Section 1.3. System Features

1.3. System Features

Linux has surpassed the features found in implementations of Unix and Windows. With the changes offered by IBM's Power Architecture, for example, Linux provides functionality for commodity hardware normally only found on the most expensive mainframes. Additionally, the latest kernels include the structure of Security Enhanced Linux (SELinux) provided by the National Security Agency (http://www.nsa.gov/selinux). SELinux provides the most trusted computing environment available today.

Now add Linux's ability to provide virtualization at the kernel level. Through Xen (http://sourceforge.net/projects/xen), Linux can securely execute multiple virtual machines, each running its own operating system, on a single physical system. This allows enterprises to stop server sprawl and increase CPU utilization.

1.3.1. A Bag of Features

This section provides a nickel tour of Linux features.

Linux is a complete multitasking , multiuser operating system (as are all other versions of Unix). This means that many users can be logged onto the same machine at once, running multiple programs simultaneously. Linux also supports multiprocessor systems (such as dual-Pentium motherboards), with support for up to 32 processors in a system,^[*] which is great for high-performance servers and scientific applications.

^[*] On a 32-bit architecture; on a 64-bit architecture, up to 64 CPUs are supported, and patches are available that support up to 256 CPUs.

The Linux system is mostly compatible with a number of Unix standards (inasmuch as Unix has standards) on the source level, including IEEE POSIX.1, System V, and BSD features. Linux was developed with source portability in mind: therefore, you will probably find features in the Linux system that are shared across multiple Unix implementations. A great deal of free Unix software available on the Internet and elsewhere compiles on Linux out of the box.

If you have some Unix background, you may be interested in some other specific internal features of Linux, including POSIX job control (used by shells such as the C shell, csh, and bash), pseudoterminals (pty devices), and support for national or customized keyboards using dynamically loadable keyboard drivers. Linux also supports virtual consoles , which allow you to switch between multiple login sessions from the system console in text mode. Users of the screen program will find the Linux virtual console implementation familiar (although nearly all users make use of a GUI desktop instead).

Linux can quite happily coexist on a system that has other operating systems installed, such as Windows 95/98, Windows NT/2000/XP, Mac OS, and Unix-like operating systems such as the variants of BSD. The Linux bootloader (LILO ) and the GRand Unified Bootloader (GRUB ) allow you to select which operating system to start at boot time, and Linux is compatible with other bootloaders as well (such as the one found in Windows XP).

Linux can run on a wide range of CPU architectures, including the Intel x86 (the whole Pentium line), Itanium, SPARC/UltraSPARC, AMD 64 ("Hammer"), ARM, PA-RISC, Alpha, PowerPC, MIPS, m68k, and IBM 390 and zSeries mainframes. Linux has also been ported to a number of embedded processors, and stripped-down versions have been built for various PDAs, including the PalmPilot and Compaq iPaq. In the other direction, Linux is being considered for top-of-the-line computers as well. Hewlett-Packard has a supercomputer with Linux as the operating system. A large number of scalable clusterssupercomputers built from arrays of PCsrun Linux as well.

Linux supports various filesystem types for storing data. Some filesystems, such as the Second Extended Filesystem (ext2fs), have been developed specifically for Linux. Other Unix filesystem types, such as the Minix-1 and Xenix filesystems, are also supported. The Windows NTFS, VFAT (Windows 95/98), and FAT (MS-DOS) filesystems have been implemented as well, allowing you to access Windows files directly. Support is included for Macintosh, OS/2, and Amiga filesystems as well. The ISO 9660 CD-ROM filesystem type, which reads all standard formats of CD-ROMs, is also supported. We talk more about filesystems in Chapter 2 and Chapter 10.

Networking support is one of the greatest strengths of Linux, in terms of both functionality and performance. Linux provides a complete implementation of TCP/IP networking. This includes device drivers for many popular Ethernet cards, PPP and SLIP (allowing you to access a TCP/IP network via a serial connection or modem), Parallel Line Internet Protocol (PLIP), and ADSL. Linux also supports the modern IPv6 protocol suite, and many other protocols, including DHCP, Appletalk, IRDA, DECnet, and even AX.25 for packet radio networks. The complete range of TCP/IP clients and services is supported, such as FTP, Telnet, NNTP, and Simple Mail Transfer Protocol (SMTP), the Sun RPC protocols allowing NFS and NIS, and the Microsoft protocols allowing participation in a Microsoft domain. The Linux kernel includes complete network firewall support, allowing any Linux machine to screen network packets and prevent unauthorized access to an intranet, for example.

