UNIX

Session 1

UNIX OS

UNIX is an Operating System just like MS-DOS.

But MS-DOS is a single user OS where as UNIX is a Network OS.

UNIX is one of the oldest OS available on computers today.

UNIX is in use from 1970’s - making it one of the most reliable and trusted OS.

Today, UNIX has over 50% of Network OS market share.

NETWORK OS ROLESA network OS like UNIX has the following roles:

Sharing of resources. For e.g. a printer connected to server can be used by all network users.Account Management. User needs to login to work with a network.Security. Security is provided by User Login, File and Directory permissions.Network Services. Users can send and receive messages across terminals.Memory Management. All users processes run in Server sharing available memory.

UNIX OS ROLES

UNIX as a Network OS, has the following roles:

Peripheral Management : Manages all peripheral devices connected to network.

Memory Management : Allocates and monitors memory to client computers.

Account Management : Manages user accounts and passwords.

Network Management : Establishes connection between nodes and server and manages the connection.

Process Management : Monitors the processes that are running on the server.

FEATURES OF UNIX OSMULTI USER : The ability to maintain a networkMULTI TASKING : The ability to run many programs at the same timeBACKGROUND PROCESSING : The ability to send a process in Background there by running another process in ForegroundTIME SHARING : The processor time in Server is shared among all nodes.UTILITIES : UNIX comes with a lot of small utility programs.PORTABILITY : A program written in Unix OS can be run on any other OS without changing.

INSIDE UNIX

UNIX OS has the following structure:

SHELL

KERNEL

UTILITIES

UNIX STRUCTURE

KERNEL : It controls the Hardware. Eliminates the need for user to know the internals of Hardware.

SHELL : The interface between the user and Kernel. It accepts user commands and executes them. Acts like a Command Interpreter.

UTILITIES : These are small shell programs run by user.

Working with UNIX

In order to work with UNIX, one has to connect to UNIX Server.

This is done by using a Communication S/W such as Telnet.

Start Telnet by typing ‘Telnet’ at StartRun.

Select Connect Remote System.

Type the IP address of Unix.

In our lab, use 200.100.100.130

as the IP Address.

After some delay, login prompt

appears.

Logging on to UNIX Network

Once you get the Login screen, type your user name and press enter.

Type your password and press enter.

If you provide user name and password correct, the Shell prompt ‘$’ appears.

If User name or Password is incorrect, you must re-enter them.

To disconnect from UNIX, type ‘logout’ at ‘$’ prompt or Press Control+D.

Select ConnectExit to close Telnet.

UNIX FILE SYSTEM

Unlike DOS, UNIX has a Hierarchical File System.

User rights are established through File Access Permissions.

The following is the directory structure of UNIX:

bina ll program s

etcconfiguration in fo

devhardw are in fo

Y our d irectory another user

usruser's hom e

/root d irectory

USERS IN UNIX ENVIRONMENT

root : This user is the Super User in UNIX. ‘root’ is the System Administrator. Responsible for startup, shutdown, user account management, backup etc.

users : All other users come under this category. For each such user, a home directory exists under /usr directory with the login name.

group : Contains a collection of users with similar interests. Groups also can be assigned permissions.

guests : Any other users who don’t have a local account but want to access the system.

UNIX Commands

Unix OS treats directories also as files. Hence, commands can be divided into: file commands, directory commands and utilities.

File commands work on files. They allow you to do common tasks like copying files, deleting files etc.

Directory commands work on directories. Tasks such as creating, changing, removing directories etc. come under this category.

Utilities enhance productivity. UNIX OS comes with lot of utilities which help the user to carry out different tasks.

Usage of Simple commands

DATE command

The date command tells us the current date and time.

WHO command

abc tty0 Jun 4 11.17

xyz tty4 Jun 4 11.30

asd tty9 Jun 5 11.40

WHO AM I

you ttya Jun 4 12.20

$

Continued …

Finger command In larger systems, you may get a far longer list than the one used in our who command , and the identifies of some of the users may not be clear to you. To get more information about specific users, use the finger command.

Eg: finger gopi (gives information about the specific user I.e gopi) Eg: finger (of all)

Continued …

Write command

This command allows you to send messages to other users over the UNIX network.

Write command allows you to communicate with other users, by copying lines from your terminal to theirs.

A write command appeared in version 6 AT&T UNIX.

Continued …

Syntax:

Write <username>

Example:

write suresh

Note:

If user has not logged in you will get a message that the user has not logged in.

