<s>
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
is	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
which	O
originated	O
from	O
the	O
Computing	O
Science	O
Research	O
Center	O
(	O
CSRC	O
)	O
at	O
Bell	O
Labs	O
in	O
the	O
mid-1980s	O
and	O
built	O
on	O
UNIX	B-Application
concepts	O
first	O
developed	O
there	O
in	O
the	O
late	O
1960s	O
.	O
</s>
<s>
Since	O
2000	O
,	O
Plan	B-Operating_System
9	I-Operating_System
has	O
been	O
free	B-License
and	I-License
open-source	I-License
.	O
</s>
<s>
Under	O
Plan	B-Operating_System
9	I-Operating_System
,	O
UNIX	B-Application
's	O
everything	B-Application
is	I-Application
a	I-Application
file	I-Application
metaphor	O
is	O
extended	O
via	O
a	O
pervasive	O
network-centric	O
filesystem	B-Application
,	O
and	O
the	O
cursor-addressed	B-General_Concept
,	O
terminal-based	O
I/O	B-General_Concept
at	O
the	O
heart	O
of	O
UNIX-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
is	O
replaced	O
by	O
a	O
windowing	B-Application
system	I-Application
and	O
graphical	B-Application
user	I-Application
interface	I-Application
without	O
cursor	O
addressing	O
,	O
although	O
rc	B-Operating_System
,	O
the	O
Plan	B-Operating_System
9	I-Operating_System
shell	B-Operating_System
,	O
is	O
text-based	O
.	O
</s>
<s>
The	O
name	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
is	O
a	O
reference	O
to	O
the	O
Ed	O
Wood	O
1957	O
cult	O
science	O
fiction	O
Z-movie	O
Plan	B-Application
9	I-Application
from	I-Application
Outer	I-Application
Space	I-Application
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
was	O
originally	O
developed	O
,	O
starting	O
in	O
the	O
late	O
1980s	O
,	O
by	O
members	O
of	O
the	O
Computing	O
Science	O
Research	O
Center	O
at	O
Bell	O
Labs	O
,	O
the	O
same	O
group	O
that	O
originally	O
developed	O
Unix	B-Application
and	O
the	O
C	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
The	O
Plan	B-Operating_System
9	I-Operating_System
team	O
was	O
initially	O
led	O
by	O
Rob	O
Pike	O
,	O
Ken	O
Thompson	O
,	O
Dave	O
Presotto	O
and	O
Phil	O
Winterbottom	O
,	O
with	O
support	O
from	O
Dennis	O
Ritchie	O
as	O
head	O
of	O
the	O
Computing	O
Techniques	O
Research	O
Department	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
replaced	O
Unix	B-Application
as	O
Bell	O
Labs	O
's	O
primary	O
platform	O
for	O
operating	O
systems	O
research	O
.	O
</s>
<s>
It	O
explored	O
several	O
changes	O
to	O
the	O
original	O
Unix	B-Application
model	O
that	O
facilitate	O
the	O
use	O
and	O
programming	O
of	O
the	O
system	O
,	O
notably	O
in	O
distributed	O
multi-user	B-Operating_System
environments	O
.	O
</s>
<s>
Three	O
years	O
later	O
,	O
Plan	B-Operating_System
9	I-Operating_System
was	O
made	O
available	O
for	O
commercial	O
parties	O
by	O
AT&T	O
via	O
the	O
book	O
publisher	O
Harcourt	O
Brace	O
.	O
</s>
<s>
By	O
early	O
1996	O
,	O
the	O
Plan	B-Operating_System
9	I-Operating_System
project	O
had	O
been	O
"	O
put	O
on	O
the	O
back	O
burner	O
"	O
by	O
AT&T	O
in	O
favor	O
of	O
Inferno	B-Operating_System
,	O
intended	O
to	O
be	O
a	O
rival	O
to	O
Sun	O
Microsystems	O
 '	O
Java	B-Device
platform	I-Device
.	O
</s>
<s>
In	O
the	O
late	O
1990s	O
,	O
Bell	O
Labs	O
 '	O
new	O
owner	O
Lucent	O
Technologies	O
dropped	O
commercial	O
support	O
for	O
the	O
project	O
and	O
in	O
2000	O
,	O
a	O
third	O
release	O
was	O
distributed	O
under	O
an	O
open-source	B-Application
license	O
.	O
</s>
<s>
A	O
fourth	O
release	O
under	O
a	O
new	O
free	B-Application
software	I-Application
license	O
occurred	O
in	O
2002	O
