.TH INTRO 1
.SH NAME
intro \- introduction to Plan 9
.SH DESCRIPTION
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.
.SS 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
.IR "name space" .
A quick tour of the standard name space is in
.IR namespace (4).
Every program running in Plan 9 belongs to a
.I process group
(see
.I rfork
in
.IR fork (2)),
and the name space for each process group can be independently
customized.
.PP
A name space is hierarchically structured.
A full file name (also called a
.IR "full path name" )
has the form
.IP
.RI / e1 / e2 /.../ en
.PP
This represents an object in a tree of files: the tree has a root,
represented by the first
.LR / ;
the root has a child file named
.IR e1 ,
which in turn has child
.IR e2 ,
and so on; the descendent
.I en
is the object represented by the path name.
.PP
There are a number of Plan 9
.I services
available, each of which provides a tree of files.
A name space is built by
.I 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
.B /dev
by convention.
Kernel services have names (outside the name space) that are a
.L #
sign followed by a single letter;
for example,
.B #c
is conventionally bound to
.BR /dev .
.PP
Plan 9 has
.IR "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
.IR 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.
.PP
The glue that holds Plan 9 together is a network protocol called
.IR 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.
.SS Booting
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
.IR env (3))
and builds an initial name space.
See
.IR namespace (4),
.IR boot (8),
and
.IR init (8)
for details, but some important aspects of the initial name space are:
.IP \(bu
The environment variable
.B $cputype
is set to the name of the kernel's CPU's architecture: one of
.BR alpha ,
.BR mips ,
.BR sparc ,
.B power
(Power PC),
.BR 386
(386, 486, Pentium, ...)
etc.
The environment variable
.B $objtype
is initially the same as
.BR $cputype .
.IP \(bu
The environment variable
.B $terminal
is set to a description of the machine running the kernel,
such as
.BR "generic pc" .
Sometimes the middle word of
.B $terminal
encodes the file from which the kernel is booted;
e.g.
.B "alpha apc axp
is bootstrapped from
.BR /alpha/bapc .
.IP \(bu
The environment variable
.B $service
is set to
.BR terminal .
(Other ways of accessing Plan 9 may set
.B $service
to one of
.BR cpu ,
.BR con ,
or
.BR rx .)
.IP \(bu
The environment variable
.B $user
is set to the name of the user who booted the terminal.
The environment variable
.B $home
is set to that user's home directory.
.IP \(bu
.B /$cputype/bin
and
.B /rc/bin
are unioned into
.BR /bin .
.PD
.PP
After booting, the terminal runs the command interpreter,
.IR rc (1),
on
.B /usr/$user/lib/profile
after moving to the user's home directory.
.PP
Here is a typical profile:
.IP
.EX
bind -a $home/bin/rc /bin
bind -a $home/bin/$cputype /bin
bind -c $home/tmp /tmp
font = /lib/font/bit/pelm/euro.9.font
upas/fs
switch($service){
case terminal
	plumber
	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% ' '	')
	news
case con
	prompt=('cpu% ' '	')
	news
}
.EE
.PD
.PP
The first three lines replace
.B /tmp
with a
.B tmp
in the user's home directory
and union personal
.B bin
directories with
.BR /bin ,
to be searched after the standard
.B bin
directories.
The next starts the mail file system; see
.IR mail (1).
Then different things happen, depending on the
.B $service
environment variable,
such as running the window system
.IR rio (1)
on a terminal.
.PP
To do heavy work such as compiling, the
.IR 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
.I cpu
was typed.
The value of
.B $service
will be
.BR cpu ,
so the second arm of the profile switch is executed.
The root of the terminal's name space is accessible through
.BR /mnt/term ,
so the
.I bind
is a way of making the window system's graphics interface (see
.IR draw (3))
available to programs running on the CPU server.
The
.IR news (1)
command reports current Plan 9 affairs.
.PP
The third possible service type,
.BR con ,
is set when the CPU server is called from a non-Plan-9 machine,
such as through
.I telnet
(see
.IR con (1)).
.SS 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.
.PP
The standard shell is
.IR rc (1),
not the Bourne shell.
The most noticeable differences appear only when programming and macro processing.
.PP
The character-delete character is backspace, and the line-kill character is
control-U; these cannot be changed.
.PP
DEL is the interrupt character: typing it sends an interrupt to processes running in that window.
See
.IR keyboard (6)
for instructions on typing characters like DEL on the various keyboards.
.PP
If a program dies with something like an address error, it enters a `Broken'
state.  It lingers, available for debugging with
.IR db (1)
or
.IR acid (1).
.I Broke
(see
.IR kill (1))
cleans up broken processes.
.PP
The standard editor is one of
.IR acme (1)
or
.IR sam (1).
There is a variant of
.I sam
that permits running the file-manipulating part of
.I sam
on a non-Plan-9 system:
.IP
.EX
sam -r tcp!kremvax
.EE
.PP
For historical reasons,
.I 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
.B $tabstop
to the desired number of spaces per tab.
.PP
Machine names may be prefixed by the network name,
here
.BR tcp ;
.B il
for the Plan 9 Internet protocol and
.B net
for the system default.
.PP
Login connections and remote execution on non-Plan-9 machines are usually
done by saying, for example,
.IP
.EX
con kremvax
.EE
.PP
or
.IP
.EX
rx deepthought chess
.EE
.PP
(see
.IR con (1)).
.PP
.I 9fs 
connects to file systems of remote systems
(see
.IR srv (4)).
For example,
.IP
.EX
9fs kremvax
.EE
.PP
sets things up so that the root of
.BR kremvax 's
file tree is visible locally in
.BR /n/kremvax .
.PP
.IR Faces (1)
gives graphical notification of arriving mail.
.PP
The Plan 9 file server has an integrated backup facility.
The command
.IP
.EX
9fs dump
.EE
.PP
binds to
.B /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,
.B /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,
.IP
.EX
cp /n/dump/1999/0615/sys/man/1/0intro .
.EE
.PP
or use
.IR yesterday (1).
.SH SEE ALSO
This section for general publicly accessible commands.
.br
Section (2) for library functions, including system calls.
.br
Section (3) for kernel devices (accessed via
.IR bind (1)).
.br
Section (4) for file services (accessed via
.IR mount ).
.br
Section (5) for the Plan 9 file protocol.
.br
Section (6) for file formats.
.br
Section (7) for databases and database access programs.
.br
Section (8) for things related to administering Plan 9.
.br
.B /sys/doc
for copies of papers referenced in this manual.
.PP
The back of this volume has a permuted index to aid searches.
.SH DIAGNOSTICS
Upon termination each program returns a string called the
.IR "exit status" .
It was either supplied by a call to
.IR exits (2)
or was written to the command's
.BI /proc/ pid /note
file
(see
.IR 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.