Linux is a completely free reimplementation of the POSIX specification, with SYSV and BSD extensions (which means it looks like Unix, but does not come from the same source code base), which is available in both source code and binary form. It is copyrighted by Linus B. Torvalds (Linus.Torvalds@Helsinki.FI) and other contributors, and is freely redistributable under the terms of the GNU Public License.
Linux is not public domain, nor is it `shareware'. It is `free' software, commonly called freeware, and you may give copies away, but you must also give the source with it or make it available in the same way. If you distribute any modifications, you are legally bound to distribute the source for those modifications. See the GNU General Public License for details. A copy is included with the Linux source, or you can get a copy via ftp from prep.ai.mit.edu in /pub/gnu/COPYING
Linux is still free as of version 2.0, and will continue to be. Because of the nature of the GNU copyright which Linux is subject to, it would be illegal for it to be made not free. Note carefully: it is perfectly legal to charge money for distributing Linux, so long as you also distribute the source code. This is a generalization; if you want the fine points, read the GPL.
Linux runs on 386/486/Pentium machines with ISA, EISA, PCI and VLB busses. MCA (IBM's proprietary bus) is not currently well-supported because there has been neither much documentation for nor significant interest in this dinosaur, although some patches are available for a few machines. If you are interested, see http://glycerine.itsmm.uni.edu/mca
There is a port in progress for multiple Motorola 680x0 platforms (currently running on some Amigas and Ataris), which now works quite well. It requires a 68020 with an MMU, a 68030, 68040, or a 68060, and also requires an FPU. Networking and X now work.
Linux runs well on DEC's Alpha CPU, currently supporting the "Jensen", "NoName", "Cabriolet", "Universal Desktop Box" (better known as the Multia), and several other platforms. For more information, see http://www.azstarnet.com/~axplinux/FAQ.html
Linux runs well on Sun SPARCs; most sun4c and sun4m machines now run Linux. Red Hat Linux is (as of this writing) the only Linux distribution available for SPARCs; see http://www.redhat.com/support/docs/rhl-sparc/
Ports to other machines, including MIPS, PowerPC, and PowerMAC, are under way and showing various amounts of progress. Don't hold your breath, but if you are interested and able to contribute, you may well find other developers who wish to work with you.
Linux is no longer considered to be in beta testing, as version 1.0 was released on March 14, 1994. There are still bugs in the system, and new bugs will creep up and be fixed as time goes on. Because Linux follows the ``open development model'', all new versions will be released to the public, whether or not they are considered ``production quality''. However, in order to help people tell whether they are getting a stable version or not, the following scheme has been implemented: Versions 1.x.y, where x is an even number, are stable versions, and only bug fixes will be applied as y is incremented. So from version 1.2.2 to 1.2.3, there were only bug fixes, and no new features. Versions 1.x.y, where x is an odd number, are beta-quality releases for developers only, and may be unstable and may crash, and are having new features added to them all the time. From time to time, as the currect development kernel stabilizes, it will be frozen as the new ``stable'' kernel, and development will continue on a new development version of the kernel.
The current stable version is 2.0.24 (this will continue to change as new device drivers get added and bugs fixed), and developement has also started on the experimental 2.1.x kernels. If 2.0.x is too new for you, you may want to stick with 1.2.13 for the time being. However, the latest releases of 2.0 have proved quite stable. Do note that in order to upgrade from 1.2 to 2.0, you need to upgrade some utilities as well; you may wish to upgrade to the latest version of your Linux distribution in order to obtain those utilities. The Linux kernel source code also contains a file, Documentation/Changes, which explains these changes and more.
Most versions of Linux, beta or not, are quite stable, and you can keep using those if they do what you need and you don't want to be on the bleeding edge. One site had a computer running version 0.97 patchlevel 1 (dating from the summer of 1992) for over 136 days without an error or crash. (It would have been longer if the backhoe operator hadn't mistaken a main power transformer for a dumpster...) Others have posted uptimes in excess of a year. One site still had a computer running Linux 0.99pl15s over 600 days at last report.
One thing to be aware of is that Linux is developed using an open and distributed model, instead of a closed and centralized model like much other software. This means that the current development version is always public (with up to a week or two's delay) so that anybody can use it. The result is that whenever a version with new functionality is released, it almost always contains bugs, but it also results in a very rapid development so that the bugs are found and corrected quickly, often in hours, as many people work to fix them.
In contrast, the closed and centralized model means that there is only one person or team working on the project, and they only release software that they think is working well. Often this leads to long intervals between releases, long waiting for bug fixes, and slower development. Of course, the latest release of such software to the public is often of higher quality, but the development speed is generally much slower.
As of October 31, 1996, the current stable version of Linux is 2.0.24, and the latest development version is 2.1.6.