.	O
</s>
<s>
In	O
early	O
2015	O
,	O
the	O
final	O
official	O
release	O
of	O
Plan	B-Operating_System
9	I-Operating_System
occurred	O
.	O
</s>
<s>
A	O
user	O
and	O
development	O
community	O
,	O
including	O
current	O
and	O
former	O
Bell	O
Labs	O
personnel	O
,	O
produced	O
minor	O
daily	O
releases	O
in	O
the	O
form	O
of	O
ISO	B-General_Concept
images	I-General_Concept
.	O
</s>
<s>
The	O
development	O
source	O
tree	O
is	O
accessible	O
over	O
the	O
9P	B-Operating_System
and	O
HTTP	B-Protocol
protocols	I-Protocol
and	O
is	O
used	O
to	O
update	O
existing	O
installations	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
official	O
components	O
of	O
the	O
OS	O
included	O
in	O
the	O
ISOs	B-General_Concept
,	O
Bell	O
Labs	O
also	O
hosts	O
a	O
repository	O
of	O
externally	O
developed	O
applications	O
and	O
tools	O
.	O
</s>
<s>
As	O
Bell	O
Labs	O
has	O
moved	O
on	O
to	O
later	O
projects	O
in	O
recent	O
years	O
,	O
development	O
of	O
the	O
official	O
Plan	B-Operating_System
9	I-Operating_System
system	O
had	O
stopped	O
.	O
</s>
<s>
On	O
March	O
23	O
,	O
2021	O
,	O
development	O
resumed	O
following	O
the	O
transfer	O
of	O
copyright	O
from	O
Bell	O
Labs	O
to	O
the	O
Plan	B-Operating_System
9	I-Operating_System
Foundation	I-Operating_System
.	O
</s>
<s>
Unofficial	O
development	O
for	O
the	O
system	O
also	O
continues	O
on	O
the	O
9front	B-Operating_System
fork	B-Application
,	O
where	O
active	O
contributors	O
provide	O
monthly	O
builds	O
and	O
new	O
functionality	O
.	O
</s>
<s>
So	O
far	O
,	O
the	O
9front	B-Operating_System
fork	B-Application
has	O
provided	O
the	O
system	O
Wi-Fi	O
drivers	O
,	O
Audio	O
drivers	O
,	O
USB	B-Protocol
support	O
and	O
built-in	O
game	O
emulator	O
,	O
along	O
with	O
other	O
features	O
.	O
</s>
<s>
Date	O
Release	O
Comment	O
1992	O
Plan	B-Operating_System
9	I-Operating_System
1st	O
edition	O
Released	O
by	O
Bell	O
Labs	O
to	O
universities	O
1995	O
Plan	B-Operating_System
9	I-Operating_System
2nd	O
edition	O
Released	O
by	O
Bell	O
Labs	O
for	O
non-commercial	O
purposes	O
2000	O
Plan	B-Operating_System
9	I-Operating_System
3rd	O
ed	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
is	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
designed	O
to	O
make	O
a	O
network	O
of	O
heterogeneous	O
and	O
geographically	O
separated	O
computers	O
function	O
as	O
a	O
single	O
system	O
.	O
</s>
<s>
In	O
a	O
typical	O
Plan	B-Operating_System
9	I-Operating_System
installation	O
,	O
users	O
work	O
at	O
terminals	O
running	O
the	O
window	B-Application
system	I-Application
rio	B-Language
,	O
and	O
they	O
access	O
CPU	O
servers	O
which	O
handle	O
computation-intensive	O
processes	B-Operating_System
.	O
</s>
<s>
The	O
first	O
idea	O
(	O
a	O
per-process	O
name	O
space	O
)	O
means	O
that	O
,	O
unlike	O
on	O
most	O
operating	O
systems	O
,	O
processes	B-Operating_System
(	O
running	O
programs	O
)	O
each	O
have	O
their	O
own	O
view	O
of	O
the	O
namespace	O
,	O
corresponding	O
to	O
what	O
other	O
operating	O
systems	O
call	O
the	O
file	B-Application
system	I-Application
;	O
a	O
single	O
path	O
name	O
may	O
refer	O
to	O
different	O
resources	O
for	O
different	O
processes	B-Operating_System
.	O
</s>
<s>
The	O
second	O
idea	O
(	O
a	O
message-oriented	O
filesystem	B-Application
)	O
means	O
that	O
processes	B-Operating_System
can	O
offer	O
their	O
services	O
to	O
other	O
processes	B-Operating_System
by	O
providing	O
virtual	O
files	O
that	O
appear	O
in	O
the	O
other	O
processes	B-Operating_System
 '	O
namespace	O
.	O
