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intro - introduction to Plan 9 from User Space
Plan 9 is a distributed computing environment built at Bell
Labs starting in the late 1980s. The system can be obtained
from Bell Labs at and runs
on PCs and a variety of other platforms. Plan 9 became a
convenient platform for experimenting with new ideas, appli-
cations, and services.
Plan 9 from User Space provides many of the ideas, applica-
tions, and services from Plan 9 on Unix-like systems. It
runs on FreeBSD (x86, x86-64), Linux (x86, x86-64, PowerPC
and ARM), Mac OS X (x86, x86-64, and PowerPC), NetBSD (x86
and PowerPC), OpenBSD (x86 and PowerPC), Dragonfly BSD
(x86-64), and SunOS (x86-64 and Sparc).
Plan 9 from User Space expects its own directory tree, con-
ventionally /usr/local/plan9. When programs need to access
files in the tree, they expect the $PLAN9 environment vari-
able to contain the name of the root of the tree. See
install(1) for details about installation.
Many of the familiar Unix commands, for example cat(1),
ls(1), and wc(1), are present, but in their Plan 9 forms:
cat takes no options, ls does not columnate its output when
printing to a terminal, and wc counts UTF characters. In
some cases, the differences are quite noticeable: grep(1)
and sed(1) expect Plan 9 regular expressions (see
regexp(7)), which are closest to what Unix calls extended
regular expressions. Because of these differences, it is
not recommended to put $PLAN9/bin before the usual system
bin directories in your search path. Instead, put it at the
end of your path and use the 9(1) script when you want to
invoke the Plan 9 version of a traditional Unix command.
Occasionally the Plan 9 programs have been changed to adapt
to Unix. Mk(1) now allows mkfiles to choose their own
shell, and rc(1) has a ulimit builtin and manages $PATH.
Many of the graphical programs from Plan 9 are present,
including sam(1) and acme(1). An X11 window manager rio(1)
mimics Plan 9's window system, with command windows imple-
mented by the external program 9term(1). Following the style
of X Windows, these programs run in new windows rather than
the one in which they are invoked. They all take a -W
option to specify the size and placement of the new window.
The argument is one of widthxheight, widthxheight@xmin,xmax,
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or xmin,ymin,xmax,ymax.
The plumber(4) helps to connect the various Plan 9 programs
together, and fittings like web(1) connect it to external
programs such as web browsers; one can click on a URL in
acme and see the page load in Firefox.
User-level file servers
In Plan 9, user-level file servers present file trees via
the Plan 9 file protocol, 9P. Processes can mount arbitrary
file servers and customize their own name spaces. These
facilities are used to connect programs. Clients interact
with file servers by reading and writing files.
This cannot be done directly on Unix. Instead the servers
listen for 9P connections on Unix domain sockets; clients
connect to these sockets and speak 9P directly using the
9pclient(3) library. Intro(4) tells more of the story. The
effect is not as clean as on Plan 9, but it gets the job
done and still provides a uniform and easy-to-understand
mechanism. The 9p(1) client can be used in shell scripts or
by hand to carry out simple interactions with servers.
Netfiles(1) is an experimental client for acme.
External databases
Some programs rely on large databases that would be cumber-
some to include in every release. Scripts are provided that
download these databases separately. These databases can be
downloaded separately. See $PLAN9/dict/README and
The shell scripts 9c and 9l (see 9c(1)) provide a simple
interface to the underlying system compiler and linker, sim-
ilar to the 2c and 2l families on Plan 9. 9c compiles
source files, and 9l links object files into executables.
When using Plan 9 libraries, 9l infers the correct set of
libraries from the object files, so that no -l options are
The only way to write multithreaded programs is to use the
thread(3) library. Rfork(3) exists but is not as capable as
on Plan 9. There are many unfortunate by necessary prepro-
cessor diversions to make Plan 9 and Unix libraries coexist.
See intro(3) for details.
The debuggers acid(1) and db(1) and the debugging library
mach(3) are works in progress. They are platform-
independent, so that x86 Linux core dumps can be inspected
on PowerPC Mac OS X machines, but they are also fairly
incomplete. The x86 target is the most mature; initial Pow-
erPC support exists; and other targets are unimplemented.
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The debuggers can only inspect, not manipulate, target pro-
cesses. Support for operating system threads and for 64-bit
architectures needs to be rethought. On x86 Linux systems,
acid and db can be relied upon to produce reasonable stack
traces (often in cases when GNU gdb cannot) and dump data
structures, but that it is the extent to which they have
been developed and exercised.
Porting programs
The vast majority of the familiar Plan 9 programs have been
ported, including the Unicode-aware troff(1).
Of the more recent additions to Plan 9, factotum(4),
secstore(1), and secstored(1), vac(1), vacfs(4), and
venti(8) are all ported.
A backup system providing a dump file system built atop
Venti is in progress; see vbackup(8).
Porting to new systems
Porting the tree to new operating systems or architectures
should be straightforward, as system-specific code has been
kept to a minimum. The largest pieces of system-specific
code are <u.h>, which must include the right system files
and set up the right integer type definitions, and
libthread, which must implement spin locks, operating system
thread creation, and context switching routines. Portable
implementations of these using <pthread.h> and <ucontext.h>
already exist. If your system supports them, you may not
need to write any system specific code at all.
There are other smaller system dependencies, such as the
terminal handling code in 9term(1) and the implementation of
getcallerpc(3), but these are usually simple and are not on
the critical path for getting the system up and running.
The rest of this manual describes Plan 9 from User Space.
Many of the man pages have been brought from Plan 9, but
they have been updated, and others have been written from
The manual pages are in a Unix style tree, with names like
$PLAN9/man/man1/cat.1 instead of Plan 9's simpler
$PLAN9/man/1/cat, so that the Unix man(1) utility can handle
it. Some systems, for example Debian Linux, deduce the man
page locations from the search path, so that adding
$PLAN9/bin to your path is sufficient to cause $PLAN9/man to
be consulted for manual pages using the system man. On other
systems, or to look at manual pages with the same name as a
system page, invoke the Plan 9 man directly, as in 9 man
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The manual sections follow the Unix numbering conventions,
not the Plan 9 ones.
Section (1) describes general publicly accessible commands.
Section (3) describes C library functions.
Section (4) describes user-level file servers.
Section (7) describes file formats and protocols. (On Unix,
section (5) is technically for file formats but seems now to
be used for describing specific files.)
Section (8) describes commands used for system administra-
Section (9p) describes the Plan 9 file protocol 9P.
These pages describe parts of the system that are new or
different from Plan 9 from Bell Labs:
9(1), 9c(1), 9p(1), 9term(1), acidtypes in acid(1),
dial(1), git(1), label(1), the MKSHELL variable in
mk(1), namespace(1), netfiles(1), page(1), psfonts(1),
rio(1), web(1), wintext(1)
intro(3), 9pclient(3), the unix network in dial(3),
exits(3), get9root(3), getns(3), notify(3),
post9pservice(3), rfork(3), searchpath(3), sendfd(3),
udpread(3), venti(3), wait(3), wctl(3)
intro(4), 9pserve(4), import(4),
In Plan 9, a program's exit status is an arbitrary text
string, while on Unix it is an integer. Section (1) of this
manual describes commands as though they exit with string
statuses. In fact, exiting with an empty status corresponds
to exiting with status 0, and exiting with any non-empty
string corresponds to exiting with status 1. See exits(3).
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