It is widely held that networking performance under Linux is superior to other operating systems. We talk more about networking in Chapter 13 and Part IV.

1.3.2. Kernel

The kernel is the guts of the operating system itself; it's the code that controls the interface between user programs and hardware devices, the scheduling of processes to achieve multitasking, and many other aspects of the system. The kernel is not a separate process running on the system. Instead, you can think of the kernel as a set of routines, constantly in memory, to which every process has access. Kernel routines can be called in a number of ways. One direct method to utilize the kernel is for a process to execute a system call, which is a function that causes the kernel to execute some code on behalf of the process. For example, the read system call will read data from a file descriptor. To the programmer, this looks like any other C function, but in actuality the code for read is contained within the kernel.

The Linux kernel is known as a monolithic kernel, in that all core functions and device drivers are part of the kernel proper. Some operating systems employ a microkernel architecture whereby device drivers and other components (such as filesystems and memory management code) are not part of the kernelrather, they are treated as independent services or regular user applications. There are advantages and disadvantages to both designs: the monolithic architecture is more common among Unix implementations and is the design employed by classic kernel designs, such as System V and BSD. Linux does support loadable device drivers (which can be loaded and unloaded from memory through user commands); this is covered in Chapter 18.

The Linux kernel on Intel platforms is developed to use the special protected-mode features of the Intel x86 processors (starting with the 80386 and moving on up to the current Pentium 4). In particular, Linux makes use of the protected-mode descriptor-based memory management paradigm and many of the other advanced features of these processors. Anyone familiar with x86 protected-mode programming knows that this chip was designed for a multitasking system such as Unix (the x86 was actually inspired by Multics). Linux exploits this functionality.

Like most modern operating systems, Linux is a multiprocessor operating system: it supports systems with more than one CPU on the motherboard. This feature allows different programs to run on different CPUs at the same time (or "in parallel"). Linux also supports threads, a common programming technique that allows a single program to create multiple "threads of control" that share data in memory. Linux supports several kernel-level and user-level thread packages, and Linux's kernel threads run on multiple CPUs, taking advantage of true hardware parallelism. The Linux kernel threads package is compliant with the POSIX 1003.1c standard.

The Linux kernel supports demand-paged loaded executables. That is, only those segments of a program that are actually used are read into memory from disk. Also, if multiple instances of a program are running at once, only one copy of the program code will be in memory. Executables use dynamically linked shared libraries, meaning that executables share common library code in a single library file found on disk. This allows executable files to occupy much less space on disk. This also means that a single copy of the library code is held in memory at one time, thus reducing overall memory usage. There are also statically linked libraries for those who wish to maintain "complete" executables without the need for shared libraries to be in place. Because Linux shared libraries are dynamically linked at runtime, programmers can replace modules of the libraries with their own routines.

In order to make the best use of the system's memory, Linux implements so-called virtual memory with disk paging. That is, a certain amount of swap space ^[*] can be allocated on disk. When applications require more physical memory than is actually installed in the machine, it will swap inactive pages of memory out to disk. (A page is simply the unit of memory allocation used by the operating system; on most architectures, it's equivalent to 4 KB.) When those pages are accessed again, they will be read from disk back into main memory. This feature allows the system to run larger applications and support more users at once. Of course, swap is no substitute for physical RAM; it's much slower to read pages from disk than from memory.

^[*] If you are a real OS geek, you will note that swap space is inappropriately named: entire processes are not swapped, but rather individual pages of memory are paged out. Although in some cases entire processes will be swapped out, this is not generally the case. The term "swap space" originates from the early days of Linux and should technically be called "paging space."

The Linux kernel keeps portions of recently accessed files in memory, to avoid accessing the (relatively slow) disk any more than necessary. The kernel uses all the free memory in the system for caching disk accesses, so when the system is lightly loaded a large number of files can be accessed rapidly from memory. When user applications require a greater amount of physical memory, the size of the disk cache is reduced. In this way physical memory is never left unused.

To facilitate debugging , the Linux kernel generates a core dump of a program that performs an illegal operation, such as accessing an invalid memory location. The core dump, which appears as a file called core in the directory that the program was running, allows the programmer to determine the cause of the crash. We talk about the use of core dumps for debugging in the section "Examining a Core File" in Chapter 21.

