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Welcome to T.he L.inux G.uide O.nline
The following is a reference to Linux. Please feel free to
contact me for any details.
Chapter 01 - Introduction and History
of Linux
1.1 Introduction
1.1.1 Operating System
WITHOUT SOFTWARE THE computer is basically a lump of plastic
and metal. It would not be able to store or process information.
It would not be able to respond to the user. It would in fact
be dead. To make the computer what it is, the software forms
the most important component of the computer system. The primary
and most fundamental of these software entities is the Operating
System (OS) which has complete control over all of the system
resources and forms the base on which later applications are
executed.
Modem Computer is a complex combination of highly variant
hardware pieces. A computer may contain a variety of components
like processors, memory, storage devices, I/O devices. Each
of these can be further divided into a number of types. For
example the I/O devices may be keyboards, monitors, mice,
scanners, printers, cameras, mobile devices and what not.
To further complicate things there are a number of manufacturers
who make these devices and therefore each of these devices
has different means by which to access them.
Imagine a situation without an OS. And imagine you were writing
a program to add two numbers and print the sum. You would
have to include in your program code to detect what hardware
is present on the client system. Then you will have to have
proper functions written to access the various keyboards that
are available in the market. You will then have to convert
the signals from the keyboard into meaningful information.
Then you will have to execute the operation taking care to
move the data from memory to Registers and back to memory.
After that you will have to Detect and initialize the monitor
of the user and present him with the output. And your program
will be perhaps 200 MB. And you still will not be able to
take care of future models and makes of keyboards and monitors
that come out into the market.
Though the above situation is simplistic, (and not very accurate)
it still suffices to emphasize the importance of the OS. The
Operating System acts as the first software layer above the
hardware that manages all parts of the hardware all by itself.
It then presents the user with a virtual machine (an interface)
that can be used by the user or any application that does
not change with the underlying hardware. This uniformity of
environment is perhaps the biggest reason for the OS to evolve.
Looking at the case of your program again. Now all you have
to do is request the OS to get two numbers from the user.
You would then add them up with all memory management done
by the OS. You would then again request the OS to print the
result. You do not care about the underlying hardware or the
method by which the OS accomplices your requests. Moreover
your OS would handle future additions to hardware without
you having to bother about it.
The above example talks only of two relatively simple (!?)
pieces of hardware. Think for a moment about the complexity
that would come in if you had to say access a floppy. Think
of having to start the motor of the floppy through the controller,
checking its status, moving the read-write head and positioning
it on the correct sector, reading signal streams. The OS makes
all this easy for the application. It provides the program
an interface. It for example allows you to read data as files,
thereby allowing the program to achieve all that is required
in a single call to the OS. Thus logically through the concept
of files and operations like reading and writing to them,
the OS abstracts and insulates the program from the hardware.
The OS hence creates a virtual or extended machine, with its
own set of different (and much simpler rules) which can be
programmed more easily to indirectly control the underlying
hardware.
1.1.2 Multi-user and Multitasking
Modern OSes are more than just an abstraction interface.
This is a more user point of view. Modern machines allow more
than one user to use the machine simultaneously. It also allows
these users to run more than one application at a time. In
this environment the OS acts as a resource manager.
It is the arbitrator that decides who gets to use what device.
It allocates the very critical resources of processor time
and memory between executing programs. It decides who has
the control of I/O devices like the keyboard and the mouse.
It provides each program with its own virtual environment
to reside and execute in. And in the worst of cases it terminates
programs that don't follow the rules of this abstracted world,
in the interest of stability and well being of other programs.
To sum up the OS has the job of keeping track of the various
resources, account for their usage and act as an arbitrator
between conflicting programs.
A few terms here would be helpful. The term multitasking
refers to the Operating System running more that one task
(program) at the same time. Modern OSes do this by running
various processes and dividing the resources among them. All
processes are not allowed to directly manipulate memory. Instead
they refer to what are known as virtual memory spaces. Through
virtual memory, the programs can not only access data
as if they are the only process running in the computer (other
than the OS of course) but they can access much more memory
than physically present in the system. The OS does all the
dirty work of working with the actual memory and mapping manipulations
in the virtual memory to the physical operations.
The processor time too has to be managed by the OS among
the various processes. The OS divides the processor time into
what are known as time slices. Each program is then
allocated a time slice for execution. When a program is executing,
and its time is up, its state is stored as it is and another
program is stored for execution. When this happens a few times
and it is again the time for execution of the first program,
its state is reloaded into the processor and execution continues
at the point left off, as if the break never happened. This
is done so quickly (hundreds of times a second) that the user
is unaware of it at all.