</s>
<s>
The	O
client	B-Operating_System
process	B-Operating_System
's	O
input/output	B-General_Concept
on	O
such	O
a	O
file	O
becomes	O
inter-process	B-Operating_System
communication	I-Operating_System
between	O
the	O
two	O
processes	B-Operating_System
.	O
</s>
<s>
This	O
way	O
,	O
Plan	B-Operating_System
9	I-Operating_System
generalizes	O
the	O
Unix	B-Application
notion	O
of	O
the	O
filesystem	B-Application
as	O
the	O
central	O
point	O
of	O
access	O
to	O
computing	O
resources	O
.	O
</s>
<s>
It	O
carries	O
over	O
Unix	B-Application
's	O
idea	O
of	O
device	B-Application
files	I-Application
to	O
provide	O
access	O
to	O
peripheral	O
devices	O
(	O
mice	B-Device
,	O
removable	O
media	O
,	O
etc	O
.	O
)	O
</s>
<s>
and	O
the	O
possibility	O
to	O
mount	B-Device
filesystems	B-Application
residing	O
on	O
physically	O
distinct	O
filesystems	B-Application
into	O
a	O
hierarchical	O
namespace	O
,	O
but	O
adds	O
the	O
possibility	O
to	O
mount	B-Device
a	O
connection	O
to	O
a	O
server	O
program	O
that	O
speaks	O
a	O
standardized	O
protocol	O
and	O
treat	O
its	O
services	O
as	O
part	O
of	O
the	O
namespace	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
original	O
window	B-Application
system	I-Application
,	O
called	O
8½	B-Application
,	O
exploited	O
these	O
possibilities	O
as	O
follows	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
represents	O
the	O
user	B-Application
interface	I-Application
on	O
a	O
terminal	B-General_Concept
by	O
means	O
of	O
three	O
pseudo-files	O
:	O
,	O
which	O
can	O
be	O
read	O
by	O
a	O
program	O
to	O
get	O
notification	O
of	O
mouse	O
movements	O
and	O
button	O
clicks	O
,	O
,	O
which	O
can	O
be	O
used	O
to	O
perform	O
textual	O
input/output	B-General_Concept
,	O
and	O
,	O
writing	O
to	O
which	O
enacts	O
graphics	O
operations	O
(	O
see	O
bit	B-Algorithm
blit	I-Algorithm
)	O
.	O
</s>
<s>
The	O
window	B-Application
system	I-Application
multiplexes	O
these	O
devices	O
:	O
when	O
creating	O
a	O
new	O
window	O
to	O
run	O
some	O
program	O
in	O
,	O
it	O
first	O
sets	O
up	O
a	O
new	O
namespace	O
in	O
which	O
,	O
and	O
are	O
connected	O
to	O
itself	O
,	O
hiding	O
the	O
actual	O
device	B-Application
files	I-Application
to	O
which	O
it	O
itself	O
has	O
access	O
.	O
</s>
<s>
The	O
window	B-Application
system	I-Application
thus	O
receives	O
all	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
commands	O
from	O
the	O
program	O
and	O
handles	O
these	O
appropriately	O
,	O
by	O
sending	O
output	O
to	O
the	O
actual	O
screen	O
device	O
and	O
giving	O
the	O
currently	O
focused	O
program	O
the	O
keyboard	O
and	O
mouse	B-Device
input	I-Device
.	O
</s>
<s>
The	O
program	O
does	O
not	O
need	O
to	O
know	O
if	O
it	O
is	O
communicating	O
directly	O
with	O
the	O
operating	O
system	O
's	O
device	B-Application
drivers	I-Application
,	O
or	O
with	O
the	O
window	B-Application
system	I-Application
;	O
it	O
only	O
has	O
to	O
assume	O
that	O
its	O
namespace	O
is	O
set	O
up	O
so	O
that	O
these	O
special	B-Application
files	I-Application
provide	O
the	O
kind	O
of	O
input	O
and	O
accept	O
the	O
kind	O
of	O
messages	O
that	O
it	O
expects	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
's	O
distributed	O
operation	O
relies	O
on	O
the	O
per-process	O
namespaces	O
as	O
well	O
,	O
allowing	O
client	B-Operating_System
and	O
server	O
processes	B-Operating_System
to	O
communicate	O
across	O
machines	O
in	O
the	O
way	O
just	O
outlined	O
.	O
</s>
<s>
The	O
command	O
exports	O
part	O
of	O
its	O
local	O
namespace	O
,	O
including	O
the	O
