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  • Understanding Unix For Solving Assignments

    What is "UNIX"? In the narrowest sense, it is a time-sharing operating system kernel: a program that controls the resources of a computer and allocates them among its users. It lets users run their programs; it controls the peripheral devices (discs, terminals, printers, and the like) connected to the machine; and it provides a file system that manages the long-term storage of information such as programs, data, and documents. UNIX is being taught in colleges all over the world. In any computer related course, student are getting assignments based on UNIX and they in turn look for programming assignment help to get it done.

    In a broader sense, "UNIX" is often taken to include not only the kernel, but also essential programs like compilers, editors, command languages, programs for copying and printing files, and so on.Still more broadly, "UNIX" may even include programs developed by you or other users to be run on your systems, such as tools for document preparation, routines for statistical analysis, and graphics packages.

    • basics logging in and out, simple commands, correcting typing mistakes. mail, inter-terminal communication.
    • day-to-day use files and the file system, printing files, directories, and commonly-used commands.
    • the command interpreter or shell filename shorthands, redirecting input and output, pipes, setting erase and kill characters, and defining your own search path for commands.

    If you've used a UNIX system before, most of this chapter should be familiar; you might want to skip straight to Chapter 2.

    You will need a copy of the UNIX Programmer's Manual, even as you read this chapter; it's often easier for us to tell you to read about something in the manual than to repeat its contents here. This book is not supposed to replace it, but to show you how to make best use of the commands described in it. Furthermore, there may be differences between what we say here and what is true on your system. The manual has a permuted index at the beginning that's indispensable for finding the right programs to apply to a problem; learn to use it.

    Finally, a word of advice: don't be afraid to experiment. If you are a beginner, there are very few accidental things you can do to hurt yourself or other users. To learn how things work by trying them. This is a long chapter, and the best way to read it is a few pages at a time, trying things out as you go.

    1.1 Getting started

    Some prerequisites about terminals and typing

    To avoid explaining everything about using computers, we must assume you have some familiarity with computer terminals and how to use them. If any of the following statements are mystifying, you should ask a local expert for help. (The UNIX system is full duplex: the characters you type on the keyboard are sent to the system, which sends them back to the terminal to be printed on the screen. Normally, this echo process copies the characters directly to the screen, so you can see what you are typing, but sometimes, such as when you are typing a secret password, the echo is turned off so the characters do not appear on the screen.

    A Session with UNIX

    Let's begin with an annotated dialog between you and your UNIX system. Throughout the examples in this book, what you type is printed in slanted letters, computer responses are in typewriter-style characters, and explanations are in italics.

    Establish a connection: dial a phone or turn on a switch as necessary. Your system should say

    Login: you                      Type your name, then press RETURN

    Password:                      Your password won't be echoed as you type it

    You have mail:              There's mail to be read after you log in

    $                                          The system is now ready for your commands

    $                                         Press RETURN a couple of times

    $                                         date What's the date and time?

    Sun Sep 25  23:02:57   EDT 1983

    $ who                                   Who's using the machine?

    Jlb      tty0         Sep 25 13 : 59

    You     tty2         Sep 25 23 :01

    mary    tty4        Sep 25 19: 03

    doug    tty5       Sep 25 19:22

    egb       tty7       Sep 25 17:17

    bob       tty8      Sep 25 20 : 48

    $ mail                                      Read your mail

    From doug Sun Sep 25 20:53 EDT 1983

    give me a call sometime monday

    ?                                                  RETURN moves on to the next message

    From mary Sun Sep 25 19:07 EDT 1983                           Next message

    Lunch at noon tomorrow ?

    ? d                                                              Delete this message

    $                                                                  No more mail

    $ mail mary                                                Send mail to mary

    lunch at 12 is fine

    ctl-d                                                              End of mail

    $                                                                   Hang up phone or turn off terminal and that's the end

    Sometimes that's all there is to a session, though occasionally people do some work too. The rest of this section will discuss the session above, plus other programs that make it possible to do useful things.

    Logging in

    You must have a login name and password, which you can get from your system administrator. The UNIX system is capable of dealing with a wide variety of terminals, but it is strongly oriented towards devices with lowercase; case distinctions matter! If your terminal produces only upper case (like some video and portable terminals). life will be so difficult that you should look for another terminal.

    Be sure the switches are set appropriately on your device: upper and lower case, full duplex, and any other settings that local experts advise, such as the speed, or baud rate. Establish a connection using whatever magic is needed for your terminal: this may involve dialinga telephone or merely flipping a switch. In either case, the system should type


    If it types garbage, you may be at the wrong speed; check the speed setting and other switches. If that fails, press the BREAK or INTERRUPT key a few times, slowly. If nothing produces a login message, you will have to get help.)