1.3.3. Commands and Shells

The most important utility to many users is the shell. The shell is a program that reads and executes commands from the user. In addition, many shells provide features such as job control (allowing the user to manage several running processes at oncenot as Orwellian as it sounds), input and output redirection, and a command language for writing shell scripts. A shell script is a file containing a program in the shell command language, analogous to a "batch file" under Windows.

Many types of shells are available for Linux. The most important difference between shells is the command language. For example, the C shell (csh) uses a command language somewhat like the C programming language. The classic Bourne shell uses a different command language. One's choice of a shell is often based on the command language it provides. The shell that you use defines, to some extent, your working environment under Linux.

No matter what Unix shell you're accustomed to, some version of it has probably been ported to Linux. The most popular shell is the GNU Bourne Again Shell (bash), a Bourne shell variant. bash includes many advanced features, such as job control, command history, command and filename completion, an Emacs-like (or optionally, a vi-like) interface for editing the command line, and powerful extensions to the standard Bourne shell language. Another popular shell is tcsh, a version of the C shell with advanced functionality similar to that found in bash. Recently, zsh, with very advanced completion facilities, has found a lot of followers. Other shells include the Korn shell (ksh), BSD's ash, and rc, the Plan 9 shell.

What's so important about these basic utilities? Linux gives you the unique opportunity to tailor a custom system to your needs. For example, if you're the only person who uses your system, and you prefer to use the vi editor and the bash shell exclusively, there's no reason to install other editors or shells. The "do it yourself" attitude is prevalent among Linux hackers and users.

1.3.4. Text Processing and Word Processing

Almost every computer user has a need for some kind of document preparation system. (In fact, one of the authors has almost entirely forgotten how to write with pen and paper.) In the PC world, word processing is the norm: it involves editing and manipulating text (often in a "what you see is what you get" [WYSIWYG] environment) and producing printed copies of the text, complete with figures, tables, and other garnishes.

As you will see in this book, Linux supports attractive and full-featured WYSIWYG tools. In Chapter 8 we'll discuss OpenOffice (a free version of a propriety product, StarOffice, released by Sun Microsystems when it bought the suite's manufacturer), and KOffice, both of which are tightly integrated suites that support word processing, spreadsheets, and other common office tasks. These don't support all the features of Microsoft Office, but by the same token, they have some valuable features that Microsoft Office lacks. If you want to run Microsoft Office, you can do so through Wine, which we mention later.

There is a role for other ways to create documents, though. The system configuration files you need to edit on Linux from time to time, as well as programming for application development, require the use of simple text processing. The most popular tools for creating such documents are vi and Emacs, described in detail in Chapter 19.

Text processing can also be used with separate formatting tools to create very readable and attractive documents. With a text processing system, the author enters text using a "typesetting language" that describes how the text should be formatted. Once the source text (in the typesetting language) is complete, a user formats the text with a separate program, which converts the source to a format suitable for printing. This is somewhat analogous to programming in a language such as C, and "compiling" the document into a printable form.

The most famous text formatting language is HTML, the markup language used by virtually every page on the World Wide Web. Another popular text processing language is DocBook XML, a kind of industry-standard set of tags for marking up technical documentation, which is also used by the Linux Documentation Project (to be discussed later in this chapter).

We'll look at several text formatting systems in Chapter 20, Text Processing: T_EX (developed by Donald Knuth of computer science fame) and its dialect LAT_EX, groff, the GNU version of the classic troff text formatter originally developed by Bell Labs); Texinfo (an extension to T_EX used for software documentation by the Free Software Foundation); and Docbook.

1.3.5. Commercial Applications

In addition to the more than fifteen hundred Linux applications maintained by Linux distributors such as Debian, a groundswell of support exists from commercial application developers for Linux. These products include office productivity suites, word processors, scientific applications, network administration utilities, ERP packages such as Oracle Financials and SAP, and large-scale database engines. Linux has become a major force in the commercial software market, so you may be surprised to find how many popular commercial applications are available for Linux. We can't possibly discuss all of them here, so we'll only touch on the most popular applications and briefly mention some of the others.

Oracle, IBM, Informix, Sybase, and Interbase have released commercial database engines for Linux. Many of the Linux database products have demonstrated better performance than their counterparts running on Windows servers.