user	O
's	O
terminal	B-General_Concept
's	O
devices	O
(	O
,	O
,	O
)	O
,	O
to	O
the	O
server	O
,	O
so	O
that	O
remote	O
programs	O
can	O
perform	O
input/output	B-General_Concept
using	O
the	O
terminal	B-General_Concept
's	O
mouse	O
,	O
keyboard	O
and	O
display	O
,	O
combining	O
the	O
effects	O
of	O
remote	B-Protocol
login	I-Protocol
and	O
a	O
shared	O
network	O
filesystem	B-Application
.	O
</s>
<s>
All	O
programs	O
that	O
wish	O
to	O
provide	O
services-as-files	O
to	O
other	O
programs	O
speak	O
a	O
unified	O
protocol	O
,	O
called	O
9P	B-Operating_System
.	O
</s>
<s>
Compared	O
to	O
other	O
systems	O
,	O
this	O
reduces	O
the	O
number	O
of	O
custom	O
programming	B-Application
interfaces	I-Application
.	O
</s>
<s>
9P	B-Operating_System
is	O
a	O
generic	O
,	O
medium-agnostic	O
,	O
byte-oriented	O
protocol	O
that	O
provides	O
for	O
messages	O
delivered	O
between	O
a	O
server	O
and	O
a	O
client	B-Operating_System
.	O
</s>
<s>
The	O
protocol	O
is	O
used	O
to	O
refer	O
to	O
and	O
communicate	O
with	O
processes	B-Operating_System
,	O
programs	O
,	O
and	O
data	O
,	O
including	O
both	O
the	O
user	B-Application
interface	I-Application
and	O
the	O
network	O
.	O
</s>
<s>
With	O
the	O
release	O
of	O
the	O
4th	O
edition	O
,	O
it	O
was	O
modified	O
and	O
renamed	O
9P2000	B-Operating_System
.	O
</s>
<s>
Unlike	O
most	O
other	O
operating	O
systems	O
,	O
Plan	B-Operating_System
9	I-Operating_System
does	O
not	O
provide	O
special	O
application	B-Application
programming	I-Application
interfaces	I-Application
(	O
such	O
as	O
Berkeley	B-Protocol
sockets	I-Protocol
,	O
X	B-Device
resources	I-Device
or	O
ioctl	B-Operating_System
system	O
calls	O
)	O
to	O
access	O
devices	O
.	O
</s>
<s>
Instead	O
,	O
Plan	B-Operating_System
9	I-Operating_System
device	B-Application
drivers	I-Application
implement	O
their	O
control	O
interface	B-Device
as	O
a	O
file	B-Application
system	I-Application
,	O
so	O
that	O
the	O
hardware	O
can	O
be	O
accessed	O
by	O
the	O
ordinary	O
file	O
input/output	B-General_Concept
operations	O
read	O
and	O
write	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
allows	O
the	O
user	O
to	O
collect	O
the	O
files	O
(	O
called	O
names	O
)	O
from	O
different	O
directory	O
trees	O
in	O
a	O
single	O
location	O
.	O
</s>
<s>
Acme	B-Application
's	O
directory	O
is	O
then	O
union	O
mounted	O
after	O
,	O
and	O
Alice	O
's	O
personal	O
directory	O
is	O
union	O
mounted	O
before	O
.	O
</s>
<s>
The	O
separate	O
process	B-Operating_System
namespaces	O
thus	O
usually	O
replace	O
the	O
notion	O
of	O
a	O
search	B-Application
path	I-Application
in	O
the	O
shell	B-Operating_System
.	O
</s>
<s>
A	O
path	O
environment	O
variable	O
(	O
)	O
still	O
exists	O
in	O
the	O
rc	B-Operating_System
shell	I-Operating_System
(	O
the	O
shell	B-Operating_System
mainly	O
used	O
in	O
Plan	B-Operating_System
9	I-Operating_System
)	O
;	O
however	O
,	O
rc	B-Operating_System
's	O
path	O
environment	O
variable	O
conventionally	O
only	O
contains	O
the	O
and	O
directories	O
and	O
modifying	O
the	O
variable	O
is	O
discouraged	O
,	O
instead	O
,	O
adding	O
additional	O
commands	O
should	O
be	O
done	O
by	O
binding	O
several	O
directories	O
together	O
as	O
a	O
single	O
.	O
</s>
<s>
Unlike	O
in	O
Plan	B-Operating_System
9	I-Operating_System
,	O
the	O
path	O
environment	O
variable	O
of	O
Unix	B-Application
shells	O
should	O
be	O
set	O
to	O
include	O
the	O
additional	O
directories	O
whose	O
executable	O
files	O
need	O
to	O
be	O
added	O
as	O
commands	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
