priority has become ready to run.

A running thread needs to wait.

When a running thread needs to wait, it relinquishes the remainder of its time slice.

 

Why Bourne Shell? 

now that you find yourself on your own, cozy slice of a Unix system, with your own shell and all its capabilities, is there any real reasonto use Bourne Shell rather than using the whole range of your shells capabilities?

Well, it depends. Probably, there isn't. For the most part of course, you are using Bourne Shell by using the whole potential of your shell.

your shell is probably that similar to the Bourne Shell. But there is one thing you might want to keep in mind: someday, you might want

to write a script that you might want to pass around to other people. Of course you can write your script using the full range of options

that your shell offers you; but then it might not work on another machine with another shell. This is where the role of Bourne Shell as the Latin of Unix command shells comes in -- and also where it is useful to know how to write scripts targeted specifically at the Bourne

Shell. If you write your scripts for the Bourne Shell and nothing but the Bourne Shell, chances are far better than equal that your script

will run straight out of the mail attachment (don't tell me you're still using boxes to ship things -- come on, get with the program) on

any command shell out there.

 

Salient Features Of Unix

The Salient features of unix are :

1. Popularity  : The unix operating  system have wide range of computing  power  from microcomputers, mainframes and different manufacture’s machine.

2. Portability : It is easier to read, understand, change and move to other machines because it is written in high-level language. The code can be changed and complied on a new machine. Users   can then choose from a wide variety of hardware vendors without being locked in with a particular vendor.

3. Machine-independence : The system is machine-independent. So , it is easier to write applications that can run on micros, mins and mainframes because the system hides the machine architecture from the user.

4. Multi-User Operations : UNIX is a multi-user system designed to support a group of users simultaneously. The system allows for the sharing of processing power and peripheral resources, white at the same time providing excellent security features.

5. Hierarchical File System :  UNIX uses a hierarchile file structure to store information. This structure has the maximum flexibility in grouping information in a way that reflects its natural state. It allows for easy maintenance and efficient implementation.

6. UNIX shell : UNIX has a simple user interface called the shell that has the power to provide the services that the user wants. It protects the user from having to know the intricate hardware details.

7. Pipes and Filters : UNIX has facilities called Pipes and Filters which permit the user to create complex programs from simple programs.

8. Utilities : UNIX has over 200 utility programs for various functions. New utilities can be built effortlessly by combining existing utilities.

9. Software Development Tools : UNIX offers an excellent variety of tools for software development for all phases, from program editing to maintenance of software.

What is vieditor?

There are many ways to edit files in Unix and for me one of the best ways is using screen-oriented text editor vi. This editor enable you to edit lines in context with other lines in the file.

Now a days you would find an improved version of vi editor which is called VIM. Here VIM stands for Vi IMproved.

The vi is generally considered the de facto standard in Unix editors because:

It's usually available on all the flavors of Unix system.

Its implementations are very similar across the board.

It requires very few resources.

It is more user friendly than any other editors like ed or ex.

You can use vi editor to edit an existing file or to create a new file from scratch. You can also use this editor to just read a text file.

Starting the vi Editor:

There are following way you can start using vi editor:

Command	Description
vi filename	Creates a new file if it already does not exist, otherwise opens existing file.
vi -R filename	Opens an existing file in read only mode.
view filename	Opens an existing file in read only mode.

What is Mutual Exclusion?

It is one of the characteristics of deadlock. When semaphores are used or mutual exclusion, the semaphore has an initial value of 1, and P() is called before the critical section, and V() is called after the critical section as shown below :

semaphore-> P();-critical section            semaphore-> V();-remainder section

let us suppose that one process A is already executing its critical section then it implies that semaphore value at that time is zero. If process B now tries to enter this critical section , it cannot enter the critical section because it will have to wait before semaphore becomes greater than zero. This is possible only when process A executes its signal operation; after executing its critical section.

Explain kill() command?

The kill command only terminates processes specified by process-id when a specific background process needs to be killed, you must usually run ps to find the process-id and then re type it as an argument to kill. Killing process is dangerous and care must be taken to kill the right processes.

Eplain fork()command?

The 'fork()' call is used to create a new process from an existing process. The new process is called the child process, and the existing process is called the parent. It return the p_id of child process and creating child process return itself is zero.