One very popular database for Linux is MySQL , a free and easy-to-use database engine. Because MySQL is easy to install, configure, and use, it has rapidly become the database engine of choice for many applications that can forego the complexity of the various proprietary engines. Furthermore, even though it's free software, MySQL is supported professionally by the company that developed it, MySQL AB. We describe the basic use of MySQL in Chapter 25.

MySQL does not include some of the more advanced features of the proprietary databases, however. Some database users prefer the open source database PostgresSQL, and Red Hat features it in some of its products. On the other hand, MySQL is catching up really quickly; the next version will contain support for distributed databases, for example.

A wide range of enterprise applications is available for Linux in addition to databases. Linux is one of the most popular platforms for Internet service hosting, so it is appropriate that high-end platforms for scalable web sites, including JBoss, BEA WebLogic, and IBM WebSphere, have been released for Linux. Commercial, high-performance Java Virtual Machines and other software are available from Sun, IBM, and other vendors. IBM has released the popular Lotus Domino messaging and web application server, as well as the WebSphere MQ (formerly MQSeries) messaging platform.

Scientists, engineers, and mathematicians will find that a range of popular commercial products are available for Linux, such as Maple, Mathematica, MATLAB, and Simulink. Other commercial applications for Linux include high-end CAD systems, network management tools, firewalls, and software development environments.

1.3.6. Programming Languages and Utilities

Linux provides a complete Unix programming environment, including all the standard libraries, programming tools, compilers, and debuggers that you would expect to find on other Unix systems. The most commonly used compiler on Linux is the GNU's Compiler Collection, or gcc. gcc is capable of compiling C, C++, Objective C (another object-oriented dialect of C), Chill (a programming language mainly used for telecommunications), FORTRAN, and Java. Within the Unix software development world, applications and systems programming is usually done in C or C++, and gcc is one of the best C/C++ compilers around, supporting many advanced features and optimizations.

Java is an object-oriented programming language and runtime environment that supports a diverse range of applications such as web page applets, Internet-based distributed systems, database connectivity, and more. Java is fully supported under Linux. Several vendors and independent projects have released ports of the Java Development Kit for Linux, including Sun, IBM, and the Blackdown Project (which did one of the first ports of Java for Linux). Programs written for Java can be run on any system (regardless of CPU architecture or operating system) that supports the Java Virtual Machine. A number of Java "just in time" (or JIT ) compilers are available, and the IBM and Sun Java Development Kits (JDKs) for Linux come bundled with high-performance JIT compilers that perform as well as those found on Windows or other Unix systems.

Some of the most popular and interesting tools associated with Java are open source. These include Eclipse, an integrated development environment (IDE) that is extendable to almost anything through plugins; JBoss, an implementation of Java 2 Enterprise Edition (J2EE) that has actually gone through the expense of becoming certified after a complaint by Sun Microsystems; and Gluecode, another application platform company bought by IBM in May 2005.

gcc is also capable of compiling Java programs directly to executables, and includes limited support for the standard JDK libraries.

Besides C, C++, and Java, many other compiled and interpreted programming languages have been ported to Linux, such as Smalltalk, FORTRAN, Pascal, LISP, Scheme, and Ada. In addition, various assemblers for writing machine code are available. An important open source project sponsored by Novell has developed an environment called Mono that provides support for Microsoft's .NET environment on Unix and Linux systems. Perhaps the most important class of programming languages for Linux is the many scripting languages, including Perl (the script language to end all script languages), Python (the first scripting language to be designed as object-oriented from the ground up), and Ruby (a fiercely object-oriented scripting language that has been heralded as very good for rapid application development ).

Linux systems make use of the advanced gdb debugger, which allows you to step through a program to find bugs or examine the cause for a crash using a core dump. gprof, a profiling utility, will give you performance statistics for your program, letting you know where your program is spending most of its time. The Emacs and vim text editors provide interactive editing and compilation environments for various programming languages. Other tools that are available for Linux include the GNU make build utility, used to manage compilation of large applications , as well as source-code control systems such as CVS and Subversion.

Linux is an ideal system for developing Unix applications. It provides a modern programming environment with all the bells and whistles, and many professional Unix programmers claim that Linux is their favorite operating system for development and debugging. Computer science students can use Linux to learn Unix programming and to explore other aspects of the system, such as kernel architecture. With Linux, not only do you have access to the complete set of libraries and programming utilities, but you also have the complete kernel and library source code at your fingertips. Chapter 20 of this book is devoted to the programming languages and tools available for Linux.