Continued …

TALK command This command is used to talk to another user This command is a visual communication program which copies lines from your terminal to that of another user. This command appeared in 4.2BSD (Berkeley Distribution) April 22, 1991(Before Use Talk Make message as y [mesg y press enter]).

Continued …

Syntax:talk <user name>

Example:talk scott

Note:If the user has not logged in a message gets displayed “your party is not logged on” or “connection not established”Press ctrl+alt+del to disconnect from talk command.

Continued …

MAIL command

This command is used to send and receive mail

To send a message to one or more people, mail can be invoked with arguments which are the names of the people to whom the mail will be sent.

Syntax:

mail <username>

Directory Commands - pwd

1. pwd (print working directory) : Since UNIX has a hierarchical file system, it would be helpful for the user to know what is his/her current working directory. ‘pwd’ command displays current working directory.

E.g.: After logging in, if you type

$ pwd <enter>

/home/<your login name>

$ cd / <enter>

$ pwd <enter>

/ (since we changed to ‘/’ root)

‘cd’ command2. This command does the following:

(a) without arguments, takes you to your HOME directory (cd)(b) changes to a given directory (cd <dir>)(c) takes you to parent of current directory (cd..)

E.g.: $ cd /etc <enter> (changes to /etc dir)$ pwd <enter>/etc$ cd .. <enter> (takes you to parent

dir)$ pwd <enter> (to know current dir)/$ cd <enter> (takes you to home

dir)

‘mkdir’ command

3. To create a directory, use ‘mkdir’ command. You should have ‘write’ permission to the parent directory to create a new directory.

E.g.:$ mkdir shell <enter> (create a dir - shell)$ cd shell <enter> (change to new dir)$ pwd <enter>/<current dir>/shell

$ mkdir /home/<your login>/c <enter>$ cd <enter> (go to your home)$ cd c <enter> (change to new dir)$ pwd <enter> (it would be ‘c’ dir)

‘rmdir’ command

4. To delete directories, provided they are empty, use this command.

E.g.:$ mkdir files <enter>$ cd files <enter>$ pwd <enter>/home/<your login>/files$ cd .. <enter> (change to parent dir)$ rmdir files <enter> (remove files dir)

‘ls’ command

5. To display a list of files in current directory, use ‘ls’ command. It is similar to MS-DOS ‘dir’.

E.g:

$ ls <enter>

… … … (displays sorted list of current dir)

The following options can be used with ‘ls’ command:

l long listing of files

r reverse listing i.e. z to a

R includes sub dirs also (same as dir/s)

x width wise displayContinued...

‘ls -l’ Command

The following is the output of ‘ls -l’ command:

total 3 (total number of blocks occupied)

-rw-r--r-- 1 user3 users 33 Jan 2 10.00 names

drw-r--r-- 2 user3 users88 Jan 12 03.45 testFile name

Date & timeSize

Group

User (Owner of file)No. of blocksFile Access permissions (First letter indicates type of file - ‘d’ for dir and ‘-’ for ordinary file. Next nine letters indicate File Access Permissions)

File commands - ‘cat’

1. ‘cat’ : displays contents of files (same as MS-DOS ‘type’ command).

E.g.:

$ cat <enter> (without arguments …)

takes input from keyboard until Control+d is pressed (same as MS-DOS ‘copy con’)

$ cat <file1> <file2> (more than one arg)

displays contents of <file1> and then contents of <file2>

The message ‘cat : can’t open <file>’ is displayed if given file doesn’t exist.

‘cp’ command

2. This command is useful to copy files.

Syntax: cp <source> <destination> - both source and destination must be given.

E.g.:

$ cp test names <enter>

the file ‘test’ is copied as ‘names’

$ cp test /home/ftp/backup <enter>

the file ‘test’ is copied into /home/ftp/backup directory

‘rm’ command3. To delete files, use ‘rm’ command.Syntax: rm <file>E.g.:

$ rm test <enter>deletes the file test under current

directory$ rm /home/<your login>/c/core <enter>

deletes the file ‘core’ under ‘c’ directory under your HOME directory (if file exists !)

