Fundamental Concepts of E-Mail Exchanging on Linux System

Basics of Mail Exchanging

E-Mail:

You can send message from one computer to another computer using electronic-mail around the world. E-mail is more powerful and more usable tools in today’s world. This E-mail is transfer via into the computer and the computer network. There are many Server are used to exchange these e-mail among the computer to computer and the one network to different network. The mail server is responsible or used to exchange this mail that means a mail server receive or store an e-mail message from client and deliver it to the other client. In this tutorial you will show that how an e-mail message is transfer from sender to receiver and you will also know that what happen during the time of exchanging the mail.

How a Mail Server Works:

Before we begin to configure the Linux Mail Server we need to understand that how an E-Mail system is work. An E-mail system is build-up some element which is described below:

Mail User Agent (MUA)

It works in user machine which is directly run by a user. This is used to compose e-mail message and send it to mail server or receive the mail message from server. It Just an application, such as Outlook, Thunderbird, Pine (used in Linux).

Mail Transfer Agent (MTA)

Mail Transfer Agent is used to transfer message between the machines. After receiving the mail from MUA, Mail Transfer Agent starts its work. In Red Hat Linux the default Mail Transfer Agent is Sendmail or you can also use Postfix to more secure your mail server. In Unix the MTA is qmail. When an email is sent, the message is routed from server to server, all the way to the recipient's email server. After receiving the mail from MUA, MTA read the receiver address from the header part of the mail and find out the receiver server IP address, then MTA try to communicate with the 25 No Port of Server IPs of receiver . If the sender server MTA could establish the connection with receiver server MTA then sender server handover the mail of receiver server MTA using the Simple Mail Transfer Protocol (SMTP).

Mail Delivery Agent (MDA)

MDA/LDA is Mail Delivery Agent or Local Delivery Agent. Both are essentially synonymous. (Actually there are subtle differences between the two).  MDA receive the message from Mail Transfer Agent and lace into the user mailbox. In Red Hat Linux MDA/LDA is procmail.

Simple Mail Transfer Protocol (SMTP)

The SMTP is responsible for transferring the mail message from one MTA to another MTA. That means this is used to transfer mail between the computers in network. It can transfer only ASCII text. It can’t handle font, color, graphics, or attachment with messages. For this you can use MIME (Multi-purpose Internet Mail Extensions or Multimedia Internet Mail Extensions). It’s an encoding protocol like BinHex in Mac and UUEncode in UNIX. That support font, color, graphics, or attachment.  At first it was used as a way of sending more than just text via email. Later the protocol was extended to manage file typing by Web servers. MUAs and MTAs use this protocol for sending e-mails.

Post Office Protocol (POP3)

POP3 stands for a Post Office Protocol version 3. POP3 is a client/server protocol. The POP3 protocol is designed to allow the users to retrieve e-mail messages when they are connected to the email server (via Internet, Ethernet or VPN network connection). Once the email messages are downloaded from the server they can be modified, read and manipulated offline. MUAs can use this protocol to sen and receive e-mails from the server.

Internet Message Access Protocol (IMAP)

The IMAP (Internet Message Access Protocol) is a newer and modern alternative to the POP3 protocol. Unlike POP3, the IMAP allows the users to work with their messages in both online and offline modes. The IMAP-capable email client programs retrieve the messages' headers from the server and can store local copies of the messages in a local (temporary) cache. All the messages are left on the server until they are deleted by the user. This mechanism allows multiple email clients to access a single mailbox and is often used for corporate / business e-mails. MUAs can use this protocol to send and receive e-mails on the server.

Mail-Boxes (Inbox)

A mail-box is container or directory of files, where incoming messages are stored.

Server Configure

In this tutorial you will see that how to configure mail server in a sub domain DNS server named mail.mydomain.com who’s FQDN is ns3.mail.mydomain.com that I have already created. You can also configure the mail server under your main DNS domain that will describe later. So let’s start.

Package required for mail server:  Server Side Packages

Package Name	Description
Sendmail	Default MTA In Red Hat/Fedora Linux
m4	Known as a Macro Processor, used to create sendmail.cf file
Postfix	Another MTA In Linux, used to Exchange secure e-mail
Dovecot	A Package for  Accessing the mailbox
POP3	It’s a Protocol to retrieve the e-mail messages from server
IMAP	It’s an alternative Protocol of POP3
SMTP	Used to transfer e-mail message from one MTA to another MTA
Squirrelmail	A Web Mail Configuration Tool
Php	It’s a scripting language for web development to produce dynamic web pages
Mysql	The most popular Open Source SQL database management system
System-switch-mail	Graphical Tools for Mail Transport Agent Switcher

Client Side Software

Software	Description
Outlook Express	POP3- and IMAP-compatible mail client with a built-in newsreader.
Thunderbird	It’s a graphical email client and newsreader developed by the Mozilla Foundation.
Pine	It’s an application for sending and receiving email in Linux/Unix
Mozilla Firefox, Internet Explorer, Opera, Safari, Or Any Suitable Browser.	Will be used to access mailbox using webmail.

Required configuration file for sendmail:

Sendmail is the default Mail Transfer Agent (MTA) in FreeBSD. sendmail's job is to accept mail from Mail User Agents (MUA) and deliver it to the appropriate mailer as defined by its configuration file. sendmail can also accept network connections and deliver mail to local mailboxes or deliver it to another program.

                sendmail uses the following configuration files:

File Name                                                                                          Function

/etc/mail/access                                                                      sendmail access database file

/etc/mail/aliases                         Mailbox aliases

/etc/mail/local-host-names                Lists of hosts sendmail accepts mail for

/etc/mail/mailer.conf                     Mailer program configuration

/etc/mail/mailertable                     Mailer delivery table

/etc/mail/sendmail.cf                     sendmail master configuration file

/etc/mail/virtusertable                   Virtual users and domain tables

Unix - Environment

An important Unix concept is the environment, which is defined by environment variables. Some are set by the system, others by you, yet others by the shell, or any program that loads another program.
A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.
For example, first we set a variables TEST and then we access its value using echo command:

$TEST="Unix Programming"
$echo $TEST
Unix Programming

Note that environment variables are set without using $ sign but while accessing them we use $sign as prefix. These variables retain their values until we come out shell.
When you login to the system, the shell undergoes a phase called initialization to set up various environment. This is usually a two step process that involves the shell reading the following files −

• /etc/profile
• profile

The process is as follows −

• The shell checks to see whether the file /etc/profile exists.
• If it exists, the shell reads it. Otherwise, this file is skipped. No error message is displayed.
• The shell checks to see whether the file .profile exists in your home directory. Your home directory is the directory that you start out in after you log in.
• If it exists, the shell reads it; otherwise, the shell skips it. No error message is displayed.

As soon as both of these files have been read, the shell displays a prompt −

$

This is the prompt where you can enter commands in order to have them execute.
Note − The shell initialization process detailed here applies to all Bourne type shells, but some additional files are used by bash and ksh.

The .profile File

The file /etc/profile is maintained by the system administrator of your UNIX machine and contains shell initialization information required by all users on a system.
The file .profile is under your control. You can add as much shell customization information as you want to this file. The minimum set of information that you need to configure includes

• The type of terminal you are using
• A list of directories in which to locate commands
• A list of variables effecting look and feel of your terminal.

You can check your .profile available in your home directory. Open it using vi editor and check all the variables set for your environment.

Setting the Terminal Type

Usually the type of terminal you are using is automatically configured by either the login or getty programs. Sometimes, the autoconfiguration process guesses your terminal incorrectly.
If your terminal is set incorrectly, the output of commands might look strange, or you might not be able to interact with the shell properly.
To make sure that this is not the case, most users set their terminal to the lowest common denominator as follows −

$TERM=vt100
$

Setting the PATH

When you type any command on command prompt, the shell has to locate the command before it can be executed.
The PATH variable specifies the locations in which the shell should look for commands. Usually it is set as follows −

$PATH=/bin:/usr/bin
$

Here each of the individual entries separated by the colon character, :, are directories. If you request the shell to execute a command and it cannot find it in any of the directories given in the PATH variable, a message similar to the following appears −

$hello
hello: not found
$

There are variables like PS1 and PS2 which are discussed in the next section.

PS1 and PS2 Variables

The characters that the shell displays as your command prompt are stored in the variable PS1. You can change this variable to be anything you want. As soon as you change it, it'll be used by the shell from that point on.
For example, if you issued the command −

$PS1='=>'
=>
=>
=>

Your prompt would become =>. To set the value of PS1 so that it shows the working directory, issue the command −

=>PS1="[\u@\h \w]\$"
[root@ip-72-167-112-17 /var/www/tutorialspoint/unix]$
[root@ip-72-167-112-17 /var/www/tutorialspoint/unix]$

The result of this command is that the prompt displays the user's username, the machine's name (hostname), and the working directory.
There are quite a few escape sequences that can be used as value arguments for PS1; try to limit yourself to the most critical so that the prompt does not overwhelm you with information.

Escape Sequence	Description
\t	Current time, expressed as HH:MM:SS.
\d	Current date, expressed as Weekday Month Date
\n	Newline.
\s	Current shell environment.
\W	Working directory.
\w	Full path of the working directory.
\u	Current user.s username.
\h	Hostname of the current machine.
\#	Command number of the current command. Increases with each new command entered.
\$	If the effective UID is 0 (that is, if you are logged in as root), end the prompt with the # character; otherwise, use the $.

You can make the change yourself every time you log in, or you can have the change made automatically in PS1 by adding it to your .profile file.
When you issue a command that is incomplete, the shell will display a secondary prompt and wait for you to complete the command and hit Enter again.
The default secondary prompt is > (the greater than sign), but can be changed by re-defining the PS2 shell variable −
Following is the example which uses the default secondary prompt −

$ echo "this is a
> test"
this is a
test
$

Following is the example which re-define PS2 with a customized prompt −

$ PS2="secondary prompt->"
$ echo "this is a
secondary prompt->test"
this is a
test
$

Environment Variables

Following is the partial list of important environment variables. These variables would be set and accessed as mentioned above −

Variable	Description
DISPLAY	Contains the identifier for the display that X11 programs should use by default.
HOME	Indicates the home directory of the current user: the default argument for the cd built-in command.
IFS	Indicates the Internal Field Separator that is used by the parser for word splitting after expansion.
LANG	LANG expands to the default system locale; LC_ALL can be used to override this. For example, if its value is pt_BR, then the language is set to (Brazilian) Portuguese and the locale to Brazil.
LD_LIBRARY_PATH	On many Unix systems with a dynamic linker, contains a colon-separated list of directories that the dynamic linker should search for shared objects when building a process image after exec, before searching in any other directories.
PATH	Indicates search path for commands. It is a colon-separated list of directories in which the shell looks for commands.
PWD	Indicates the current working directory as set by the cd command.
RANDOM	Generates a random integer between 0 and 32,767 each time it is referenced.
SHLVL	Increments by one each time an instance of bash is started. This variable is useful for determining whether the built-in exit command ends the current session.
TERM	Refers to the display type
TZ	Refers to Time zone. It can take values like GMT, AST, etc.
UID	Expands to the numeric user ID of the current user, initialized at shell startup.

Following is the sample example showing few environment variables −

$ echo $HOME
/root
]$ echo $DISPLAY

$ echo $TERM
xterm
$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/home/amrood/bin:/usr/local/bin
$

 

Unix - File Permission / Access Modes

File ownership is an important component of UNIX that provides a secure method for storing files. Every file in UNIX has the following attributes −

• Owner permissions − The owner's permissions determine what actions the owner of the file can perform on the file.
• Group permissions − The group's permissions determine what actions a user, who is a member of the group that a file belongs to, can perform on the file.
• Other (world) permissions − The permissions for others indicate what action all other users can perform on the file.

The Permission Indicators

While using ls -l command it displays various information related to file permission as follows −

$ls -l /home/amrood
-rwxr-xr--  1 amrood   users 1024  Nov 2 00:10  myfile
drwxr-xr--- 1 amrood   users 1024  Nov 2 00:10  mydir

Here first column represents different access mode ie. permission associated with a file or directory.

The permissions are broken into groups of threes, and each position in the group denotes a specific permission, in this order: read (r), write (w), execute (x) −

• The first three characters (2-4) represent the permissions for the file's owner. For example -rwxr-xr-- represents that onwer has read (r), write (w) and execute (x) permission.
• The second group of three characters (5-7) consists of the permissions for the group to which the file belongs. For example -rwxr-xr-- represents that group has read (r) and execute (x) permission but no write permission.
• The last group of three characters (8-10) represents the permissions for everyone else. For example -rwxr-xr-- represents that other world has read (r) only permission.

File Access Modes

The permissions of a file are the first line of defense in the security of a Unix system. The basic building blocks of Unix permissions are the read, write, and execute permissions, which are described below −

1. Read

Grants the capability to read ie. view the contents of the file.

2. Write

Grants the capability to modify, or remove the content of the file.

3. Execute

User with execute permissions can run a file as a program.

Directory Access Modes

Directory access modes are listed and organized in the same manner as any other file. There are a few differences that need to be mentioned:

1. Read

Access to a directory means that the user can read the contents. The user can look at the filenames inside the directory.

2. Write

Access means that the user can add or delete files to the contents of the directory.

3. Execute

Executing a directory doesn't really make a lot of sense so think of this as a traverse permission.

A user must have execute access to the bin directory in order to execute ls or cd command.

Changing Permissions

To change file or directory permissions, you use the chmod (change mode) command. There are two ways to use chmod: symbolic mode and absolute mode.

Using chmod in Symbolic Mode

The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table.

Chmod operator	Description
+	Adds the designated permission(s) to a file or directory.
-	Removes the designated permission(s) from a file or directory.
=	Sets the designated permission(s).

Here's an example using testfile. Running ls -1 on testfile shows that the file's permissions are as follows −

$ls -l testfile
-rwxrwxr--  1 amrood   users 1024  Nov 2 00:10  testfile

Then each example chmod command from the preceding table is run on testfile, followed by ls -l so you can see the permission changes −

$chmod o+wx testfile
$ls -l testfile
-rwxrwxrwx  1 amrood   users 1024  Nov 2 00:10  testfile
$chmod u-x testfile
$ls -l testfile
-rw-rwxrwx  1 amrood   users 1024  Nov 2 00:10  testfile
$chmod g=rx testfile
$ls -l testfile
-rw-r-xrwx  1 amrood   users 1024  Nov 2 00:10  testfile

Here's how you could combine these commands on a single line:

$chmod o+wx,u-x,g=rx testfile
$ls -l testfile
-rw-r-xrwx  1 amrood   users 1024  Nov 2 00:10  testfile

Using chmod with Absolute Permissions

The second way to modify permissions with the chmod command is to use a number to specify each set of permissions for the file.

Each permission is assigned a value, as the following table shows, and the total of each set of permissions provides a number for that set.

Number	Octal Permission Representation	Ref
0	No permission	---
1	Execute permission	--x
2	Write permission	-w-
3	Execute and write permission: 1 (execute) + 2 (write) = 3	-wx
4	Read permission	r--
5	Read and execute permission: 4 (read) + 1 (execute) = 5	r-x
6	Read and write permission: 4 (read) + 2 (write) = 6	rw-
7	All permissions: 4 (read) + 2 (write) + 1 (execute) = 7	rwx

Here's an example using testfile. Running ls -1 on testfile shows that the file's permissions are as follows −

$ls -l testfile
-rwxrwxr--  1 amrood   users 1024  Nov 2 00:10  testfile

Then each example chmod command from the preceding table is run on testfile, followed by ls -l so you can see the permission changes −

$ chmod 755 testfile
$ls -l testfile
-rwxr-xr-x  1 amrood   users 1024  Nov 2 00:10  testfile
$chmod 743 testfile
$ls -l testfile
-rwxr---wx  1 amrood   users 1024  Nov 2 00:10  testfile
$chmod 043 testfile
$ls -l testfile
----r---wx  1 amrood   users 1024  Nov 2 00:10  testfile

Changing Owners and Groups

While creating an account on Unix, it assigns a owner ID and a group ID to each user. All the permissions mentioned above are also assigned based on Owner and Groups.

Two commands are available to change the owner and the group of files −

• chown − The chown command stands for "change owner" and is used to change the owner of a file.
• chgrp − The chgrp command stands for "change group" and is used to change the group of a file.

Changing Ownership

The chown command changes the ownership of a file. The basic syntax is as follows −

$ chown user filelist

The value of user can be either the name of a user on the system or the user id (uid) of a user on the system.

Following example −

$ chown amrood testfile
$

Changes the owner of the given file to the user amrood.

NOTE: The super user, root, has the unrestricted capability to change the ownership of a any file but normal users can change only the owner of files they own.

Changing Group Ownership

The chrgp command changes the group ownership of a file. The basic syntax is as follows −

$ chgrp group filelist

The value of group can be the name of a group on the system or the group ID (GID) of a group on the system.

Following example −

$ chgrp special testfile
$

Changes the group of the given file to special group.

SUID and SGID File Permission

Often when a command is executed, it will have to be executed with special privileges in order to accomplish its task.

As an example, when you change your password with the passwd command, your new password is stored in the file /etc/shadow.

As a regular user, you do not have read or write access to this file for security reasons, but when you change your password, you need to have write permission to this file. This means that the passwd program has to give you additional permissions so that you can write to the file /etc/shadow.

Additional permissions are given to programs via a mechanism known as the Set User ID ( SUID) and Set Group ID ( SGID) bits.

When you execute a program that has the SUID bit enabled, you inherit the permissions of that program's owner. Programs that do not have the SUID bit set are run with the permissions of the user who started the program.

This is true for SGID as well. Normally programs execute with your group permissions, but instead your group will be changed just for this program to the group owner of the program.

The SUID and SGID bits will appear as the letter "s" if the permission is available. The SUID "s" bit will be located in the permission bits where the owners execute permission would normally reside. For example, the command

$ ls -l /usr/bin/passwd
-r-sr-xr-x  1   root   bin  19031 Feb 7 13:47  /usr/bin/passwd*
$

Which shows that the SUID bit is set and that the command is owned by the root. A capital letter S in the execute position instead of a lowercase s indicates that the execute bit is not set.

If the sticky bit is enabled on the directory, files can only be removed if you are one of the following users −

• The owner of the sticky directory
• The owner of the file being removed
• The super user, root

To set the SUID and SGID bits for any directory try the following −

$ chmod ug+s dirname
$ ls -l
drwsr-sr-x 2 root root  4096 Jun 19 06:45 dirname
$

 

File Management in UNIX

All data in UNIX is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem.
When you work with UNIX, one way or another you spend most of your time working with files. This tutorial would teach you how to create and remove files, copy and rename them, create links to them etc.
In UNIX there are three basic types of files −

• Ordinary Files − An ordinary file is a file on the system that contains data, text, or program instructions. In this tutorial, you look at working with ordinary files.
• Directories − Directories store both special and ordinary files. For users familiar with Windows or Mac OS, UNIX directories are equivalent to folders.
• Special Files − Some special files provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Other special files are similar to aliases or shortcuts and enable you to access a single file using different names.