1.3.7. The X Window System

The X Window System is the standard GUI for Unix systems. It was originally developed at MIT in the 1980s with the goal of allowing applications to run across a range of Unix workstations from different vendors. X is a powerful graphical environment supporting many applications. Many X-specific applications have been written, such as games, graphics utilities, programming and documentation tools, and so on.

Unlike Microsoft Windows, the X Window System has built-in support for networked applications: for example, you can run an X application on a server machine and have its windows display on your desktop, over the network. Also, X is extremely customizable: you can easily tailor just about any aspect of the system to your liking. You can adjust the fonts, colors, window decorations, and icons for your personal taste. You can go so far as to configure keyboard macros to run new applications at a keystroke. It's even possible for X to emulate the Windows and Macintosh desktop environments , if you want to keep a familiar interface.

The X Window System is freely distributable. However, many commercial vendors have distributed proprietary enhancements to the original X software. The version of X available for Linux is known as X.org , which is a port of X11R6 (X Window System Version 11, Release 6) made freely distributable for PC-based Unix systems, such as Linux.^[*] X.org supports a wide range of video hardware, including standard VGA and many accelerated video adapters. X.org is a complete distribution of the X software, containing the X server itself, many applications and utilities, programming libraries, and documentation. It comes bundled with nearly every Linux distribution.

^[*] X.org actually derives from another PC-based version of the X Window System, XFree86. Political quarrels that we do not want to go into here have led to a split into XFree86 and X.org; most Linux distributions these days ship the X.org version. This is not relevant for you, though, unless you plan to help with the continued development of the X Window System.

The look and feel of the X interface are controlled to a large extent by the window manager. This friendly program is in charge of the placement of windows, the user interface for resizing, iconifying, and moving windows, the appearance of window frames, and so on.

The X distribution and the major Linux distributions also contain programming libraries and include files for those wily programmers who wish to develop X applications. All the standard fonts, bitmaps, manual pages, and documentation are included.

Chapter 16 discusses how to install and use the X Window System on your Linux machine.

1.3.8. KDE and GNOME

Although the X Window System provides a flexible windowing system, many users want a complete desktop environment, with a customizable look and feel for all windows and widgets (such as buttons and scrollbars), a simplified user interface, and advanced features such as the ability to "drag and drop" data from one application to another. The KDE and GNOME projects are separate efforts that are striving to provide such an advanced desktop environment for Linux. By building up a powerful suite of development tools, libraries, and applications that are integrated into the desktop environment, KDE and GNOME aim to usher in the next era of Linux desktop computing. In the spirit of the open source community, these projects work together to provide complete interoperability so that applications originating in one environment will work on the other. Both systems provide a rich GUI, window manager, utilities, and applications that rival or exceed the features of systems such as the Windows XP desktop.

With KDE and GNOME, even casual users and beginners will feel right at home with Linux. Most distributions automatically configure one of these desktop environments during installation, making it unnecessary to ever touch the text-only console interface.

Both KDE and GNOME aim to make the Linux environment more user-friendly, and each has its fans and partisans. We discuss both in Chapter 3. As with X, both KDE and GNOME provide open source libraries that let you write programs conforming to their behavior and their look and feel.

1.3.9. Networking

Linux boasts one of the most powerful and robust networking systems in the worldmore and more people are finding that Linux makes an excellent choice as a network server. Linux supports the TCP/IP networking protocol suite that drives the entire Internet, as well as many other protocols, including IPv6 (a new version of the IP protocol for the next-generation Internet), and UUCP (used for communication between Unix machines over serial lines). With Linux, you can communicate with any computer on the Internet, using Ethernet (including Fast and Gigabit Ethernet), Token Ring, dial-up connection, wireless network, packet radio, serial line, ADSL, ISDN, ATM, IRDA, AppleTalk, IPX (Novell NetWare), and many other network technologies. The full range of Internet-based applications is available, including World Wide Web browsers, web servers, FTP, email, chat, news, ssh, Telnet, and more.

Most Linux users use either a dial-up or a DSL connection through an ISP to connect to the Internet from home. Linux supports the popular PPP and SLIP protocols, used by most ISPs for dial-in access. If you have a broadband connection, such as a T1 line, cable modem, DSL, or other service, Linux supports those technologies as well. You can even configure a Linux machine to act as a router and firewall for an entire network of computers, all connecting to the Internet through a single dial-up or broadband connection.

