Introduction to UNIX: 
Lecture One

1.1 Objectives
This lecture covers:
1.2 What is an Operating System?
An operating system (OS) is a resource manager. It takes the form of a set of software routines that allow users and application programs to access system resources (e.g. the CPU, memory, disks, modems, printers network cards etc.) in a safe, efficient and abstract way.

For example, an OS ensures safe access to a printer by allowing only one application program to send data directly to the printer at any one time. An OS encourages efficient use of the CPU by suspending programs that are waiting for I/O operations to complete to make way for programs that can use the CPU more productively. An OS also provides convenient abstractions (such as files rather than disk locations) which isolate application programmers and users from the details of the underlying hardware.

Fig. 1.1:  General operating system architecture

Fig. 1.1 presents the architecture of a typical operating system and shows how an OS succeeds in presenting users and application programs with a uniform interface without regard to the details of the underlying hardware. We see that:

Operating systems (and different flavours of the same operating system) can be distinguished from one another by the system calls, system utilities and user interface they provide, as well as by the resource scheduling policies implemented by the kernel.
1.3 A Brief History of UNIX
UNIX has been a popular OS for more than two decades because of its multi-user, multi-tasking environment, stability, portability and powerful networking capabilities. What follows here is a simplified history of how UNIX has developed (to get an idea for how complicated things really are, see the web site

Fig. 1.2: Simplified UNIX FamilyTree

In the late 1960s, researchers from General Electric, MIT and Bell Labs launched a joint project to develop an ambitious multi-user, multi-tasking OS for mainframe computers known as MULTICS (Multiplexed Information and Computing System). MULTICS failed (for some MULTICS enthusiasts "failed" is perhaps too strong a word to use here), but it did inspire Ken Thompson, who was a researcher at Bell Labs, to have a go at writing a simpler operating system himself. He wrote a simpler version of MULTICS on a PDP7 in assembler and called his attempt UNICS (Uniplexed Information and Computing System). Because memory and CPU power were at a premium in those days, UNICS (eventually shortened to UNIX) used short commands to minimize the space needed to store them and the time needed to decode them - hence the tradition of short UNIX commands we use today, e.g. ls, cp, rm, mv etc.

Ken Thompson then teamed up with Dennis Ritchie, the author of the first C compiler in 1973. They rewrote the UNIX kernel in C - this was a big step forwards in terms of the system's portability - and released the Fifth Edition of UNIX to universities in 1974. The Seventh Edition, released in 1978, marked a split in UNIX development into two main branches: SYSV (System 5) and BSD (Berkeley Software Distribution). BSD arose from the University of California at Berkeley where Ken Thompson spent a sabbatical year. Its development was continued by students at Berkeley and other research institutions. SYSV was developed by AT&T and other commercial companies. UNIX flavours based on SYSV have traditionally been more conservative, but better supported than BSD-based flavours.

The latest incarnations of SYSV (SVR4 or System 5 Release 4) and BSD Unix are actually very similar. Some minor differences are to be found in  file system structure, system utility names and options and system call libraries as shown in Fig 1.3.

  Feature           Typical SYSV            Typical BSD
  kernel name       /unix                   /vmunix
  boot init         /etc/rc.d directories   /etc/rc.* files
  mounted FS        /etc/mnttab             /etc/mtab
  default shell     sh, ksh                 csh, tcsh
  FS block size     512 bytes->2K           4K->8K
  print subsystem   lp, lpstat, cancel      lpr, lpq, lprm
  echo command      echo "\c"               echo -n
   (no new line)
  ps command        ps -fae                 ps -aux
  multiple wait     poll                    select
  memory access     memset, memcpy          bzero, bcopy
Fig. 1.3: Differences between SYSV and BSD

Linux is a free open source UNIX OS for PCs that was originally developed in 1991 by Linus Torvalds, a Finnish undergraduate student. Linux is neither pure SYSV or pure BSD. Instead, incorporates some features from each (e.g. SYSV-style startup files but BSD-style file system layout) and aims to conform with a set of IEEE standards called POSIX (Portable Operating System Interface). To maximise code portability, it typically supports SYSV, BSD and POSIX system calls (e.g. poll, select, memset, memcpy, bzero and bcopy are all supported).

The open source nature of Linux means that the source code for the Linux kernel is freely available so that anyone can add features and correct deficiencies. This approach has been very successful and what started as one person's project has now turned into a collaboration of hundreds of volunteer developers from around the globe. The open source approach has not just successfully been applied to kernel code, but also to application programs for Linux (see e.g.

As Linux has become more popular, several different development streams or distributions have emerged, e.g. Redhat, Slackware, Mandrake, Debian, and Caldera. A distribution comprises a prepackaged kernel, system utilities, GUI interfaces and application programs.

Redhat is the most popular distribution because it has been ported to a large number of hardware platforms (including Intel, Alpha, and SPARC), it is easy to use and install and it comes with a comprehensive set of utilities and applications including the X Windows graphics system, GNOME and KDE GUI environments, and the StarOffice suite (an open source MS-Office clone for Linux).

1.4 Architecture of the Linux Operating System
Linux has all of the components of a typical OS (at this point you might like to refer back to Fig 1.1):
1.5 Logging into (and out of) UNIX Systems
Text-based (TTY) terminals:

When you connect to a UNIX computer remotely (using telnet) or when you log in locally using a text-only terminal, you will see the prompt:


At this prompt, type in your usename and press the enter/return/ key. Remember that UNIX is case sensitive (i.e. Will, WILL and will are all different logins). You should then be prompted for your password:

    login: will

Type your password in at the prompt and press the enter/return/ key. Note that your password will not be displayed on the screen as you type it in.

If you mistype your username or password you will get an appropriate message from the computer and you will be presented with the login: prompt again. Otherwise you should be presented with a shell prompt which looks something like this:


To log out of a text-based UNIX shell, type "exit" at the shell prompt (or if that doesn't work try "logout"; if that doesn't work press ctrl-d).

Graphical terminals:

If you're logging into a UNIX computer locally, or if you are using a remote login facility that supports graphics, you might instead be presented with a graphical prompt with login and password fields. Enter your user name and password in the same way as above (N.B. you may need to press the TAB key to move between fields).

Once you are logged in, you should be presented with a graphical window manager that looks similar to the Microsoft Windows interface. To bring up a window containing a shell prompt look for menus or icons which mention the words "shell", "xterm", "console" or "terminal emulator".

To log out of a graphical window manager, look for menu options similar to "Log out" or "Exit".

1.6 Changing your password
One of the things you should do when you log in for the first time is to change your password.

The UNIX command to change your password is passwd:

      $ passwd

The system will prompt you for your old password, then for your new password. To eliminate any possible typing errors you have made in your new password, it will ask you to reconfirm your new password.

Remember the following points when choosing your password:

1.7 General format of UNIX commands
A UNIX command line consists of the name of a UNIX command (actually the "command" is the name of a built-in shell command, a system utility or an application program) followed by its "arguments" (options and the target filenames and/or expressions). The general syntax for a UNIX command is

    $ command -options targets

Here command can be though of as a verb, options as an adverb and targets as the direct objects of the verb. In the case that the user wishes to specify several options, these need not always be listed separately (the options can sometimes be listed altogether after a single dash).


© September 2001 William Knottenbelt (