Listing Files

To list the files and directories stored in the current directory. Use the following command −

$ls

Here is the sample output of the above command −

$ls

bin        hosts  lib     res.03
ch07       hw1    pub     test_results
ch07.bak   hw2    res.01  users
docs       hw3    res.02  work

The command ls supports the -l option which would help you to get more information about the listed files −

$ls -l
total 1962188

drwxrwxr-x  2 amrood amrood      4096 Dec 25 09:59 uml
-rw-rw-r--  1 amrood amrood      5341 Dec 25 08:38 uml.jpg
drwxr-xr-x  2 amrood amrood      4096 Feb 15  2006 univ
drwxr-xr-x  2 root   root        4096 Dec  9  2007 urlspedia
-rw-r--r--  1 root   root      276480 Dec  9  2007 urlspedia.tar
drwxr-xr-x  8 root   root        4096 Nov 25  2007 usr
drwxr-xr-x  2    200    300      4096 Nov 25  2007 webthumb-1.01
-rwxr-xr-x  1 root   root        3192 Nov 25  2007 webthumb.php
-rw-rw-r--  1 amrood amrood     20480 Nov 25  2007 webthumb.tar
-rw-rw-r--  1 amrood amrood      5654 Aug  9  2007 yourfile.mid
-rw-rw-r--  1 amrood amrood    166255 Aug  9  2007 yourfile.swf
drwxr-xr-x 11 amrood amrood      4096 May 29  2007 zlib-1.2.3
$

Here is the information about all the listed columns −

• First Column: represents file type and permission given on the file. Below is the description of all type of files.
• Second Column: represents the number of memory blocks taken by the file or directory.
• Third Column: represents owner of the file. This is the Unix user who created this file.
• Fourth Column: represents group of the owner. Every Unix user would have an associated group.
• Fifth Column: represents file size in bytes.
• Sixth Column: represents date and time when this file was created or modified last time.
• Seventh Column: represents file or directory name.

In the ls -l listing example, every file line began with a d, -, or l. These characters indicate the type of file that's listed.

Prefix	Description
-	Regular file, such as an ASCII text file, binary executable, or hard link.
b	Block special file. Block input/output device file such as a physical hard drive.
c	Character special file. Raw input/output device file such as a physical hard drive
d	Directory file that contains a listing of other files and directories.
l	Symbolic link file. Links on any regular file.
p	Named pipe. A mechanism for interprocess communications
s	Socket used for interprocess communication.

Meta Characters

Meta characters have special meaning in Unix. For example * and ? are metacharacters. We use * to match 0 or more characters, a question mark ? matches with single character.
For Example −

$ls ch*.doc

Displays all the files whose name start with ch and ends with .doc −

ch01-1.doc   ch010.doc  ch02.doc    ch03-2.doc 
ch04-1.doc   ch040.doc  ch05.doc    ch06-2.doc
ch01-2.doc ch02-1.doc c

Here * works as meta character which matches with any character. If you want to display all the files ending with just .doc then you can use following command −

$ls *.doc

Hidden Files

An invisible file is one whose first character is the dot or period character (.). UNIX programs (including the shell) use most of these files to store configuration information.
Some common examples of hidden files include the files −

• .profile − the Bourne shell ( sh) initialization script
• .kshrc − the Korn shell ( ksh) initialization script
• .cshrc − the C shell ( csh) initialization script
• .rhosts − the remote shell configuration file

To list invisible files, specify the -a option to ls −

$ ls -a

.         .profile       docs     lib     test_results
..        .rhosts        hosts    pub     users
.emacs    bin            hw1      res.01  work
.exrc     ch07           hw2      res.02
.kshrc    ch07.bak       hw3      res.03
$

• Single dot . − This represents current directory.
• Double dot .. − This represents parent directory.

Note: I have put stars (*) just to show you the location where you would need to enter the current and new passwords otherwise at your system, it would not show you any character when you would type.

Creating Files

You can use vi editor to create ordinary files on any Unix system. You simply need to give following command −

$ vi filename

Above command would open a file with the given filename. You would need to press key i to come into edit mode. Once you are in edit mode you can start writing your content in the file as below −

This is unix file....I created it for the first time.....
I'm going to save this content in this file.