‘mv’ command

4. To rename or to move files, use ‘mv’ command.

Syntax: to rename: mv <file> <new file name>

to move: mv <file> <directory>

E.g.:

$ mv names names1 <enter>

rename ‘names’ as ‘names1’

$ mv names1 /home <enter>

move ‘names1’ file to ‘/home’ directory

Head and Tail command

This command is used to display the initial part of a text file. This can be thought of as a complement command to the tail command which displays the last part of a text file. By default head command displays first 10 lines of a file and the tail command the last 10 lines.Syntax:

$ head [ -count] [filename …] Example:

$ head – 4 fruits (will display the first four lines of a file called fruits)

UNIX Wildcards

UNIX OS supports the following wildcards:

* - indicates all files in current directory or more than one char

? - indicates one char

[ ] - used for range of chars

\ - removes the wildcard meaning.

E.g: $ cat * <enter> (displays contents of all

files)

Continued …

$ cp a* /ftp <enter> (copy all files starting with ‘a’ to /ftp)

$ ls -l f?? <enter> (files starting with ‘f’ then two chars)

$ ls -l [ab]* <enter> (files starting with either ‘a’ or ‘b’)

$ rm [a-g]* <enter> (files starting with any letter between a to g)

$ cat ab\? <enter> (display contents of file ‘ab?’)

Getting help through ‘man’ command

UNIX OS displays online help using ‘man’ command.

Syntax: man <command>.

Use arrow keys to move forward and backward.

Type ‘q’ to exit from help.

Use ‘/<search>’ to search in displayed help.

Use ‘/’ to repeat search.

E.g.:

$ man cd <enter>

displays help about ‘cd’ command.

Changing password using ‘passwd’If you wish to change your password, use ‘passwd’ command.At ‘$’ prompt, type ‘passwd’ and press <enter>.UNIX will ask for your old password.Type your old password and press <enter>.If old password is typed correctly, UNIX will ask for new password. Type the new password and press <enter>.UNIX will ask you again to type new password for confirmation. Type it and press <enter>.The new password would come into effect from next time you login.Passwords are stored in ‘passwd’ file in /etc dir in encrypted form.

Rules for Passwords

The following are general rules for passwords (which may change from network to network due to the password policies in effect):

min length : 6 chars

max length : 14 chars

should contain : alphabets, digits and atleast one special character

can’t be : user name, dictionary word such as name of persons, places etc.

If a user forgets password, system administrator can assign a generic password and user can change it later.

FILE ACCESS PERMISSIONS

As you are already aware, UNIX offers security to files and directories through File Access Permissions (FAP).The permissions that can be given to files and directories are:

‘r’ read. User can copy, view files‘w’ write. User can edit, rename,delete files‘x’ execute. User can run the programme.

These permissions can be given to ‘User’, ‘Group’ and ‘Others’ by the creator of the file or system administrator (root).

Continued...

Giving FAP using ‘chmod’ command

To give FAP, use ‘chmod’ command.Syntax: chmod <permissions> <files>

Use ‘+’ to give a permission and ‘-’ to remove a permission.Use ‘u’ for user,‘g’ for group and ‘o’ for others

In the output of ‘ls -l’ command, from second letter, first three letters indicate FAP for user, next three for group and last three for others.You can use the following numerics to assign permission: r4, w2 and x1. For user, multiply these by 100, for group multiply by 10 and for others multiply by 1.

‘chmod’ command examples

FAP beforeFAP before chmod commandchmod command FAP afterFAP afterr w - r - - - - - chmod 644 <file>r w - r - - r - -r w - - - - - - - chmod 764 <file>r w x r w - r - -r w x r - - r - - chmod 600 <file>r w - - - - - - -r w - r - - r - - chmod g +w <file> r w - r w - r -

-r w - r w - - - -chmod ug +x <file> r w x r w x

--- r - - - - - - - - chmod +r <file> r - - r - - r - -r w - r - - r - - chmod 611 <file>r w - - - x - - x

Setting group using ‘chgrp’ command

To change the group of file(s), use ‘chgrp’ command.

Only the owner of file(s) or system administrator can change the group.

Syntax: chgrp <group> file(s)

E.g.:

$ chgrp students test <enter>

changes the group of file ‘test’ to ‘students’

To see whether the group has changed or not, use

command.

Redirection

Redirection changes the assignments for standard input and standard output.

Output Redirection

When output from a process is redirected to sources other than the standard output, it is called Output Redirection. The Shell command line format for redirecting output to a file is,

Syntax:

$ command > file

Continued …

Example:

$ date > today

$ cat today

Input Redirection

We can redirect the input for a command using input redirection.

Syntax:

Command < file

Here file is the source of the data for command.

Continued …

Example:

Cat < names

Siny

Siju

Simi

Suni

Redirecting both Standard Input & Output: