- multitasking: several programs running at once.
- multiuser: several users on the same machine at once
(and no two-user licenses!).
- multiplatform: runs on many different CPUs, not just Intel.
- multiprocessor: SMP support is available on the Intel and
SPARC platforms (with work currently in progress on other
platforms), and Linux is used in several loosely-coupled MP
applications, including Beowulf systems (see
http://cesdis.gsfc.nasa.gov/linux-web/beowulf/beowulf.html)
and the Fujitsu AP1000+ SPARC-based supercomputer.
- runs in protected mode on the 386.
- has memory protection between processes, so that one program can't
bring the whole system down.
- demand loads executables: Linux only reads from disk those parts of a
program that are actually used.
- shared copy-on-write pages among executables. This means that
multiple process can use the same memory to run in. When one
tries to write to that memory, that page (4KB piece of memory)
is copied somewhere else. Copy-on-write has two benefits:
increasing speed and decreasing memory use.
- virtual memory using paging (not swapping whole processes) to
disk: to a separate partition or a file in the filesystem, or
both, with the possibility of adding more swapping areas during
runtime (yes, they're still called swapping areas). A total of 16
of these 128 MB swapping areas can be used at once, for a
theoretical total of 2 GB of useable swap space. It is simple
to increase this if necessary, by changing a few lines of source code.
- a unified memory pool for user programs and disk cache, so
that all free memory can be used for caching, and the cache
can be reduced when running large programs.
- dynamically linked shared libraries (DLL's), and static libraries
too, of course.
- does core dumps for post-mortem analysis, allowing
the use of a debugger on a program not only while it is running
but also after it has crashed.
- mostly compatible with POSIX, System V, and BSD at the source
level.
- through an iBCS2-compliant emulation module, mostly compatible with SCO,
SVR3, and SVR4 at the binary level.
- all source code is available, including the whole kernel and
all drivers, the development tools and all user programs;
also, all of it is freely distributable. Plenty of
commercial programs are being provided for Linux without
source, but everything that has been free, including the
entire base operating system, is still free.
- POSIX job control.
- pseudoterminals (pty's).
- 387-emulation in the kernel so that programs don't need to
do their own math emulation. Every computer running Linux
appears to have a math coprocessor. Of course, if your
computer already contains an FPU, it will be used instead
of the emulation, and you can even compile your own kernel
with math emulation removed, for a small memory gain.
- support for many national or customized keyboards, and it is
fairly easy to add new ones dynamically.
- multiple virtual consoles: several independent login sessions
through the console, you switch by pressing a hot-key
combination (not dependent on video hardware). These are
dynamically allocated; you can use up to 64.
- Supports several common filesystems, including minix,
Xenix, and all the common system V filesystems, and has an advanced
filesystem of its own, which offers filesystems of up to 4 TB,
and names up to 255 characters long.
- transparent access to MS-DOS partitions (or OS/2 FAT
partitions) via a special filesystem: you don't need any
special commands to use the MS-DOS partition, it looks just
like a normal Unix filesystem (except for funny restrictions
on filenames, permissions, and so on). MS-DOS 6 compressed
partitions do not work at this time without a patch (dmsdosfs).
VFAT (WNT, Windows 95) support is available in Linux 2.0
- special filesystem called UMSDOS which allows Linux to be
installed on a DOS filesystem.
- read-only HPFS-2 support for OS/2 2.1
- HFS (Macintosh) file system support is available separately as
a module.
- CD-ROM filesystem which reads all standard formats of CD-ROMs.
- TCP/IP networking, including ftp, telnet, NFS, etc.
- Appletalk server
- Netware client and server
- Lan Manager (SMB) client and server
- Many networking protocols: the base protocols available in the
latest development kernels include TCP, IPv4, IPv6, AX.25, X.25,
IPX, DDP (Appletalk), NetBEUI, Netrom, and others. Stable network
protocols included in the stable kernels currently include TCP, IPv4,
IPX, DDP, and AX.25.