Page 407 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes 14.1 Overview of Linux

In this section, we will provide a general introduction to Linux and how it is used, for the
benefit of readers not already familiar with it. Nearly all of this material applies to just about all
UNIX variants with only small deviations. Although Linux has several graphical interfaces, the
focus here is how Linux appears to a programmer working in a shell window on X. Subsequent
sections will focus on system calls and how it works inside.

14.1.1 Linux Goals
UNIX was always an interactive system designed to handle multiple processes and multiple users
at the same time. It was designed by programmers, for programmers, to use in an environment
in which the majority of the users are relatively sophisticated and are engaged in (often quite
complex) software development projects. In many cases, a large number of programmers are
actively cooperating to produce a single system, so UNIX has extensive facilities to allow people
to work together and share information in controlled ways. The model of a group of experienced
programmers working together closely to produce advanced software is obviously very different
from the personal computer model of a single beginner working alone with a word processor,
and this difference is reflected throughout UNIX from start to finish. It is only natural that
Linux inherited many of these goals, even though the first version was for a personal computer.
What is it that good programmers want in a system? To start with, most like their systems to be
simple, elegant, and consistent. For example, at the lowest level, a file should just be a collection
of bytes. Having different classes of files for sequential access, random access, keyed access,
remote access, etc. (as mainframes do) just gets in the way.
Similarly, if the command
ls A*
means list all the files beginning with ‘’A’’ then the command
rm A*
should mean remove all the files beginning with “A” and not remove the one file whose name
consists of an ‘’A’’ and an asterisk. This characteristic is sometimes called the principle of least
surprise.
Another thing that experienced programmers generally want is power and flexibility. This
means that a system should have a small number of basic elements that can be combined in an
infinite variety of ways to suit the application. One of the basic guidelines behind Linux is that
every program should do just one thing and do it well. Thus compilers do not produce listings,
because other programs can do much better.
Finally, most programmers have a strong dislike for useless redundancy. Why type copy
when cp is enough? To extract all the lines containing the string “ard” from the file f, the Linux
programmer types grep ard f. The opposite approach is to have the programmer first select the
grep program (with no arguments), and then have grep announce itself by saying: “Hi, I’m grep,
I look for patterns in files. Please enter your pattern.” After getting the pattern, grep prompts for
a file name. Then it asks if there are any more file names. Finally, it summarizes what it is going
to do and ask if that is correct. While this kind of user interface may or may not be suitable for
rank novices, it irritates skilled programmers to no end.
14.1.2 Interfaces to Linux

A Linux system can be regarded as a kind of pyramid, as illustrated in Figure 14.1. At the
bottom is the hardware, consisting of the CPU, memory, disks, a monitor and keyboard, and
other devices running on the bare hardware is the operating system. Its function is to control
the hardware and provide a system call interface to all the programs. These system calls allow
user programs to create and manage processes, files, and other resources.

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