    When you get the login: message, type your login name in lowercase. Follow it by pressing RETURN. If a password is required, you will be asked for it, and printing will be turned off while you type it.The culmination of your login efforts is a prompt, usually a single character, indicating that the system is ready to accept commands from you. The prompt is most likely to be a dollar sign $ or a percent sign %, but you can change it to anything you like; we'll show you how a little later. The prompt is actually printed by a program called the command interpreter or shell, which is your main interface to the system.There may be a message of the day just before the prompt, or a notification that you have mail. You may also be asked what kind of terminal you are using; your answer helps the system to use any special properties the terminal might have.Typing commands

    Once you receive the prompt, you can type commands, which are requests that the system do something. We will use program as a synonym for command. When you see the prompt (let's assume it's $), type date and press RETURN. The system should reply with the date and time, then print another prompt, so the whole transaction will look like this on your terminal:

    $ date

    Mon Sep 26 12:20:57 EDT 1983


    Don't forget RETURN, and don't type the $. If you think you're being ignored, press RETURN; something should happen. RETURN won't be mentioned again, but you need it at the end of every line.

    The next command to try is who, which tells you everyone who is currently logged in:

    $ who

    rim             tty0             Sep 26      11 : 17

    pjw             tty4             Sep 26     11 : 30

    gerard         tty7            Sep 26     10 : 26

    mark            tty9           Sep 26      07 : 59

    you               ttya           Sep 26     12 : 20


    The first column is the user name. The second is the system's name for the connection being used ("tty" stands for "teletype," an archaic synonym for "Terminal"). The rest tells when the user logged on. You might also try

    $ who am i

    You     ttya      Sep 26      12:20


    If you make a mistake typing the name of a command, and refer to a non-existent command, you will be told that no command of that name can be found:

    $ whom                                          Misspelled command name...

    whom: not found                     …so system didn't know how to run it

    Of course, if you inadvertently type the name of an actual command, it will run, perhaps with mysterious results.

    Strange terminal behavior

    Sometimes your terminal will act strangely, for example, each letter may be typed twice, or RETURN may not put the cursor at the first column of the next line. You can usually fix this by turning the terminal off and on, or by logging out and logging back in. Or you can read the description of the command stty ("set terminal options") in Section 1 of the manual. To get intelligent treatment of tab characters if your terminal doesn't have tabs, type the command

    $ sty -     tabs

    and the system will convert tabs into the right number of spaces. If your terminal does have computer-settable tab stops, the command tabs will set them correctly for you. (You may actually have to say

    $ tabs terminal -    type

    to make it work-see the tabs command description in the manual.)

    Mistakes in typing

    If you make a typing mistake, and see it before you have pressed RETURN, there are two ways to recover: erase characters one at a time or kill the whole line and re-type it.

    If you type the line kill character, by default an at-sign, it causes the whole line to be discarded, just as if you'd never typed it, and starts you over on a new line:

    $ ddtae@                                              Completely botched; start over

    date                                                        on a new line

    Mon Sep 26 12:23:39 EDT 1983


    The sharp character erases the last character typed; each erases one more character, back to the beginning of the line (but not beyond). So if you type badly, you can correct as you go:

    $ dd#atte##e                                              Fix it as you go

    Mon Sep 26   12:24:02  EDT 1983

    The particular erase and line kill characters are very system dependent. On many systems (including the one we use), the erase character has been changed to backspace, which works nicely on video terminals. You can quickly check which is the case on your system:

    $ date <-                                                                      Try <-

    Date <- not found                                                      Its not <-

    $ date#                                                                       Try #

    Mon Sep 26 12:26:08 EDT 1983 It

    (We printed the backspace as so you can see it.) Another common choice is ctl-u for line kill.

    We will use the sharp as the erase character for the rest of this section because it's visible, but make the mental adjustment if your system is different. Later on, in "tailoring the environment," we will tell you how to set the erase and line kill characters to whatever you like, once and for all.

    What if you must enter an erase or line kill character as part of the text? If you precede either or@ by a backslash \, it loses its special meaning. So to enter a # or @, type \# or \@. The system may advance the terminal's cursor to the next line after your @, even if it was preceded by a backslash. Don't worry the at-sign has been recorded.

    The backslash, sometimes called the escape character, is used extensively to indicate that the following character is in some way special. To erase a backslash, you have to type two erase characters: \##. Do you see why? The characters you type are examined and interpreted by a sequence of programs before they reach their destination, and exactly how they are interpreted depends not only on where they end up but how they got there. Every character you type is immediately echoed to the terminal, unless echoing is turned off, which is rare. Until you press RETURN, the characters are held temporarily by the kernel, so typing mistakes can be corrected with the erase and line kill characters. When an erase or line kill character is preceded by a backslash, the kernel discards the backslash and holds the following character without interpretation.

    When you press RETURN, the characters being held are sent to the program that is reading from the terminal. That program may in turn interpret the characters in special ways; for example, the shell turns off any special interpretation of a character if it is preceded by a backslash. We'll come back to this in Chapter 3. For now, you should remember that the kernel processes erase and line kill, and backslash only if it precedes erase or line kill; whatever characters are left after that may be interpreted by other programs as well.


    The kernel reads what you type as you type it, even if it's busy with something else, so you can type as fast as you want, whenever you want, even when some command is printing at you. If you type while the system is printing, your input characters will appear intermixed with the output characters, but they will be stored away and interpreted in the correct order. (You can type commands one after another without waiting for them to finish or even to begin.