The following is probably the smallest possible configuration that Linux will work on: 386SX/16, 1 MB RAM, 1.44 MB or 1.2 MB floppy, any supported video card (+ keyboards, monitors, and so on of course). This should allow you to boot and test whether it works at all on the machine, but you won't be able to do anything useful. See http://rsphy1.anu.edu.au/~gpg109/mem.html for minimal Linux configurations
In order to do something, you will want some hard disk space as well, 5 to 10 MB should suffice for a very minimal setup (with only the most important commands and perhaps one or two small applications installed, like, say, a terminal program). This is still very, very limited, and very uncomfortable, as it doesn't leave enough room to do just about anything, unless your applications are quite limited. It's generally not recommended for anything but testing if things work, and of course to be able to brag about small resource requirements.
If you are going to run computationally intensive programs, such as gcc, X, and TeX, you will probably want a faster processor than a 386SX/16, but even that should suffice if you are patient.
In practice, you need at least 4 MB of RAM if you don't use X, and 8 MB if you do. Also, if you want to have several users at a time, or run several large programs (compilations for example) at a time, you may want more than 4 MB of memory. It will still work with a smaller amount of memory (should work even with 2 MB), but it will use virtual memory (using the hard drive as slow memory) and that will be so slow as to be unusable. If you use many programs at once, 16 MB will reduce swapping considerably. If you don't want to swap appreciably under any normal load, 32 MB will probably suffice. Of course, if you run memory-hungry applications, you may want more.
The amount of hard disk you need depends on what software you want to install. The normal basic set of Unix utilities, shells, and administrative programs should be comfortable in less than 10 MB, with a bit of room to spare for user files. For a more complete system, get Red Hat, Debian, Slackware, or another distribution, and assume that you will need 60 to 300 MB, depending on what you choose to install and what distribution you get. Add whatever space you want to reserve for user files to these totals. With today's prices on hard drives, if you are buying a new system, it makes no sense to buy a drive that is too small. Get at least 500 MB, preferably 1GB or more, and you will not regret it.
Add more memory, more hard disk, a faster processor and other stuff depending on your needs, wishes and budget to go beyond the merely usable. In general, one big difference from DOS is that with Linux, adding memory makes a large difference, whereas with DOS, extra memory doesn't make that much difference. This of course has something to do with DOS's 640KB limit, which is completely nonexistent under Linux.
CPU:
Anything that runs 386 protected mode programs (all models of 386's 486's, 586's, and 686's should work. 286s and below may someday be supported on a smaller kernel called ELKS (Embeddable Linux Kernel Subset), but don't expect the same capabilities. A version for the 680x0 CPU (for x = 2 with external MMU, 3, 4, and 6) which runs on Amigas and Ataris can be found at tsx-11.mit.edu in the 680x0 directory. Many DEC Alphas are supported. Some Sparcs are now supported. Ports are also being done to the PowerPC, ARM, and MIPS architectures. More details are available elsewhere.
Architecture:
ISA or EISA bus. MCA (mostly true blue PS/2's) support is incomplete (see above). Local busses (VLB and PCI) work. Linux puts higher demands on hardware than DOS, Windows, and in fact most operating systems. This means that some marginal hardware that doesn't fail when running less demanding operating system may fail when running Linux. Linux is an excellent memory tester...
RAM:
Theoretically up to 1 GB; more on 64-bit platforms. This has not been tested. (If you test it, tell me!) Some people (including Linus) have noted that adding ram without adding more cache at the same time has slowed down their machine extremely, so if you add memory and find your machine slower, try adding more cache. Some machines can only cache certain amounts of memory regardless of how much RAM is installed (64 MB is the most one popular chipset can cache). Over 64 MB of memory will require a boot-time parameter, as the BIOS cannot report more than 64MB, because it is ``broken as designed.''
Data storage:
Generic AT drives (EIDE, IDE, 16 bit HD controllers with MFM or RLL, or ESDI) are supported, as are SCSI hard disks and CD-ROMs, with a supported SCSI adaptor. Generic XT controllers (8 bit controllers with MFM or RLL) are also supported. Supported SCSI adaptors: Adaptec 1542, 1522, 1740, 27xx, and 29xx series, Buslogic MultiMaster controllers (Flashpoint support is in beta-testing), NCR53c810-based controllers, DPT controllers, Seagate ST-01 and ST-02, Future Domain TMC-88x series (or any board based on the TMC950 chip) and TMC1660/1680, Ultrastor 14F, 24F and 34F, Western Digital wd7000, and others. SCSI, QIC-02, and some QIC-80 tapes are also supported. Several CD-ROM devices are also supported, including Matsushita/Panasonic, Mitsumi, Sony, Soundblaster, Toshiba, ATAPI (EIDE), SCSI, and others. For exact models, check the hardware compatibility HOWTO.