 

Access Control

exit - terminate a shell (see "man sh" or "man csh")

logout - sign off; end session (C shell and bash shell only;)

passwd - change login password

rlogin - log in remotely to another UNIX system

ssh - secure shell

slogin - secure version of rlogin

yppasswd - change login password in yellow pages

Communications

mail - send and receive mail

mesg - permit or deny terminal messages and talk requests

pine - send and receive mail 

talk - talk to another logged-in user

write - write to another logged-in user

Programming Tools

as - assembler, specific to each machine architecture

awk - pattern scanning and processing language

bc - online calculator

cc - C compiler 

csh - C shell command interpreter

dbx - source-level debugging program

f77 - Fortran compiler 

gdb - GNU Project debugger

gprof - display profile of called routines

kill - kill a process

ld - the UNIX loader

lex - generate lexical analysis programs

lint - check C source code

make - maintain large programs

maple - symbolic mathematics program

math - symbolic mathematics program

nice - run a command at low priority (see "man nice" or "man csh")

nohup - run a command immune to hangups

pc - Pascal compiler (xlp on ADS)

perl - Popular script interpreter

prof - display profile data

python - Python programming language

sh - Bourne shell command interpreter

yacc - generate input parsing programs

xcalc - graphical calulator under x

Documentation

apropos - locate commands by keyword lookup

find - locate file (i.e. find . -name *.tex -print)

info - start the info explorer program 

man - find manual information about commands

whatis - describe what a command is

whereis - locate source, binary, or man page for a program

Editors

emacs - screen-oriented text editor

pico - screen-oriented text editor (renamed called nano)

sed - stream-oriented text editor

vi - full-screen text editor

vim - full-screen text editor ("vi-improved")

File and Directory Management

cd - change working directory

chmod - change the protection of a file or directory

chown - change owner (or group) of a file or directory

chgrp - change group of a file or directory

cmp - compare two files

comm - select/reject lines common to two sorted files

cp - copy files

crypt - encrypt/decrypt files (CCWF only)

diff - compare the contents of two ASCII files

file - determine expand files

ln - make a link to a file file type

grep - search a file for a pattern

gzip - compress or 

ls - list the contents of a directory

lsof - list of open files    

mkdir - create a directory

mv - move or rename files and directories

pwd - show the full pathname of your working directory

quota - display disk usage and limits

rm - delete (remove) files

rmdir - delete (remove) directories

stat - status of file (i.e. last access)

sync - flush filesystem buffers   

sort - sort or merge files

tar - create or extract archives

tee - copy input to standard output and other files

tr - translate characters

umask - change default file protections

uncompress - restore compressed file

uniq - report (or delete) repeated lines in a file

wc - count lines, words, and characters in a file

File Display and Printing

cat - show the contents of a file; catenate files

fold - fold long lines to fit output device

head - show first few lines of a file

lpq - examine the printer spooling queue

lpr - print a file

lprm - remove jobs from the printer spooling queue

more - display a file, one screen at a time

less - like more with more features 

page - like "more", but prints screens top to bottom

pr - paginate a file for printing

tail - show the last part of a file

zcat - display a compressed file

xv - show print, manipulate images

gv - show ps and pdf files

xpdf = shopw pdf files (use gv) 

File Transfer

ftp - transfer files between network hosts

rsync -  fast and flexible sync between computers

scp - secure version of rcp

Miscellaneous

alias - define synonym commands

chquota - change disk quota on ACITS UNIX systems

chsh - change default login shell

clear - clear terminal screen

echo - echo arguments

pbm - portable bitmap manipulation programs

popd - pop the directory stack (C shell only)

pushd - push directory on stack (C shell only)

script - make typescript of terminal session

setenv - set an environment variable (C shell only)

stty - set terminal options

News/Networks

netstat - show network status

rsh - run shell or command on another UNIX system

ssh - secure-shell version of rsh

Process Control

 

bg - put suspended process into background

fg - bring process into foreground

jobs - list processes

^y - suspend process at next input request

^z - suspend current process 

Status Information

clock - determine processor time

date - show date and time

df - summarize free disk space

du - summarize disk space used

env - display environment

finger - look up user information

history - list previously issued commands 

last - indicate last login of users

lpq - examine spool queue

manpath - show search path for man pages

printenv - print out environment

ps - show process status

pwd - print full pathname of working directory

set - set shell variables (C shell, bash, or ksh)

spend - lists year-to-date ACITS UNIX charges

stty - set terminal options

time - timing programs

top - list  top  cpu processes

uptime - show system load, how long system has been up

w - show who is on system, what command each job is executing

who - show who is logged onto the system

whois - Internet user name directory service

whoami - who owns the shell

Image Processing

gimp - photoshop type image processing program

xfig - drawing program

xv - image viewer

xvscan - scan picture 

xpaint - paint program

kpaint - kde paint program 

Sound

mplayer - mpg player 

realplay - realaudio player

timidity - midi to wav converter and player

xmms - mp3 player 

Text Processing

abiword - open source word processor

addbib - create or extend bibliographic database

col - filter reverse line feeds

diction - identify wordy sentences 

diffmk - mark differences between files

dvips - convert TeX DVI files into PostScript

explain - explain phrases found by diction program

grap - pic preprocessor for drawing graphs 

hyphen - find hyphenated words 

ispell - check spelling interactively

latex - format text in LaTeX (based on TeX)

pdfelatex - latex with pdf output 

latex2html - Latex to html

lookbib - find bibliography references

macref - make cross-reference listing of nroff/troff macro files

ndx - create a subject-page index for a document

neqn - format mathematics with nroff

nroff - format text for simple display

pic - make simple pictures for troff input

psdit - filter troff output for Apple LaserWriter

ptx - make permuted index (not on CCWF)