Linux supports a wide range of web browsers, including Mozilla (the open source spin-off of the Netscape browser), Konquerer (another open source browser packaged with KDE), and the text-based Lynx browser. The Emacs text editor even includes a small text-based web browser.

Linux also hosts a range of web servers. Linux played an important role in the emergence of the popular and free Apache web server. In fact, it's estimated that Apache running on Linux systems drives more web sites than any other platform in the world. Apache is easy to set up and use; we show you how in Chapter 22.

A full range of mail and news readers is available for Linux, such as MH, Elm, Pine, and mutt, as well as the mail/news readers included with the Mozilla web browser. Many of these are compatible with standard mail and news protocols such as IMAP and POP. Whatever your preference, you can configure your Linux system to send and receive electronic mail and news from all over the world.

A variety of other network services are available for Linux. Samba is a package that allows Linux machines to act as a Windows file and print server. NFS allows your system to share files seamlessly with other machines on the network. With NFS, remote files look to you as if they were located on your own system's drives. FTP allows you to transfer files to and from other machines on the network. Other networking features include NNTP-based electronic news systems such as C News and INN; the Sendmail, Postfix, and Exim mail transfer agents; ssh, telnet, and rsh, which allow you to log in and execute commands on other machines on the network; and finger, which allows you to get information on other Internet users. There are tons of TCP/IP-based applications and protocols out there.

If you have experience with TCP/IP applications on other systems, Linux will be familiar to you. The system provides a standard socket programming interface, so virtually any program that uses TCP/IP can be ported to Linux. The Linux X server also supports TCP/IP, allowing you to display applications running on other systems on your Linux display. Administration of Linux networking will be familiar to those coming from other Unix systems, as the configuration and monitoring tools are similar to their BSD counterparts.

In Chapter 13, we discuss the configuration and setup of TCP/IP, including PPP, for Linux. We also discuss configuration of web browsers, web servers, and mail software.

1.3.10. Laptop Support

Linux includes a number of laptop-specific features, such as PCMCIA (or "PC Card") support and APM and the newer ACPI, as well as the wireless networking built into Centrino laptops. The PCMCIA Tools package for Linux includes drivers for many PCMCIA devices, including modems, Ethernet cards, and SCSI adapters. APM allows the kernel to keep track of the laptop's battery power and perform certain actions (such as an automated shutdown) when power is low; it also allows the CPU to go into "low-power" mode when not in use. This is easy to configure as a kernel option. Various tools interact with APM, such as apm (which displays information on battery status) and apmd (which logs battery status and can be used to trigger power events). These should be included with most Linux distributions. ACPI has a similar purpose, but is newer and more featureful. With ACPI, you can even use the so-called "suspend to disk" facility with it, where the current state of the computer is written to your hard disk, and the computer turned off. You can then turn it on later and resume your work exactly where you left off. GUI tools such as kpowersave let you control this from a friendly graphical environment.

1.3.11. Interfacing with Windows

Various utilities exist to interface with the world of Windows and MS-DOS . The most well-known application is a project known as Winea platform for Microsoft Windows applications on the X Window System under Linux. Wine allows Microsoft Windows applications to run directly under Linux and other Intel-based operating systems. Wine is in a process of continual development, and now runs a wide variety of Windows software, including many desktop applications and games. We discuss Wine in Chapter 28.

Linux provides a seamless interface for transferring files between Linux and Windows systems. You can mount a Windows partition or floppy under Linux, and directly access Windows files as you would any others. In addition, there is the mtools package, which allows direct access to MS-DOS-formatted floppies, as well as htools , which does the same for Macintosh floppy disks.

Another legacy application is the Linux MS-DOS Emulator, or DOSEMU, which allows you to run many MS-DOS applications directly from Linux. Although MS-DOS-based applications are rapidly becoming a thing of the past, there are still a number of interesting MS-DOS tools and games that you might want to run under Linux. It's even possible to run the old Microsoft Windows 3.1 under DOSEMU.

Although Linux does not have complete support for emulating Windows and MS-DOS environments, you can easily run these other operating systems on the same machine with Linux, and choose which operating system to run when you boot the machine. Many distributions know how to preserve another operating system that's already installed when you add Linux to the computer, and set up a working LILO or GRUB bootloader to let you to select between Linux, Windows, and other operating systems at boot time. In this book we'll show you how to set up the LILO bootloader, in case you need to do it yourself.

