Practical UNIX and Internet Security, 3rd Edition

What Is an Operating System?

For most people, a computer is a tool for solving problems. When running a word processor, a computer becomes a machine for arranging words and ideas. With a spreadsheet, the computer is a financial-planning machine, one that is vastly more powerful than a pocket calculator. Connected to an electronic network, a computer becomes part of a powerful communications system.

At the heart of every computer is a master set of programs called the operating system. This is the software that communicates with the system hardware to control the computer’s input/output systems, such as keyboards and disk drives, and that loads and runs other programs. The operating system is also a set of mechanisms and policies that help define controlled sharing of system resources.

Along with the operating system is (usually) a large set of standard utility programs for performing common functions such as copying files and listing the contents of directories. Although these programs are not technically part of the operating system according to some formal definitions, the popular notion of an operating system includes them. Whether they are part of the definition or not, they can have a dramatic impact on a computer system’s security.

All of Unix can be divided into four parts:

The kernel

The kernel, or heart of the Unix system, is the operating system. The kernel is a special program that is loaded into the computer when it is first started. It controls all of the computer’s input ...