refer - insert references from bibliographic databases 

roffbib - run off bibliographic database 

sortbib - sort bibliographic database 

spell - find spelling errors

ispell - interactive spell checker

style - analyze surface characteristics of a document 

tbl - format tables for nroff/troff

tex - format text

tpic - convert pic source files into TeX commands 

wget -  grab webpage

X windows

grabmode - info on screen: i.e. "1152x864"  51.213kHz/56.59Hz

import - grab window (i.e. import ppm:- >out.ppm)

xdpyinfo -  number of colors 

xkill - kill xwindow

xlock - lock screen

xterm - xterminal

xwininfo - information on open window

Web

html2ps - html to ps

latex2html - latex to html translator

lynx - text based webbrowser

netscape - webbrowser

sitecopy - sitecopy is for easily maintaining remote web sites.

weblint - html sytax and style checker

 

What is threading ?

Native threads

A thread implementation that is supported by a unix kernel. For example, Linux's native threads are created with the clone() system call without the COPYVM flag set.

Threading Library

A user-space library and API that provides user-level access to native or user level threads. For example, pthreads (also known as Posix Threads) is the most popular threading library. Some Unix flavors (Solaris and UnixWare) also have an older API to their native threads. This API is called Unix International (UI) threads, and uses the thr_ prefix.

How threading use?

1. pthread_create()

2. Forking creates two processes, each having a separate thread of control. Creating a thread creates an extra thread of control within a single process.

3. No - it is generally harder to get threaded applications right than it is to get separate processes right. And by quite a large margin

(DNS)  Domain Name Service

Domain name services resolves names to the ip addresses of clients and vice verse. Domain name system provides a convenient way of finding computer systems in network based on its name and ip address . With increased internet usage and globalization of companies setting up of DNS servers has become a major responsibility of system administrators worldwide .

NIS / NIS+

Network Information Service (formerly known as Yellow Pages) provides account data (names, passwords, etc.) in a relatively convenient, but insecure manner. The data is moved across the network in it's native fashion, meaning usernames go across as clear text, and the password hashes are sent without any additional protection (thus if you use shadow passwords to protect the passwords, using NIS negates that rather quickly). NIS+ provides an additional layer of security in the form of DES encryption (56 bits) which will prevent any casual attackers, but not a determined attacker. NIS+ has traditionally been a pain to setup, however there has been much work on it in the last few year so installation and configuration is much easier then in past.

Page replacement techniques

In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a page fault occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.

When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.

The page replacing problem is a typical online problem from the competitive analysis perspective in the sense that optimal deterministic algorithm is known.

Features of operating system?

The features of the operating systems are totally dependent on the type of software. Every operating system differs from each other in some features while there are some features which are common in all operating systems. 

Some of the features of operating systems are detailed here.

Software and hardware management

The operating system is the bridge between computer hardware and software and makes the communication between them possible. Also communication between different softwares in the computer is also taken care by operating system.

Constant API

Application Program Interface (API) is a software that allows different applications that run on a computer to work on other computers also. But they should have same operating system. So it is very vital to have consistent API in the operating system.

Execution of programs

Programs running in the computer are completely dependent on the operating system. But program execution is a tough process. The multitasking and multithreading features of the operating system are dependent upon the type of program execution feature of O.S.

Interruptions

Interruption may happen at any time while using the computers. So the operating system should allow and handle many numbers of interrupts. Whenever an interruption occurs, the operating system should respond to it by saving and stopping the current execution and work on the new execution. This is the most hard-hitting process for the operating system.

Managing memory

The operating system provides the memory for the programs that are executed at any moment. So the operating system should have good memory allocation facility to execute the programs smoothly. The prioritization and allocation of memory to the applications running should be taken care by the operating systems.

Networking

Today computers are nothing without internet connection or some network connection. This is the age of networking. So if computers are connected to a network, the there should be definitely communication between one computer and another. So the operating system is what makes it possible for one computer to communicate with other computers.

Security

Security is the important feature that should be looked for in an operating system. An operating system in the computer takes care of all security issues of computer and data in it. Log in passwords, firewall settings, and every such aspect related to security depends on the ability of the operating system. Some of the computers in network may involve in file sharing, and other data sharing. So it is important in such cases to have powerful secured operating systems.

Different betweenmultitasking & multiprogramming

Multiprogramming is a method of running several different programs in a computer apparently at the same time.

Usually on a mainframe - the computer has a number of programs loaded into memory and the operating system switches quickly between them, processing a little bit of each one in turn. The high speed of the processor makes it seem like more than one program is being run at the same time.

On a PC it is usually called multitasking.  

-multitasking is the process which occur when more than one task(job) are executed by CPU at a time....-.Multitasking should not be confusing with multiple program where by in multiprogram more than one program(task) are in the mein memory but each one are executed by CPU at different time...It is impossible CPU to running more than one program at the same time