Introduction to Unix Operating System

What is UNIX?

UNIX is an operating system which was first
developed in the 1960s, and has been under
constant development ever since. By operating
system, we mean the suite of programs which
make the computer work. It is a stable, multi-
user, multi-tasking system for servers,
desktops and laptops.
UNIX systems also have a graphical user
interface (GUI) similar to Microsoft Windows
which provides an easy to use environment.
However, knowledge of UNIX is required for
operations which aren't covered by a graphical
program, or for when there is no windows
interface available, for example, in a telnet
session.

Types of UNIX

There are many different versions of UNIX,
although they share common similarities. The
most popular varieties of UNIX are Sun Solaris,
GNU/Linux, and MacOS X.
Here in the School, we use Solaris on our
servers and workstations, and Fedora Linux on
the servers and desktop PCs.
The UNIX operating system
The UNIX operating system is made up of three
parts; the kernel, the shell and the programs.
The kernel
The kernel of UNIX is the hub of the operating
system: it allocates time and memory to
programs and handles the filestore and
communications in response to system calls.
As an illustration of the way that the shell and
the kernel work together, suppose a user types
rm myfile (which has the effect of removing
the file myfile ). The shell searches the filestore
for the file containing the program rm , and then
requests the kernel, through system calls, to
execute the program rm on myfile . When the
process rm myfile has finished running, the
shell then returns the UNIX prompt % to the
user, indicating that it is waiting for further
commands.

The shell

The shell acts as an interface between the user
and the kernel. When a user logs in, the login
program checks the username and password,
and then starts another program called the
shell. The shell is a command line interpreter
(CLI). It interprets the commands the user
types in and arranges for them to be carried
out. The commands are themselves programs:
when they terminate, the shell gives the user
another prompt (% on our systems).
The adept user can customise his/her own
shell, and users can use different shells on the
same machine. Staff and students in the school
have the tcsh shell by default.
The tcsh shell has certain features to help the
user inputting commands.
Filename Completion - By typing part of the
name of a command, filename or directory and
pressing the [Tab ] key, the tcsh shell will
complete the rest of the name automatically. If
the shell finds more than one name beginning
with those letters you have typed, it will beep,
prompting you to type a few more letters before
pressing the tab key again.
History - The shell keeps a list of the
commands you have typed in. If you need to
repeat a command, use the cursor keys to
scroll up and down the list or type history for a
list of previous commands.
Files and processes
Everything in UNIX is either a file or a process.
A process is an executing program identified by
a unique PID (process identifier).
A file is a collection of data. They are created
by users using text editors, running compilers
etc.

Examples of files:
a document (report, essay etc.)
the text of a program written in some high-level
programming language
instructions comprehensible directly to the
machine and incomprehensible to a casual
user, for example, a collection of binary digits
(an executable or binary file);
a directory, containing information about its
contents, which may be a mixture of other
directories (subdirectories) and ordinary files.
The Directory Structure
All the files are grouped together in the directory
structure. The file-system is arranged in a
hierarchical structure, like an inverted tree. The
top of the hierarchy is traditionally called root
(written as a slash / )
In the diagram above, we see that the home
directory of the undergraduate student
"ee51vn" contains two sub-directories ( docs
and pics ) and a file called report.doc .
The full path to the file report.doc is "/home/
its/ug1/ee51vn/report.doc"
Starting an UNIX terminal
To open an UNIX terminal window, click on the
"Terminal" icon from Applications/Accessories
menus.
An UNIX Terminal window will then appear with
a % prompt, waiting for you to start entering
commands.