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Unix FAQ

What is an operating system?
What is the kernel?
What is the shell?
What is a process?
How old is UNIX?

An operating system is the program that controls all the other parts of a computer system - both the hardware and the software. Most importantly, it allows you to make use of the facilities provided by the system. Every computer has an operating system.

Strictly speaking, UNIX is not one single operating system, it is a family of operating systems. Different computer manufacturers produce their own versions of UNIX.

Although these are mostly similar, there are small differences which can cause problems. The most obvious examples are the layout of the file system and the exact format of certain commands.

The UNIX operating system has three important features; a kernel, the shell and a filesystem.

As its name implies, the kernel is at the core of each UNIX system and is loaded in whenever the system is started up - referred to as a boot of the system.

It manages the entire resources of the system, presenting them to you and every other user as a coherent system. You do not need to know anything about the kernel in order to use a UNIX system. This information is provided for your information only.

The functions performed by the kernel include:

• managing the machine's memory and allocating it to each process.
• scheduling the work done by the CPU so that the work of each user is carried out as efficiently as is possible.
• organizing the transfer of data from one part of the machine to another.
• accepting instructions from the shell and carrying them out.
• enforcing the access permissions that are in force on the file system.

Whenever you login to a Unix system you are placed in a program called the shell. You can see its prompt at the bottom left of your screen. To get your work done, you enter commands at this prompt.

The shell acts as a command interpreter; it takes each command and passes it to the operating system kernel to be acted upon. It then displays the results of this operation on your screen.

The shell provides you with one or more of the following features. You can:

• create an environment that meets your needs
• write shell scripts
• define command aliases
• manipulate the command history
• automatically complete the command line
• edit the command line

Some shells provide more of these facilites than others.

A file system is a logical method for organizing and storing large amounts of information in a way which makes it easy manage. The file is the smallest unit in which information is stored. The UNIX file system has several important features.

• Different types of file
• Structure of the file system
• Your home directory
• Your current directory
• Pathnames
• Access permissions

When you enter a command it invokes a program. While this program is running it is called a process. It is important to grasp that although there is only one copy of a program held in the file system, any number of processes can be invoked which run this program.

When the operating system is started after a boot, a single process is started. This process is the parent of all subsequent processes. Each process created on the system has a unique number, known as its PID, associated with it.

When you login to the system a process is started to run your shell program. Any processes that are started from within your shell - such as entering a command - are the children of this process. A process can have many children, but only one parent.

The first version of UNIX was created in 1969 by Kenneth Thompson and Dennis Ritchie, system engineers at AT&T's Bell Labs. It went through many revisions and gained in popularity until 1977, when it was first made commercially available by Interactive Systems Corporation.

At the same time a team from the University of California at Berkeley was working to improve UNIX. In 1977 it released the first Berkeley Software Distribution, which became known as BSD. Over time this won favour through innovations such as the C shell.

Meanwhile the AT&T version was developing in different ways. The 1978 release of Version 7 included the Bourne Shell for the first time. By 1983 commercial interest was growing and Sun Microsystems produced a UNIX workstation. System V appeared, directly descended from the original AT&T UNIX and the prototype of the more widely used variant today.

There are two main versions of UNIX in use today (not counting the extremely popular Linux): System V and BSD. System V is the more popular of the two.

From a user's perspective they are very similar and you are unlikely to have difficulty unless you use more than one type of system. In this case you might notice differences in the structure of the file system or in how certain commands behave. The on-line manual should be helpful if you have problems.

Although UNIXhelp is now based on a System V variant of UNIX, you should see only minor differences in the example input and output if your system is a BSD one.

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