The Operating System also gives the program, through the
extended machine, what are known as system calls. These
are multi purpose functions interfaced to the program that
allow it to accomplish a task quickly. These device independent
calls allow the user to accomplish disk access, printer access
etc. These also allow process management, signaling between
processes, time management and security.
1.1.3 Unix
The Unix is the oldest and probably one of the best Operating
Systems out there. Unix formed the basis for the development
of the OS as we know it. Unix was developed by Ken Thompson,
Dennis Ritchie and others at the AT&T, and is the trademark
of The Open Group. Unix is a real Operating System fulfilling
the criteria for multi-user and multitasking. Unix has been
ported to many hardware platforms.
Having been developed in academic institutions primarily
gives Unix a head start in incorporating the cutting edge
of technology. Bought up with the network, Unix has networking
built into it and is undoubtedly the best when it comes to
networking. Also it was available at a time when 'personal'
computers were undreamt of, it has security and privacy built
into it.
But among its downsides is the fact that it is costly (too
costly for a desktop). And secondly the massive proliferation
of Unix gave birth to a number of variant (clones) that caused
the differences in the OS to become very widespread and made
application development non-standard. All good Unix therefore
adopted a common standard later and now adhere to the IEEE
POSIX.1 standard.
1.2 What is Linux
Linus Torvalds a Finnish developer wanted a version of Unix
for his computer at home. The available free clone of Unix,
called MINIX, he found, was inadequate for his needs. He took
the easy way out (which is what differentiated the present
Linux users from other OS users) and started to write one
himself. He posted requests for help on the Usenet requesting
some information about the Posix systems for the x86 systems.
He developed the Linux kernel and released the first version
0.0.2 in 1991. Then he not only put the entire source code
on the web but invited people to take it off and develop it
further. And what happened later is, as they say, history.
The unique method of cooperative work on Linux added its
own strengths. People could borrow, share and modify other
people's ideas. Time was there fore spent in not reinventing
the wheel. Working on the kernel and the other applications
continued in parallel and Linux grew from strength to strength.
This strangely however did not lead to any chaos during the
development as would normally be expected. The spirit that
drove thousands to spend their free time in building and modifying
the OS also ensured that the work would be done according
to standard. With Linux falling under the Open General License
(OGL) various people all over the world carefully monitored
all changes. The same multitude also helped in testing to
such an extent that most of the bugs in software were discovered
during the initial phases.
And no longer is the myth that Linux is best left to geeks
true. Though initially Linux was the products of people with
a very strong technical background, great pains are being
taken to allow even ordinary users to harness the incredible
power of Linux. With the introduction of the Xserver, Linux
boasts of a full-fledged GUI, giving it the much needed user
friendliness. Once the early learning is accomplished exploring
Linux is very easy and rewarding. This book is therefore aimed
at building that initial information database that would embark
you on the great journey to explore the fascinating world
of Linux later on.
1.3 Advantages of Linux
1.3.1 Full fledged OS
Make no mistake about it - Linux is good. And is very comparable
with the very best out there. Linux, which till a few years
back was restricted to the academic institutions, is now not
only active as a desktop alternative but also forms a viable
corporate alternative. In fact the Apache server running on
a Linux server probably forms that largest percentage of the
web servers on the Internet.
There are a large number of applications currently available
for Linux. There are databases, web servers, networking tools,
office suites, Windowing interfaces and last but not the least
games for this platform.
Virtually every utility one would expect of a standard UNIX
implementation has been ported to Linux, including basic commands
like ls, awk, tr, sed, bc, and more. The familiar working
environment of other UNIX systems is duplicated on Linux.
All standard commands and utilities are included. Many text
editors are available, including vi, ex, pico, jove, and GNU
emacs, and variants like Lucid emacs, which incorporates extensions
of the X Window System, and joe. The text editor you're accustomed
to using has more than likely been ported to Linux.
Most of the basic Linux utilities are GNU software. GNU utilities
support advanced features that are not found in the standard
versions of BSD and UNIX System Vprograms. For example, the
GNU vi clone, elvis, includes a structured macro language
that differs from the original implementation. However, GNU
utilities are intended to remain compatible with their BSD
and System V counterparts. Many people consider the GNU versions
to be superior to the originals.
From the technical point of view, Linux easily measures up
to all the specifications of an OS specified earlier. It is
pre-emptive multitasking. It allows multiple users. Being
a Unix clone means that it has security built into the system.