Once you are done, do the following steps −

• Press key esc to come out of edit mode.
• Press two keys Shift + ZZ together to come out of the file completely.

Now you would have a file created with filemame in the current directory.

$ vi filename
$

Editing Files

You can edit an existing file using vi editor. We would cover this in detail in a separate tutorial. But in short, you can open existing file as follows −

$ vi filename

Once file is opened, you can come in edit mode by pressing key i and then you can edit file as you like. If you want to move here and there inside a file then first you need to come out of edit mode by pressing key esc and then you can use following keys to move inside a file −

• l key to move to the right side.
• h key to move to the left side.
• k key to move up side in the file.
• j key to move down side in the file.

So using above keys you can position your cursor where ever you want to edit. Once you are positioned then you can use i key to come in edit mode. Edit the file, once you are done press esc and finally two keys Shift + ZZ together to come out of the file completely.

Display Content of a File

You can use cat command to see the content of a file. Following is the simple example to see the content of above created file −

$ cat filename
This is unix file....I created it for the first time.....
I'm going to save this content in this file.
$

You can display line numbers by using -b option along with cat command as follows −

$ cat -b filename
1   This is unix file....I created it for the first time.....
2   I'm going to save this content in this file.
$

Counting Words in a File

You can use the wc command to get a count of the total number of lines, words, and characters contained in a file. Following is the simple example to see the information about above created file −

$ wc filename
2  19 103 filename
$

Here is the detail of all the four columns −

• First Column: represents total number of lines in the file.
• Second Column: represents total number of words in the file.
• Third Column: represents total number of bytes in the file. This is actual size of the file.
• Fourth Column: represents file name.

You can give multiple files at a time to get the information about those file. Here is simple syntax −

$ wc filename1 filename2 filename3

Copying Files:

To make a copy of a file use the cp command. The basic syntax of the command is −

$ cp source_file destination_file

Following is the example to create a copy of existing file filename.

$ cp filename copyfile
$

Now you would find one more file copyfile in your current directory. This file would be exactly same as original file filename.

Renaming Files

To change the name of a file use the mv command. Its basic syntax is −

$ mv old_file new_file

Following is the example which would rename existing file filename to newfile:

$ mv filename newfile
$

The mv command would move existing file completely into new file. So in this case you would fine only newfile in your current directory.

Deleting Files

To delete an existing file use the rm command. Its basic syntax is −

$ rm filename

Caution: It may be dangerous to delete a file because it may contain useful information. So be careful while using this command. It is recommended to use -i option along with rm command.
Following is the example which would completely remove existing file filename:

$ rm filename
$

You can remove multiple files at a tile as follows −

$ rm filename1 filename2 filename3
$

Standard Unix Streams

Under normal circumstances every Unix program has three streams (files) opened for it when it starts up −

• stdin − This is referred to as standard input and associated file descriptor is 0. This is also represented as STDIN. Unix program would read default input from STDIN.
• stdout − This is referred to as standard output and associated file descriptor is 1. This is also represented as STDOUT. Unix program would write default output at STDOUT
• stderr − This is referred to as standard error and associated file descriptor is 2. This is also represented as STDERR. Unix program would write all the error message at STDERR.

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, HP Unix and BSD are few examples. Linux is also a flavor 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. These directories are organized into a tree-like structure called the filesystem.

System Bootup

If you have a computer which has UNIX operating system installed on it, then you simply need to turn on its power to make it live.

As soon as you turn on the power, system starts booting up and finally it prompts you to log into the system, which is an activity to log into the system and use it for your day to day activities.

