UNIX Introduction

What is UNIX?

UNIX® License PlateUNIX 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 ssh session.

Types of UNIX

The Linux PenguinThere are many different versions of UNIX, although they share common similarities. The most popular varieties of UNIX are 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.

The adept user can customize his/her own shell, and users can use different shells on the same machine. Users may have the tcsh shell or bash shell by default.

These shells have certain features to help the user inputting 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:

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 / )

Unix File Structure

In the diagram above, we see that the directory "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 a terminal (from inside Linux)

To open a terminal window, click on the "Terminal" icon from the drop-down menus.

Gnome Menus


An UNIX Terminal window will then appear with a % prompt, waiting for you to start entering commands.

Unix Terminal window

Starting a terminal (from inside Windows)

To use UNIX from a Microsoft Windows computer requires additional effort. The two main approaches include

Both of these approaches require you to install software on your Windows computer. Further information on both of the above approaches may be found here.

M. Stonebank, 9 October 2000,     D.R. Reynolds, 20 January 2009