Networking too is a strong point of Linux.
The Linux system is mostly compatible with several UNIX standards
(inasmuch as UNIX has standards) at the source level, including
IEEE POSIX.1, UNIX System V, and Berkely System Distribution
UNIX. Linux was developed with source code portability in
mind, and it's easy to find commonly used features that are
shared by more than one platform. Much of the free UNIX software
available on the Internet and elsewhere compiles under Linux
``right out of the box.'' In addition, all of the source code
for the Linux system, including the kernel, device drivers,
libraries, user programs, and development tools, is freely
distributable.
Other specific internal features of Linux include POSIX job
control (used by shells like csh and bash), pseudoterminals
( pty devices), and support for dynamically loadable national
or customized keyboard drivers. Linux supports virtual
consoles that let you switch between login sessions on
the same system console. Users of the screen program will
find the Linux virtual console implementation familiar.
The kernel can emulate 387-FPU instructions, and systems
without a math coprocessor can run programs that require floating-point
math capability.
1.3.2 Free for use
Linux has many of its components covered by the Open General
License (OGL). This is a unique and very powerful method of
copyrighting that allows software to be modified that does
not infringe upon the copyrights of the initial author.
What this means to the user is that since everyone contributed
to it, Linux is not officially owned by anyone. Linus Trovalds
owns GPL one some parts of it and a number of people hold
a GPL on others. And since it is not a copyrighted product
it can be installed, copied and distributed freely any number
of times. Without these restrictions the concept of piracy
does not exist on Linux.
This naturally leads to the question 'Where is the money
if all this is free?' GPL only states that you cannot be charged
for the software but you can be charged for a number of accompanying
items like support, distributions etc. This process is 'copyleft'
as opposed to 'copyright' on the software. This has spawned
a number of Linux distributions. One should keep in mind that
no matter what the distribution is, the software is still
Linux. The kernel still is one of the official versions approved
by Linus Trovalds. But the accompanying applications and installation
may vary
One additional point in favor of Linux is the fact that along
with the Operating System comes the whole of its source code.
That is the actual program compiled to produce the kernel
and the other modules. This means that you can use this source
to not only customize your copy of Linux (specific to your
hardware) but also use it to extend the operations of the
OS itself. Linux kernel hacking is a fascinating subject of
study and a lot can be learnt from studying the code of a
well written software like the kernel.
1.3.3 Stable and robust
One thing that Linux offers, which many commercial Operating
Systems cannot offer is unmatched stability and resistance
to crashes. The core of Linux is the kernel. It is a very
well written piece of code that has been tested thousands
of times and is nearly bug-free. This kernel has complete
control over the system resources. And all processes are directly
in control of the kernel. This allows the kernel to regain
control over the machine not only when a program misbehaves
but also during the crash of an application. No matter what
happens it is always possible to signal the kernel to kill
the errant process.
This high reliability of the kernel also makes the platform
an excellent choice for mission critical applications. System
that run intensive applications like servers, databases etc
have an excellent choice in Linux. Systems with up-times (time
during which the system has been on continuously without a
reboot) of weeks and months prefer a Unix clone with Linux
being the first choice. People having legacy software that
runs on Unix can now use Linux to run it with better results.
1.3.4 Native support for networking
Linux as its predecessor, Unix, has networking built into
the operating system. TCP/IP, the protocol that runs the Internet
has been an integral part of the creation of Linux. Linux
also has a number of tools that not only make the process
of networking easy, but Linux also allows the implementation
of other software that make full use of the networking facility.
Linux can be used for networking both in an intranet and
to the Internet. With telnet, users can log onto and work
on Linux machines from other remote machines. It can serve
as HTTP or file server. With support for SMTP and POP mailing
in never difficult. With the use of Networked File Server
and the Network Information Service complete use can be make
of network wide resources. Using the SAMBA server Linux can
also connect to machines running Windows.
A regular distribution of Linux also has a number of tools
that make full use of these capabilities. Tools include regular
and graphical configuration tools, browsers, mail and newsreaders
etc. There is also support for very secure transfer of files.
Tools also exist to monitor and regulate the activity on a
machine from a network. Such a rich collection of tools is
an added advantage not only to the users but also to the system
administrators. And combined with its unmatched stability,
Linux offers complete networking solution on one platform.
1.3.5 Efficient file system, virtual memory, libraries
Linux uses a file system called the ext2 native to it. This
file system is very easy to maintain, doesn't fragment easily
and has security features built in. Linux also supports a
number of other file systems that allows it to recognize,
read and write into other file systems like the vfat, msdos
and the iso9660 (cdrom).