    Stopping a program

    You can stop most commands by typing the character DELETE. The BREAK key found on most terminals may also work, although this is system: dependent. In a few programs, like text editors, DELETE stops whatever the program is doing but leaves you in that program. Turning off the terminal or hanging up the phone will stop most programs. If you just want output to pause, for example to keep something critical from disappearing off the screen, type ctl-s. The output will stop almost immediately; your program is suspended until you start it again. When you want to resume, type ctl-q.

    Logging out

    The proper way to log out is to type ctl-d instead of a command; this tells the shell that there is no more input. (How this actually works will be explained in the next chapter.) You can usually just turn off the terminal or hang up the phone, but whether this really logs you out depends on your system.


    The system provides a post. system for communicating with other users, so someday when you log in, you will see the message

           You have mail.

    before the first prompt. To read your mail, type

             $ mail

    Your mail will be printed, one message at a time, most recent first. After each item, mail waits for you to say what to do with it. The two basic responses are d, which deletes the message, and RETURN, which does not (so it will still be there the next time you read your mail). Other responses include p to reprint a message, s filename to save it in the file you named, and q to quit from mail. (If you don't know what a file is, think of it as a place where you can store information under a name of your choice, and retrieve it later. Files are the topic of Section 1.2 and indeed of much of this book.)

    mail is one of those programs that is likely to differ from what we describe here; there are many variants. Look in your manual for details.

    Sending mail to someone is straightforward. Suppose it is to go to the person with the login name nico. The easiest way is this:

    $ mail     nico

    Now type in the text of the letter

    on as many lines as you like…

    After the last line of the letter

    type a control-d.



    The crl-d signals the end of the letter by telling the mail command that there is ao more input. If you change your mind halfway through composing the letter, press DELETE instead of ctl-d. The half-formed letter will be stored in a file called dead. letter instead of being sent.

    For practice, send mail to yourself, then type mail to read it. (This isn't as aberrant as it might sound-it's a handy reminder mechanism.)

    There are other ways to send mail - you can send a previously prepared letter, you can mail to a number of people all at once, and you may be able to send mail to people on other machines. For more details see the description of the mail command in Section 1 of the UNIX Programmer's Manual. Hen forth we'll use the notation mail(1) to mean the page describing mail in Section 1 of the manual. All of the commands discussed in this chapter are found in Section 1.

    There may also be a calendar service (see calendar(1)); we'll show you in Chapter 4 how to set one up if it hasn't been done already.

    Writing to other users

    If your UNIX system has multiple users, someday, out of the blue, your terminal will print something like

    Message from mary tty7...

    accompanied by a startling beep. Mary wants to write to you, but unless you take explicit action you won't be able to write back. To respond, type

    $ write mary

    This establishes a two-way communication path. Now the lines that Mary types on her terminal will appear on yours and vice versa, although the path is slow, rather like talking to the moon.

    If you are in the middle of something, you have to get to a state where you can type a command. Normally, whatever program you are running has to stop or be stopped, but some programs, such as the editor and write itself, have a 'I' command to escape temporarily to the shell see Table 2 in Appendix 1.

    The write command imposes no rules, so a protocol is needed to keep what you type from getting garbled up with what Mary types. One convention is to take turns, ending each turn with (o), which stands for "over," and to signal your intent to quit with (oo), for "over and out."

    Mary's terminal.                                                  Your terminal:

    $ write you

                                                                                              $Message form mary tty7…

                                                                                               Write mary

    Message from you ttya…

    Did you forget lunch ? (o)

                                                                                                did you forget lunch ? (o)


                                                                                                ten minutes (o)

    ten minutes (o)

    ok (oo)

                                                                                                ok (oo)




                                                                                                $ EOF


    You can also exit from write by pressing DELETE. Notice that your typing errors do not appear on Mary's terminal.

    If you try to write to someone who isn't logged in, or who doesn't want to be disturbed, you'll be told. If the target is logged in but doesn't answer after a decent interval, the person may be busy or away from the terminal; simply type ctl-d or DELETE. If you don't want to be disturbed, use mesg(1).


    Many UNIX systems provide a news service, to keep users abreast of interesting and not so interesting events. Try typing

    $ news

    There is also a large network of UNIX systems that keep in touch through telephone calls; ask a local expert about netnews and USENET.

    The manual

    The UNIX Programmer's Manual describes most of what you need to know about the system. Section I deals with commands, including those we discuss in this chapter. Section 2 describes the system calls, the subject of Chapter 7, and Section 6 has information about games. The remaining sections talk about functions for use by C programmers, file formats, and system maintenance. (The numbering of these sections varies from system to system.) Don't forget the permuted index at the beginning; you can skim it quickly for commands that might be relevant to what you want to do. There is also an introduction to the system that gives an overview of how things work.  Often the manual is kept online so that you can read it on your terminal. If you get stuck on something, and can't find an expert to help, you can print any manual page on your terminal with the command man command name.

    Thus to read about the who command, type

                                     $ man who

    and, of course,

                                     $ man man

    tells about the man command.

    terminal doesn't have tabs, type the

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