Video:
VGA, EGA, CGA, or Hercules (and compatibles) work in text mode. For graphics and X, there is support for (at least) normal VGA, some super-VGA cards (most of the cards based on ET3000, ET4000, Paradise, and some Trident chipsets), S3, 8514/A, ATI MACH8, ATI MACH32, and hercules. (Linux uses the Xfree86 X server, so that determines what cards are supported.)
Networking:
Ethernet support includes 3COM 503/509/579/589 (501/505/507 are supported but not recomended), AT&T GIS (ne้ NCR) WaveLAN, most WD8390-based cards, most WD80x3-based cards, NE1000/2000 and most clones, AC3200, Apricot 82596, AT1700, ATP, DE425/434/435/500, D-Link DE-600/620, DEPCA, DE100/101, DE200/201/202 Turbo, DE210, DE422, Cabletron E2100 (not recommended), Intel EtherExpress (not recommended), DEC EtherWORKS 3, HP LAN, HP PCLAN/plus, most AMD LANCE-based cards, NI5210, ni6510, SMC Ultra, DEC 21040 (tulip), Zenith Z-Note ethernet, All Zircom cards and all Cabletron cards other than the E2100 are unsupported, due to the manufacturers unwillingness to release programming information freely.
FDDI support currently includes the DEFxx cards from DEC.
Point-to-Point networking support include PPP, SLIP, CSLIP, and PLIP.
Serial:
Most 16450 and 16550 UART-based boards, including AST Fourport, the Usenet Serial Card II, and others. Intelligent boards supported include Cyclades Cyclom series (supported by the manufacturer), Comtrol Rocketport series (supported by the manufacturer), Stallion (most boards; supported by the manufacturer), and Digi (some boards; not manufacturer-supported). Some ISDN, frame relay, and leased line hardware is supported.
Other hardware:
SoundBlaster, ProAudio Spectrum 16, Gravis Ultrasound, most other sound cards, most (all?) flavours of bus mice (Microsoft, Logitech, PS/2), etc.
Most of the common Unix tools and programs have been ported to Linux, including almost all of the GNU stuff and many X clients from various sources. Actually, ported is often too strong a word, since many programs compile out of the box without modifications, or only small modifications, because Linux tracks POSIX quite closely. Unfortunately, there are not very many end-user applications at this time, but recently, this has begun to change. Here is an incomplete list of software that is known to work under Linux.
Basic Unix commands:
ls, tr, sed, awk and so on (you name it, Linux probably has it).
Development tools:
gcc, gdb, make, bison, flex, perl, rcs, cvs, prof.
Languages and Environments:
C, C++, Objective C, Java, Modula-3, Modula-2, Oberon, Ada95, Pascal, Fortran, ML, scheme, Tcl/tk, Perl, Python, Common Lisp, and many others.
Graphical environments:
X11R5 (XFree86 2.x), X11R6 (XFree86 3.x), MGR.
Editors:
GNU Emacs, XEmacs, MicroEmacs, jove, ez, epoch, elvis (GNU vi), vim, vile, joe, pico, jed, and others.
Shells:
bash (POSIX sh-compatible), zsh (includes ksh compatiblity mode), pdksh, tcsh, csh, rc, es, ash (mostly sh-compatible shell used as /bin/sh by BSD), and many more.
Telecommunication:
Taylor (BNU-compatible) UUCP, SLIP, CSLIP, PPP, kermit, szrz, minicom, pcomm, xcomm, term (runs multiple shells, redirects network activity, and allows remote X, all over one modem line), Seyon (popular X-windows communications program), and several fax and voice-mail (using ZyXEL and other modems) packages are available. Of course, remote serial logins are supported.
News and mail:
C-news, innd, trn, nn, tin, smail, elm, mh, pine, etc.
Textprocessing:
TeX, groff, doc, ez, Linuxdoc-SGML, and others.
Games:
Nethack, several Muds and X games, and lots of others. One of those games is looking through all the games available at tsx-11 and sunsite.
Suites:
AUIS, the Andrew User Interface System. ez is part of this suite.
All of these programs (and this isn't even a hundredth of what is available) are freely available. Commercial software is becoming widely available; ask the vendor of your favorite package if they support Linux.
Although Linux is supplied with the complete source code, it is copyrighted software, not public domain. However, it is available for free under the GNU General Public License, sometimes referred to as the ``copyleft''. See the GPL for more information. The programs that run under Linux each have their own copyright, although many of them use the GPL as well. X uses the MIT X copyright, and some utilities are under the BSD copyright. In any case, all of the software on the FTP site is freely distributable (or else it shouldn't be there).