Chap 2 Basics of UNIX

8/3/2019 Chap 2 Basics of UNIX

1/27BASICS OF UNIX -FOILS ON THE UNIX OS & SHELL PROG. 14/ 399

BASICS OF UNIX

STARTING TO USE UNIX

All UNIX systems are administered andmaintained by a SYSTEM MANAGER or a

SYSTEM ADMINISTRATOR(SA).

USER ACCOUNT

Every user is given a password- a secret

code he must type in every time heuses the system

A unique identity which identifies him tothe system - called USERID

Once a user is given a userid and apassword in the system, he has anACCOUNT on that system

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User Account contd...

Account keeps track of most activitiesperformed by the user

An Account has certain predefinedlimits also, like how much diskstorage space is allowed for files

USERID

Normally chosen by SystemAdministrator

Used by UNIX to identify user

For example, any data files created byuser A will be owned by his UserID

NOT secret

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THE PASSWORD

IS secret

Encrypted by the system, nobody canread it

If forgotten, only SA can give out afresh password

LOGGING IN AND OUT

Terminal needs correct connections tocomputer running UNIX

User is online when connected

When connection is lost, he goesoffline

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Logging In And Out cond...

Terminal may be

- local which is directly cabled (hard-wired) to the computer

- remotely connected via modemsand telephone lines

LOGGING IN

The process user goes through to starthis work is called logging In

Consists of typing UserID and the

password

When the UNIX terminal is ready touse, it displays :

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login : _

Logging In contd...

User types in his UserID, and pressesReturn

login : Amit

Backspace or Delete key allowed insome systems to correct errors

On others, press the Return key any-way and wait for message whichinforms login was incorrect, and tryagain

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Logging In contd...

Once the UserID has been entered,UNIX prompts for password:

Password : _

Type password and press Return

As password is typed, UNIX does notecho it on screen

This helps in security and the feature is

known as echo suppression

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THE SHELL PROMPT

After successful login, a prompt isdisplayed

User gives series of commands to do

his work

Shell reads and interprets commands

Each shell displays a different prompt:

Bourne Shell $ (default, authorStephen Bourne)

Korn Shell $

C Shell % (bsd UNIX support)

Superuser # (SystemAdministrator)

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LOGGING OUT

User can terminate session by pressingCtrl + D

Sends end-of-file signal to UNIX

Tells the shell no more data coming

Shell terminates

UNIX logs the user out

Screen returns to the login prompt

login : _

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NOTES

Each command in UNIX needs to beterminated by a Return key-press toexecute it

UNIX DOES distinguish between upperand lower case letters

If a UserID is given as amit, typing Amitat the login prompt will result in an

incorrect login attempt

UNIX, by default, uses lowercase

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SHELL COMMANDS

UNIX can be simple to complex,common to very, very rare

A command is brief set of letters,

followed by options or arguments

General command format:

command options expression filename

Arguments

Options modify behavior of command

Marked with leading hyphen

Example:

$ ls -a10



Shell Commands contd...

Expression is group of charactersused as input to command

Filename is name of file whichcommand refers to in some way

All arguments are not mandatory inevery command

Checking Date

System date in UNIX checked throughthe date command

UNIX uses 24 hour clock and givesthe time upto the second

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$ dateThu Jan 08 14:07:43 IST 1998$ _

Output

Checking Date contd...

Who Is There?

The Who command displays:

- A list of all active users in system

- Terminals they are logged into

- Date and time they last logged on

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$ whostheer tty01 Jan 08 14:01anasua tty02 Jan 08 14:50indira tty04 Jan 08 15:06amit tty09 Jan 08 15:10

User Names

Terminal

Month of login

Date of login

Time of login

The Who Command contd...

Argument can be added to the whocommand

am i - If user shares his terminal withother users, and previous user has not

logged out, who am i used to check whohas abandoned the terminal

$ who am istheer tty01 Jan 08 14:01

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MULTIPLE COMMANDS

UNIX allows two or more commands tobe entered on the same line

Put a semi-colon between each

command, then press Enter after thefinal command

$ date; whoThu Jan 08 15:12:43 IST 1998

stheer tty01 Jan 08 14:01anasua tty02 Jan 08 14:50indira tty04 Jan 08 15:06amit tty09 Jan 08 15:10

$ _

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Non Existent Command !!