Login Unix

When you first connect to a UNIX system, you usually see a prompt such as the following −

login:

To log in

• Have your userid (user identification) and password ready. Contact your system administrator if you don't have these yet.
• Type your userid at the login prompt, then press ENTER. Your userid is case-sensitive, so be sure you type it exactly as your system administrator instructed.
• Type your password at the password prompt, then press ENTER. Your password is also case-sensitive.
• If you provided correct userid and password then you would be allowed to enter into the system. Read the information and messages that come up on the screen something as below.

login : amrood
amrood's password:
Last login: Sun Jun 14 09:32:32 2009 from 62.61.164.73
$

You would be provided with a command prompt ( sometime called $ prompt ) where you would type your all the commands. For example to check calendar you need to type cal command as follows −

$ cal
     June 2009
Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6
 7  8  9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30

$

Change Password

All Unix systems require passwords to help ensure that your files and data remain your own and that the system itself is secure from hackers and crackers. Here are the steps to change your password −

• To start, type passwd at command prompt as shown below.
• Enter your old password the one you're currently using.
• Type in your new password. Always keep your password complex enough so that no body can guess it. But make sure, you remember it.
• You would need to verify the password by typing it again.

$ passwd
Changing password for amrood
(current) Unix password:******
New UNIX password:*******
Retype new UNIX password:*******
passwd: all authentication tokens updated  successfully

$

Note − I have put stars (*) just to show you the location where you would need to enter the current and new passwords otherwise at your system, it would not show you any character when you would type.

Listing Directories and Files

All data in UNIX is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem.

You can use ls command to list out all the files or directories available in a directory. Following is the example of using ls command with -l option.

$ ls -l
total 19621
drwxrwxr-x  2 amrood amrood      4096 Dec 25 09:59 uml
-rw-rw-r--  1 amrood amrood      5341 Dec 25 08:38 uml.jpg
drwxr-xr-x  2 amrood amrood      4096 Feb 15  2006 univ
drwxr-xr-x  2 root   root        4096 Dec  9  2007 urlspedia
-rw-r--r--  1 root   root      276480 Dec  9  2007 urlspedia.tar
drwxr-xr-x  8 root   root        4096 Nov 25  2007 usr
-rwxr-xr-x  1 root   root        3192 Nov 25  2007 webthumb.php
-rw-rw-r--  1 amrood amrood     20480 Nov 25  2007 webthumb.tar
-rw-rw-r--  1 amrood amrood      5654 Aug  9  2007 yourfile.mid
-rw-rw-r--  1 amrood amrood    166255 Aug  9  2007 yourfile.swf

$

Here enteries starting with d..... represent directories. For example uml, univ and urlspedia are directories and rest of the enteries are files.

Who Are You?

While you're logged in to the system, you might be willing to know : Who am I?

The easiest way to find out "who you are" is to enter the whoami command −

$ whoami
 amrood

$

Try it on your system. This command lists the account name associated with the current login. You can try who am i command as well to get information about yourself.

Who is Logged In?

Sometime you might be interested to know who is logged in to the computer at the same time.

There are three commands are available to get you this information, based on how much you'd like to learn about the other users: users, who, and w.

$ users
 amrood bablu qadir

$ who
amrood ttyp0 Oct 8 14:10 (limbo)
bablu  ttyp2 Oct 4 09:08 (calliope)
qadir  ttyp4 Oct 8 12:09 (dent)

$

Try w command on your system to check the output. This would list down few more information associated with the users logged in the system.

Logging Out

When you finish your session, you need to log out of the system to ensure that nobody else accesses your files while masquerading as you.

To log out

• Just type logout command at command prompt, and the system will clean up everything and break the connection

System Shutdown

The most consistent way to shut down a Unix system properly via the command line is to use one of the following commands −

Command	Description
halt	Brings the system down immediately.
init 0	Powers off the system using predefined scripts to synchronize and clean up the system prior to shutdown
init 6	Reboots the system by shutting it down completely and then bringing it completely back up
poweroff	Shuts down the system by powering off.
reboot	Reboots the system.
shutdown	Shuts down the system.

You typically need to be the superuser or root (the most privileged account on a Unix system) to shut down the system, but on some standalone or personally owned Unix boxes, an administrative user and sometimes regular users can do so.