intro – introduction to Plan 9


Plan 9 is a distributed computing environment assembled from separate machines acting as terminals, CPU servers, and file servers. A user works at a terminal, running a window system on a raster display. Some windows are connected to CPU servers; the intent is that heavy computing should be done in those windows but it is also possible to compute on the terminal. A separate file server provides file storage for terminals and CPU servers alike.

Name Spaces

In Plan 9, almost all objects look like files. The object retrieved by a given name is determined by a mapping called the name space . A quick tour of the standard name space is in namespace(4). Every program running in Plan 9 belongs to a process group (see rfork in fork(2)), and the name space for each process group can be independently customized.

A name space is hierarchically structured. A full file name (also called a full path name ) has the form


This represents an object in a tree of files: the tree has a root, represented by the first /; the root has a child file named e1, which in turn has child e2, and so on; the descendent en is the object represented by the path name.

There are a number of Plan 9 services available, each of which provides a tree of files. A name space is built by binding services (or subtrees of services) to names in the name-space-so-far. Typically, a user’s home file server is bound to the root of the name space, and other services are bound to conventionally named subdirectories. For example, there is a service resident in the operating system for accessing hardware devices and that is bound to /dev by convention. Kernel services have names (outside the name space) that are a # sign followed by a single letter; for example, #c is conventionally bound to /dev.

Plan 9 has union directories : directories made of several directories all bound to the same name. The directories making up a union directory are ordered in a list. When the bindings are made (see bind(1)), flags specify whether a newly bound member goes at the head or the tail of the list or completely replaces the list. To look up a name in a union directory, each member directory is searched in list order until the name is found. A bind flag specifies whether file creation is allowed in a member directory: a file created in the union directory goes in the first member directory in list order that allows creation, if any.

The glue that holds Plan 9 together is a network protocol called 9P, described in section 5 of this manual. All Plan 9 servers read and respond to 9P requests to navigate through a file tree and to perform operations such as reading and writing files within the tree.


When a terminal is powered on or reset, it must be told the name of a file server to boot from, the operating system kernel to boot, and a user name and password. How this dialog proceeds is environment- and machine-dependent. Once it is complete, the terminal loads a Plan 9 kernel, which sets some environment variables (see env(3)) and builds an initial name space. See namespace(4), boot(8), and init(8) for details, but some important aspects of the initial name space are:

The environment variable $cputype is set to the name of the kernel’s CPU’s architecture: one of mips, sparc, power (Power PC), 386 (386, 486, Pentium, ...) etc. The environment variable $objtype is initially the same as $cputype.  

The environment variable $terminal is set to a description of the machine running the kernel, such as generic pc . Sometimes the middle word of $terminal encodes the file from which the kernel is booted.  

The environment variable $service is set to terminal. (Other ways of accessing Plan 9 may set $service to one of cpu, con, or rx.)  

The environment variable $user is set to the name of the user who booted the terminal. The environment variable $home is set to that user’s home directory.  

/$cputype/bin and /rc/bin are unioned into /bin.  

After booting, the terminal runs the command interpreter, rc(1), on /usr/$user/lib/profile after moving to the user’s home directory.

Here is a typical profile:


bind -a $home/bin/rc /bin
bind -a $home/bin/$cputype /bin
bind -c $home/tmp /tmp
font = /lib/font/bit/pelm/euro.9.font
case terminal
	prompt=('term% ' '	')
	exec rio -f $font
case cpu
	bind /mnt/term/dev/cons /dev/cons
	bind /mnt/term/dev/consctl /dev/consctl
	bind -a /mnt/term/mnt/wsys /dev
	prompt=('cpu% ' '	')
case con
	prompt=('cpu% ' '	')

The first three lines replace /tmp with a tmp in the user’s home directory and union personal bin directories with /bin, to be searched after the standard bin directories. The next starts the mail file system; see mail(1). Then different things happen, depending on the $service environment variable, such as running the window system rio(1) on a terminal.

To do heavy work such as compiling, the cpu(1) command connects a window to a CPU server; the same environment variables are set (to different values) and the same profile is run. The initial directory is the current directory in the terminal window where cpu was typed. The value of $service will be cpu, so the second arm of the profile switch is executed. The root of the terminal’s name space is accessible through /mnt/term, so the bind is a way of making the window system’s graphics interface (see draw(3)) available to programs running on the CPU server. The news(1) command reports current Plan 9 affairs.

The third possible service type, con, is set when the CPU server is called from a non-Plan-9 machine, such as through telnet (see con(1)).

Using Plan 9

The user commands of Plan 9 are reminiscent of those in Research Unix, version 10. There are a number of differences, however.

The standard shell is rc(1), not the Bourne shell. The most noticeable differences appear only when programming and macro processing.

The character-delete character is backspace, and the line-kill character is control-U; these cannot be changed.

DEL is the interrupt character: typing it sends an interrupt to processes running in that window. See keyboard(6) for instructions on typing characters like DEL on the various keyboards.

If a program dies with something like an address error, it enters a ‘Broken’ state. It lingers, available for debugging with db(1) or acid(1). Broke (see kill(1)) cleans up broken processes.

The standard editor is one of acme(1) or sam(1). There is a variant of sam that permits running the file-manipulating part of sam on a non-Plan-9 system:


sam -r tcp!kremvax

For historical reasons, sam uses a tab stop setting of 8 spaces, while the other editors and window systems use 4 spaces. These defaults can be overridden by setting the value of the environment variable $tabstop to the desired number of spaces per tab.

Machine names may be prefixed by the network name, here tcp; and net for the system default.

Login connections and remote execution on non-Plan-9 machines are usually done by saying, for example,


con kremvax



rx deepthought chess

(see con(1)).

9fs connects to file systems of remote systems (see srv(4)). For example,


9fs kremvax

sets things up so that the root of kremvax’s file tree is visible locally in /n/kremvax.

Faces(1) gives graphical notification of arriving mail.

The Plan 9 file server has an integrated backup facility. The command


9fs dump

binds to /n/dump a tree containing the daily backups on the file server. The dump tree has years as top level file names, and month-day as next level file names. For example, /n/dump/2000/0120 is the root of the file system as it appeared at dump time on January 20, 2000. If more than one dump is taken on the same day, dumps after the first have an extra digit. To recover the version of this file as it was on June 15, 1999,


cp /n/dump/1999/0615/sys/man/1/0intro .

or use yesterday(1).


This section for general publicly accessible commands.
Section (2) for library functions, including system calls.
Section (3) for kernel devices (accessed via bind(1)).
Section (4) for file services (accessed via mount).
Section (5) for the Plan 9 file protocol.
Section (6) for file formats.
Section (7) for databases and database access programs.
Section (8) for things related to administering Plan 9.
/sys/doc for copies of papers referenced in this manual.

The back of this volume has a permuted index to aid searches.


Upon termination each program returns a string called the exit status . It was either supplied by a call to exits(2) or was written to the command’s /proc/pid/note file (see proc(3)), causing an abnormal termination. The empty string is customary for successful execution; a non-empty string gives a clue to the failure of the command.