The Linux kernel is developed to use protected-mode features
of Intel 80386 and better processors. In particular, Linux
uses the protected-mode, descriptor based, memory-management
paradigm, and other advanced features. Anyone familiar with
80386 protected-mode programming knows that this chip was
designed for multitasking systems like UNIX. Linux exploits
this functionality.
The kernel supports demand-paged, loaded executables. Only
those segments of a program which are actually in use are
read into memory from disk. Also, copy-on-write pages are
shared among executables. If several instances of a program
are running at once, they share physical memory, which reduces
overall usage.
In order to increase the amount of available memory, Linux
also implements disk paging. Up to one gigabyte of swap space
may be allocated on disk (upt to 8 partitions of 128 megabytes
each). When the system requires more physical memory, it swaps
inactive pages to disk, letting you run larger applications
and support more users. However, swapping data to disk is
no substitute for physical RAM, which is much faster.
The Linux kernel also implements a unified memory pool for
user programs and disk cache. All free memory is used by the
cache, which is reduced when running large programs.
Linux offers asynchronous file operations that allow file
I/O to be done in the background, giving control back to the
user before than the actual transfer takes place. This extensive
use of buffers allows full CPU utilization and reduces wait
time for I/O operations.
Executables use dynamically linked, shared libraries: code
from a single library on disk. This is not unlike the SunOS
shared library mechanism. Executable files occupy less disk
space, especially those which use many library functions.
There are also statically linked libraries for object debugging
and maintaining ``complete'' binary files when shared libraries
are not installed. The libraries are dynamically linked at
run time, and the programmer can use his or her own routines
in place of the standard library routines. These libraries
not only reduce the size of a new application but also make
the process of maintenance easy and centralized.
1.3.6 Highly customizable GUI
One main contention against Unix and its clones was the absence
of a user friendly GUI. The XFree86 X server allows a very
stable and customizable GUI environment. This X server only
supports the basic functions to accomplish graphical I/O.
Thus by leaving the actual look of the system to the various
Window Managers, XFree86 offers unmatched customizability.
There are a number of managers to choose from allowing users
to make the environment specific to their needs. This user-friendly
program is in charge of the placement of windows, the user
interface for resizing and moving them, changing windows to
icons, and the appearance of window frames, among other tasks.
XFree86 includes twm, the classic MIT window manager, and
advanced window managers like the Open Look Virtual Window
Manager (olvwm) are available. Popular among Linux users is
fvwm--a small window manager that requires less than half
the memory of twm. It provides a 3-dimensional appearance
for windows and a virtual desktop. The user moves the mouse
to the edge of the screen, and the desktop shifts as though
the display were much larger than it really is. fvwm is greatly
customizable and allows access to functions from the keyboard
as well as mouse. Many Linux distributions use fvwm as the
standard window manager. A version of fvwm called fvwm95-2
offers Microsoft Windows 95-like look and feel.
There also exists a programs called the glade and a KDE GUI
builder that allows developments of program using the Linux
GUI.
1.3.7 A chance to learn the internals of an OS
This is probably the main reason that drives students from
all over the world flocking to Linux. Because its source code
is given for free it forms a good mode to study how an OS
actually works. The practice, in which configuration files
are stored as text files, not only allows an intuitive understanding
of the concerned application but also gives a chance to tweak
performance that is not possible with front-end graphical
tools.
Also how many of us can atually run and configure high end
server software like Web servers, and mail servers etc. These
are available on default Linux distributions. And these are
not scaled down versions, they are full-fledged servers which
are actually employed on the Internet.
1.4 Drawbacks of Linux
New users often have a few misconceptions and false expectations
about Linux. It is important to understand the philosophy
and design of Linux in order to use it effectively. We'll
start by describing how Linux is not designed.
In commercial UNIX development houses, the entire system
is developed under a rigorous quality assurance policy that
utilizes source and revision control systems, documentation,
and procedures to report and resolve bugs. Developers may
not add features or change key sections of code on a whim.
They must validate the change as a response to a bug report
and subsequently ``check in'' all changes to the source control
system, so that the changes may be reversed if necessary.
Each developer is assigned one or more parts of the system
code, and only that developer can alter those sections of
the code while it is ``checked out'' (that is, while the code
is under his or her control).
Organizationally, a quality assurance department runs rigorous
tests on each new version of the operating system and reports
any bugs. The developers fix these bugs as reported. A complex
system of statistical analysis is used to ensure that a certain
percentage of bugs are fixed before the next release, and
that the operating system as a whole passes certain release
criteria.
