INTRO(1) INTRO(1)
NAME
intro - introduction to Plan 9
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.
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
/e1/e2/.../en
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 con-
vention. 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
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INTRO(1) INTRO(1)
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 read-
ing and writing files within the tree.
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 dia-
log 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 ini-
tial name space. See namespace(4), boot(8), and init(8) for
details, but some important aspects of the initial name
space are:
+o The environment variable $cputype is set to the name of
the kernel's CPU's architecture: one of alpha, mips,
sparc, power (Power PC), 386 (386, 486, Pentium, ...)
etc. The environment variable $objtype is initially the
same as $cputype.
+o The environment variable $terminal is set to a descrip-
tion 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; e.g., alpha apc axp
is bootstrapped from /alpha/9apc.
+o The environment variable $service is set to terminal.
(Other ways of accessing Plan 9 may set $service to one
of cpu, con, or rx.)
+o The environment variable $user is set to the name of the
user who booted the terminal. The environment variable
$home is set to that user's home directory.
+o /$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
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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
}
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 vari-
able, such as running the window system rio(1) on a termi-
nal.
To do heavy work such as compiling, the cpu(1) command con-
nects a window to a CPU server; the same environment vari-
ables 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 exe-
cuted. 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 pro-
grams 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 differ-
ences, however.
The standard shell is rc(1), not the Bourne shell. The most
noticeable differences appear only when programming and
macro processing.
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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 debug-
ging 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;
il for the Plan 9 Internet protocol and and net for the sys-
tem default.
Login connections and remote execution on non-Plan-9
machines are usually done by saying, for example,
con kremvax
or
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
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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).
SEE ALSO
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.
Section (9) for kernel functions.
Section (10) for site-specific commands.
/sys/doc for copies of papers referenced in this manual.
The back of this volume has a permuted index to aid
searches.
DIAGNOSTICS
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.
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