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Part 2. Accessing a Unix System

There are many ways that you can access a Unix system. If you want the fullest possible access to the computer's commands and utilities, you must initiate a login session. The main mode of initiating a login session to a Unix machine is through a terminal, which usually includes a keyboard, and a video monitor.

When a terminal establishes a connection to the Unix system, the Unix kernel runs a process called a
tty to accept input from the terminal, and send output to the terminal. When the tty process is created,
it must be told the capabilities of the terminal, so it can correctly read from, and write to, the
terminal. If the tty process receives incorrect information about the terminal type, unexpected results
can occur.

Console

Every Unix system has a main console that is connected directly to the machine. The console is a special type of terminal that is recognized when the system is started. Some Unix system operations must be performed at the console. Typically, the console is only accessible by the system operators, and administrators.

Dumb terminals

Some terminals are referred to as "dumb" terminals because they have only the minimum amount of power required to send characters as input to the Unix system, and receive characters as output from the Unix system. Personal computers are often used to emulate dumb terminals, so that they can be connected to a Unix system. Dumb terminals can be connected directly to a Unix machine, or may be connected remotely, through a modem, a terminal server, or other network connection.

Smart terminals

Smart terminals, like the X terminal, can interact with the Unix system at a higher level. Smart
terminals have enough on-board memory and processing power to support graphical interfaces. The interaction between a smart terminal and a Unix system can go beyond simple characters to include icons, windows, menus, and mouse actions.

Network-based access modes

Unix computers were designed early in their history to be network-aware. The fact that Unix
computers were prevalent in academic and research environments led to their broad use in the
implementation of the Department of Defense's Advanced Research Projects Administration
(DARPA) computer network. The DARPA network laid the foundations for the Internet.

FTP
The FTP (File Transfer Protocol) provides a simple means of transferring files to and from a
Unix computer. FTP access to a Unix machine may be authenticated by means of a username and password pair, or may be anonymous. An FTP session provides the user with a limited set of commands with which to manipulate and transfer files.


Telnet
Telnet is a means by which one can initiate a Unix shell login across the Internet. The normal
login procedure takes place when the telnet session is initiated.


HTTP
The HTTP protocol has become important in recent years, because it is the primary way in
which the documents that constitute the World Wide Web are served. HTTP servers are most often publicly accessible. In some cases, access to documents provided by HTTP servers will require some
form of authentication.


HTTPS
A variation of HTTP that is likely to become increasingly important in the future. The "S" stands for "secure." When communications are initiated via the HTTPS protocol, the sender and recipient use an encryption scheme for the information to be exchanged. When the sending computer transmits the message, the information is encrypted so that outside parties cannot examine it. Once the message is received by the destination machine, decryption restores the original information.

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