When non-existent command isentered

- UNIX gives a message saying the

command is not found

- It then returns to the shell prompt

$ what is the datewhat is the date: not found

$ who aam I

who aam I : not found

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Display On The Screen

echo command displays a string onthe screen

$ echo this is fun!!this is fun!!$_

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Changing The Password

When a users UNIX account is set up,the SA assigns a UserID and apassword

Password can be later changed byuser by passwd command

UNIX asks for old password first (why?)

Then passwd prompts for newpassword

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SETTING PASSWORDS

Some systems need all passwords tomeet certain specifications, like thelength of the password should be aminimum of 8 characters

If new password does not meetrequired criteria, you are so informedand asked to enter a new choice

Finally, passwd asks you to reenter thenew password to ensure match with firstpassword

No password, either old or new, is

displayed on the screen as they aretyped

The passwords are stored in a filecalled the /etc/password file

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Setting Passwords contd...

SCO UNIX and some other UNIXsystems offer automatic generation ofpronounceable but obscure passwordslike klibrufar !!

SA can delete relevant entry frompassword file to allow user to enter hislogin if he forgets his password

$ passwd

Changing password for stheer

Old password : mineonly

New password : annieta1

R-enter new password : annieta1

$ _ NotEchoed O

Screen

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LOGIN PROFILES

SA provides global actions, or defaultsettings that affect all users.

They are placed in a file called the

global login profile

Shell executes commands in this filewhenever a user logs in

User can modify some or all of thesedefaults either temporarily orpermanently for all sessions

He runs commands of his choose to

modify his environment during a givensession

These changes last till he logs out

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Login Profiles contd...

Or, he can place these modifications inhis home directory (covered in the nextsession) in his personal login profile

This gives the user a permanent startupenvironment each time he logs in

Having a global profile for everyone,followed by personalized profiles for

each user gives a lot of flexibility

Similar to AUTOEXEC.BAT files in DOS,it is run automatically when user logs in

Bourne shell looks for a file called profileunder directory etc

If found, all commands in it are executed

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Login Profiles contd...

Next, it looks for a file called .profile(the users personal login profile) inthe users home directory

If found, it executes it

Then shell prompt appears on screen

User 1 User 2

Global Login Profile

User Login

Profile For User 2User 3 - has

NO user

login profile

executed every time

user1/2/3 logs on

User Login

Profile For User 1

executedwhen user1logs in

executedwhen user2

logs in22



SHELL VARIABLES

Shell variables are like variables used inconventional programming languages

Shell variables allow user to provide a

name or variable identifier for someentity (example, PATH or doc)

Variable names with capital letters areusually reserved for standard variables

with pre-determined UNIX meanings

Contents of all shell variables can beseen with the set command

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Shell Variables contd...

$ setHOME = /usr/stheer

IFS=

MAIL=/usr/mail/stheer

PATH=:/bin:/usr/bin

PS1=$

PS2=>

TERM=ansi$

bin : binary object filesTERM :terminal type,

normally ansi or vt100

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$ PS1=Enter your command :Enter your command : _

The promptchanges toEnter your command :

THE PRIMARY PROMPT

PS1 stores the primary prompt symbol($, by default)

Setting PS1 to any character string will

change the primary prompt

No spaces are allowed before or afterthe assignment operator =

The single forward quotes when settingPS1 ensures that spaces within thenew prompt string are transferred to PS1

Any variable name, prefixed with a $displays content of variable rather than

the variable name itself25



The Primary Prompt contd...

Enter your command:echo PS1PS1

Enter your command : echo $PS1Enter your command : Content Of Primary Prompt

PS1 is displayed

THE SECONDARY PROMPT (PS2)

Used when UNIX thinks you havestarted a new line without finishing acommand

New lines can also be included in a

quoted string

After pressing Enter at end of first line,shell displays its secondary promptand waits for more input

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The Secondary Prompt cond...

After final quote and newline, shellunderstands command is complete,and executes it

By default, PS2 stores the character >

Example:

Enter your command : PS1=$

$ echo Friends, Romans, Countrymen > lend me your ears !!

Friends, Romans, Countrymenlend me your ears !!$ echo $PS2>$ PS2=More Input :

$ echo $PS2More Input :$ echo Friends, Romans, Countrymen

More Input : lend me your ears !!

The secondary prompt, >by default, waiting for the

final quotes

Changing the secondaryprompt to More Input

:.

The secondary prompt,now changed to MoreInput:, waiting for the

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