What is UNIX?
 UNIX
is an operating system which was first developed in the 1960s, and has been
under constant development ever since.
A D V E R T I S E M E N T
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 the
drop-down menus.
An UNIX Terminal window will then appear with a % prompt, waiting for you to
start entering commands.
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