UNIX Introduction

This session concerns UNIX, which is a common operating system. By operating system, we mean the suite of programs which make the computer work. UNIX is used by most of the servers within the department and the university.

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).

A user can select any of a number of different shells installed on the system (e.g., you may use sh, bash, csh, tcsh, etc. on the CS machines), and the adept user can customise his/her own shell. Most of the instructions in this tutorial work with bash and tcsh, but might require slight modification for the other shells due to differences in syntax.

The tcsh and bash shells have certain features to help the user inputting commands: (i) 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. (ii) 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:

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.

The Unix file structure

In the diagram above, we see that the directory ee51ab contains the subdirectory unixstuff and a file proj.txt

Starting a login session

To start a login session, you should remotely login onto one of the CS Dept. servers from your personal computer. If your personal computer is running linux or Mac OS X, please open a terminal window on your computer, and then use the "ssh" command to login. If your computer is running Windows, please use SecureCRT (or equivalent) software installed on your computer. In all cases, we recommend you use the following server:   login.cs.unc.edu.


Attribution: This tutorial was originally designed and written by M.Stonebank@surrey.ac.uk, 9 October 2000; modified by Montek Singh, September 1, 2010.

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This tutorial is licensed under a Creative Commons License.