Another popular option is to run a system-level virtual machine, which literally allows you to run Linux and Windows at the same time. A virtual machine is a software application that emulates many of the hardware features of your system, tricking the operating system into believing that it is running on a physical computer. Using a virtual machine, you can boot Linux and then run Windows at the same timewith both Linux and Windows applications on your desktop at once. Alternatively, you can boot Windows and run Linux under the virtual machine. Although there is some performance loss when using virtual machines, many people are very happy employing them for casual use, such as running a Windows-based word processor within a Linux desktop. The most popular virtual machines are VMware (http://www.vmware.com), which is a commercial product, and Bochs (http://bochs.sourceforge.net), which is an open source project. We describe VMware in Chapter 28.

Finally, remote logins allow you to work on another system from your Linux system. Any two computers running the X Window System (mostly Linux, BSD, and Unix systems) can share work this way, with a user on one system running a program on another, displaying the graphical output locally, and entering commands from the local keyboard and mouse. RDP, an acronym that has been expanded to both Remote Desktop Protocol and Remote Display Protocol, allows a Linux system to run programs on remote Windows systems in the same way. A Virtual Network Connection (VNC ) client and server perform the same task with even greater flexibility, letting different operating systems on different computers work together. In "Remote Desktop Access to Windows Programs" we show you how to set up these services, and in "FreeNX: Linux as a Remote Desktop Server" we discuss the FreeNX remote communication system, which allows the same transparent networking as X with a tremendous speed advantage. Both of these sections are in Chapter 28.

1.3.12. Other Applications

A host of miscellaneous applications are available for Linux, as one would expect from an operating system with such a diverse set of users. Linux's primary focus is currently for personal Unix computing, but this is rapidly changing. Business and scientific software are expanding, and commercial software vendors have contributed a growing pool of applications.

The scientific community has wholly embraced Linux as the platform of choice for inexpensive numerical computing. A large number of scientific applications have been developed for Linux, including the popular technical tools MATLAB and Mathematica. A wide range of free packages is also available, including FELT (a finite-element analysis tool), Spice (a circuit design and analysis tool), and Khoros (an image/digital signal processing and visualization system). Many popular numerical computing libraries have been ported to Linux, including the LAPACK linear algebra library. There is also a Linux-optimized version of the BLAS code upon which LAPACK depends.

Linux is one of the most popular platforms for parallel computing using clusters , which are collections of inexpensive machines usually connected with a fast (gigabit-per-second or faster) network. The NASA Beowulf project first popularized the idea of tying a large number of Linux-based PCs into a massive supercomputer for scientific and numerical computing. Today, Linux-based clusters are the rule, rather than the exception, for many scientific applications. In fact, Linux clusters are finding their way into increasingly diverse applicationsfor example, the Google search engine runs on a cluster of Linux machines (over 250,000 of them in December 2004, according to an MIT paper)!

As with any operating system, Linux has its share of games. A number of popular commercial games have been released for Linux, including Quake, Quake II, Quake III Arena, Doom, SimCity 3000, Descent, and more. Most of the popular games support play over the Internet or a local network, and clones of other commercial games are popping up for Linux. There are also classic text-based dungeon games such as Nethack and Moria; MUDs (multiuser dungeons, which allow many users to interact in a text-based adventure) such as DikuMUD and TinyMUD; and a slew of free graphical games, such as xtetris, netrek, and Xboard (the X11 frontend to gnuchess).

For audiophiles, Linux has support for a wide range of sound hardware and related software, such as CDplayer (a program that can control a CD-ROM drive as a conventional CD player, surprisingly enough), MIDI sequencers and editors (allowing you to compose music for playback through a synthesizer or other MIDI-controlled instrument), and sound editors for digitized sounds. You can play your MP3 and OGG/Vorbis files on Linux, and with the tools in some distributions you can handle more proprietary formats as well.

Can't find the application you're looking for? A number of web sites provide comprehensive directories of Linux applications. The best known is Freshmeat (http://www.freshmeat.net); a couple others are listed in Appendix A. Take a look at these sites just to see the enormous amount of code that has been developed for Linux.

If you absolutely can't find what you need, you can always attempt to port the application from another platform to Linux. Or, if all else fails, you can write the application yourself. That's the spirit of free softwareif you want something to be done right, do it yourself! While it's sometimes daunting to start a major software project on your own, many people find that if they can release an early version of the software to the public, many helpers pop up in the free software community to carry on the project.