Page 406 - DCAP103_Principle of operating system

Unit 14: Case Study of Linux Operating System

releases followed frequently. The version numbers of the Linux kernel consist of four numbers— Notes
A, B, C and D, the first number denotes the kernel version. The second number denotes the
major revision. Prior to the 2.6 kernel, even revision numbers corresponded to stable kernel
releases, whereas odd ones corresponded to unstable revisions, under development. With the
2.6 kernel that is no longer the case. The third number corresponds to the minor revisions,
such as support for new drivers. The fourth number corresponds to minor bug fixes or security
patches. A large array of standard UNIX software has been ported to Linux, including the
X Window System and a great deal of networking software. Two different GUIs (GNOME and
KDE) have also been written for Linux. In short, it has grown to a full-blown UNIX clone with
all the bells and whistles a UNIX lover might want.

One unusual feature of Linux is its business model—it is free software. It can be downloaded
from various sites on the Internet, for example: www.kernel.org. Linux comes with a license
devised by Richard Stallman, founder of the Free Software Foundation. Despite the fact that
Linux is free, this license, the GPL (GNU Public License), is longer than Microsoft’s Windows
license and specifies what you can and cannot do with the code. Users may use, copy, modify,
and redistribute the source and binary code freely. The main restriction is that all works derived
from the Linux kernel may not be sold or redistributed in binary form only; the source code
must either be shipped with the product or be made available on request.
Although Torvalds still controls the kernel fairly closely, a large amount of user-level software
has been written by numerous other programmers, many of them originally migrated over from
the MINIX, BSD, and GNU online communities. However, as Linux evolves, a steadily smaller
fraction of the Linux community want to hack source code (witness hundreds of books telling
how to install and use Linux and only a handful discussing the code or how it works). Also,
many Linux users now forego the free distribution on the Internet to buy one of many CD-ROM
distributions available from numerous competing commercial companies.
A Website listing the current top 100 top Linux distributions is www.distrowatch.org. As more
and more software companies start selling their own versions of Linux and more and more
hardware companies offer to preinstall it on the computers they ship, the line between commercial
software and free software is beginning to blur substantially. As a footnote to the Linux story,
it is interesting to note that just as the Linux bandwagon was gaining steam, it got a big boost
from an unexpected source AT&T. In 1992, Berkeley, by now running out of funding, decided
to terminate BSD development with one final release, 4.4BSD, (which later formed the basis of
FreeBSD). Since this version contained essentially no AT&T code, Berkeley issued the software
under an open source license (not GPL) that let everybody do whatever they wanted with it
except one thing—sue the University of California. The AT&T subsidiary controlling UNIX
promptly reacted by—you guessed it—suing the University of California. It simultaneously
sued a company, BSDI, set up by the BSD developers to package the system and sell support,
much as Red Hat and other companies now do for Linux. Since virtually no AT&T code was
involved, the lawsuit was based on copyright and trademark infringement, including items
such as BSDI’s 1-800-ITS-UNIX telephone number. Although the case was eventually settled
out of court, this legal action kept FreeBSD off the market just long enough for Linux to get
well established. Had the lawsuit not happened, starting around 1993 there would have been
a serious competition between two free, open source UNIX systems: the reigning champion,
BSD, a mature and stable system with a large academic following dating back to 1977 versus
the vigorousyoung challenger, Linux, just two years old but with a grow by following among
individual users. Who knows how this battle of the free UNICES would have turned out?

Linux began in 1991 with the commencement of a personal project by a Finnish
student, Linus Torvalds.

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