Linux Introduction

This tutorial covers the basics of using the Linux operating system from the command line. An operating system is the suite of programs that make everything else on a computer work. Linux is patterned after an earlier set of systems collectively referred to as UNIX, and so Linux will often be desribed as a UNIX system.

When you are sitting at the computer's console, the X Window System and programs built on top of it provide you with a graphical user interface. There are many things, however, for which you will need to use the text-only command interface. You can also easily use the command-line interface when you are connected remotely over a secure shell (SSH) session.

The UNIX operating system

The UNIX operating system is made up of three parts; the kernel, the shell and other 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 again presents its 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 CS lab have the shell bash by default.

bash has several features to help the user entering commands.

Filename Completion

By typing part of the name of a command, filename or directory and pressing the [Tab] key, the 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.

Command-line editing

As you would expect, you can use the [Delete] key to delete a character and back up. Less obviously, you can use the forward- and back-arrow keys to move around within a line; you can type other characters to insert them at the current position. A few other useful characters:

^A go to the beginning of the line
^E go to the end of the line
^U delete back to the beginning of the line
^K delete the rest of the line, going forward

Other programs

The number of other programs available is mind-boggling. But you can do lots of useful things efficiently if you learn the basics of a handful of standard programs, which is what the rest of this tutorial will teach you.

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. (On Mac OS and MS Windows, directories are usually presented as folders.

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. Each user has a personal directory called the home directory; on our systems the home directories are in a directory named home that is, in turn, found in the root directory.

A directory tree

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

Starting a terminal session

When you log in on the console, you'll see your desktop. If you type [Ctrl][Alt]T (that is, while holding down the keys marked Ctrl and Alt, hit T). That should produce a Terminal window that looks something like this:

A Terminal window at
the shell prompt

That shows you a shell prompt, waiting for you to type a command. You might notice that the prompt contains three pieces of information: a username, the name of the computer, and the current directory.

If the Terminal program was not already in your dock, you can add it so that it will be easier to launch it in the future: right click on the Terminal icon in the dock and select “Add to Favorites”.