The software company, quite reasonably, must have quantitative
proof that the next revision of the operating system is ready
to be shipped; hence, the gathering and analysis of statistics
about the performance of the operating system. It is a big
job to develop a commercial UNIX system, often large enough
to employ hundreds, if not thousands, of programmers, testers,
documenters, and administrative personnel. Of course, no two
commercial UNIX vendors are alike, but that is the general
picture.
The Linux model of software development discards the entire
concept of organized development, source code control systems,
structured bug reporting, and statistical quality control.
Linux is, and likely always will be, a hacker's operating
system. (By hacker, I mean a feverishly dedicated programmer
who enjoys exploiting computers and does interesting things
with them. This is the original definition of the term, in
contrast to the connotation of hacker as a computer wrongdoer,
or outlaw.)
There is no single organization responsible for developing
Linux. Anyone with enough know-how has the opportunity to
help develop and debug the kernel, port new software, write
documentation, and help new users. For the most part, the
Linux community communicates via mailing lists and Usenet
newsgroups. Several conventions have sprung up around the
development effort. Anyone who wishes to have their code included
in the ``official'' kernel, mails it to Linus Torvalds. He
will test and include the code in the kernel as long as it
doesn't break things or go against the overall design of the
system.
1.4.1 Inadequate hardware support
All hardware is accessed by means of special software called
the device drivers. The manufacturers of a device generally
prepare these device drivers for a particular platform. Since
Linux was not a commercial venture, it depended on generic
drivers to access the hardware or drivers written by somebody
across the Internet. Hence it is natural that the number brands
that were available for use under Linux was restricted. Linux
therefore couldn't work to its full potential in case of unsupported
hardware. Also there was a performance loss in partly supported
hardware that made its use unsatisfactory.
Things are changing as more and more hardware companies are
realizing the power of Linux, and are porting their drivers
to Linux. The day is not far off when this will discontinue
to be a problem with Linux.
1.4.2 Inadequate customer oriented support
Also due to the above reasons and the fact that the user
base of the OS has been low, there has not been extensive
customer support. With the rise of companies like Red Hat,
SuSe and Caldera things are now changing.
1.4.3 Primarily command line driven
Despite the existence of the GUI with the XFree86 server,
many configurations are still done manually in configuration
files. There is a lack of good tools for the X system. There
also is an acute shortage of user programs like office suites
etc that prevent making Linux a popular desktop alternative.
1.4.4 Learning Curve
Also there exists a learning curve for any user making a
transition from a regular GUI based OS to Linux, which may
be disconcerting.
1.5 GNU Public License
Linux is copy righted under the GNU General Public License
(GPL). The main highlights if the license are as follows.
- The original author retains the copyright
- Others can use the software as they wish, including modifying
it, basing other programs on it, and redistributing or reselling
it. The software may even be sold commercially for a profit.
The source code must accompany the software as well
- The copyright cannot be restricted down the line. Any
user automatically is bound to impose on other potential
users of the software, after modifications, if at all, the
same GPL. Hence all derived works automatically falls under
the GPL thereby preventing other people from taking credit
for the original authors work.
Such a unique licensing is in place because the original
authors of Linux didn't mean Linux as a means to make money
or profit from it. It was intended to be freely available
to everyone without warranty. Although there is not warranty
for the software, having problems does not mean that there
is no help available. There are a number of sources of information
on the web - websites, newsgroups and the Linux User Groups
(LUGs). The no warranty only limits the liability of the author.
The GPL has since proved to be the best way of distributing
free, but good, software on the net.
1.6 System Requirements
The Linux has some minimum system configuration for it to
execute on a platform. Thankfully though this is way below
those required by other modern OSes. Because it is not very
demanding on system resources, even old machines that have
been discarded as slow to use can now be reactivated with
Linux.
1.6.1 Intel platforms
The latest requirements and the list of supported hardware
are listed at the web site of the various distributions. Such
a listing from the website of Red Hat Linux is given below
for the Intel platform.
- Intel 386 or greater, through Pentium Pro and Pentium
II
- 40 MB of hard disk space in character mode, or 100 MB
with X Window
- 8MB of RAM (16 MB recommended)
- Most video cards are supported
- SCSI or IDE CD-ROM drive
- 3.5" floppy drive
1.6.2 SPARC and Alpha etc
Red Hat Linux supports a variety of hardware including the
SPARC and the Alpha variety of hardware. The listing of the
supported platforms can be found at the website www.redhat.com
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