kernel	O
can	O
keep	O
separate	O
mount	B-Device
tables	O
for	O
each	O
process	B-Operating_System
,	O
and	O
can	O
thus	O
provide	O
each	O
process	B-Operating_System
with	O
its	O
own	O
file	B-Application
system	I-Application
namespace	O
.	O
</s>
<s>
Processes	B-Operating_System
 '	O
namespaces	O
can	O
be	O
constructed	O
independently	O
,	O
and	O
the	O
user	O
may	O
work	O
simultaneously	O
with	O
programs	O
that	O
have	O
heterogeneous	O
namespaces	O
.	O
</s>
<s>
Namespaces	O
may	O
be	O
used	O
to	O
create	O
an	O
isolated	O
environment	O
similar	O
to	O
chroot	B-Application
,	O
but	O
in	O
a	O
more	O
secure	O
way	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
's	O
union	O
directory	O
architecture	O
inspired	O
4.4BSD	O
and	O
Linux	B-Application
union	B-Application
file	I-Application
system	I-Application
implementations	O
,	O
although	O
the	O
developers	O
of	O
the	O
BSD	O
union	O
mounting	O
facility	O
found	O
the	O
non-recursive	O
merging	O
of	O
directories	O
in	O
Plan	B-Operating_System
9	I-Operating_System
"	O
too	O
restrictive	O
for	O
general	O
purpose	O
use	O
"	O
.	O
</s>
<s>
Instead	O
of	O
having	O
system	O
calls	O
specifically	O
for	O
process	B-Operating_System
management	I-Operating_System
,	O
Plan	B-Operating_System
9	I-Operating_System
provides	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Each	O
process	B-Operating_System
appears	O
as	O
a	O
directory	O
containing	O
information	O
and	O
control	O
files	O
which	O
can	O
be	O
manipulated	O
by	O
the	O
ordinary	O
file	B-General_Concept
IO	I-General_Concept
system	O
calls	O
.	O
</s>
<s>
The	O
file	B-Application
system	I-Application
approach	O
allows	O
Plan	B-Operating_System
9	I-Operating_System
processes	B-Operating_System
to	O
be	O
managed	O
with	O
simple	O
file	O
management	O
tools	O
such	O
as	O
ls	B-Application
and	O
cat	B-Device
;	O
however	O
,	O
the	O
processes	B-Operating_System
cannot	O
be	O
copied	O
and	O
moved	O
as	O
files	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
does	O
not	O
have	O
specialised	O
system	O
calls	O
or	O
ioctls	B-Operating_System
for	O
accessing	O
the	O
networking	O
stack	O
or	O
networking	O
hardware	O
.	O
</s>
<s>
Instead	O
,	O
the	O
file	B-Application
system	I-Application
is	O
used	O
.	O
</s>
<s>
Sub-directories	O
such	O
as	O
and	O
are	O
used	O
as	O
an	O
interface	B-Device
to	O
their	O
respective	O
protocols	O
.	O
</s>
<s>
To	O
reduce	O
the	O
complexity	O
of	O
managing	O
character	B-General_Concept
encodings	I-General_Concept
,	O
Plan	B-Operating_System
9	I-Operating_System
uses	O
Unicode	O
throughout	O
the	O
system	O
.	O
</s>
<s>
The	O
initial	O
Unicode	O
implementation	O
was	O
ISO/IEC	O
10646-1:1993	O
.	O
</s>
<s>
Ken	O
Thompson	O
invented	O
UTF-8	O
,	O
which	O
became	O
the	O
native	B-Language
encoding	O
in	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
UTF-8	O
preserves	O
backwards	O
compatibility	O
with	O
traditional	O
null-terminated	B-Data_Structure
strings	I-Data_Structure
,	O
enabling	O
more	O
reliable	O
information	O
processing	O
and	O
the	O
chaining	O
of	O
multilingual	O
string	O
data	O
with	O
Unix	B-Operating_System
pipes	I-Operating_System
between	O
multiple	O
processes	B-Operating_System
.	O
</s>
<s>
Using	O
a	O
single	O
UTF-8	O
encoding	O
with	O
characters	O
for	O
all	O
cultures	O
and	O
regions	O
eliminates	O
the	O
need	O
for	O
switching	O
between	O
code	B-General_Concept
sets	I-General_Concept
.	O
</s>
<s>
Though	O
interesting	O
on	O
their	O
own	O
,	O
the	O
design	O
concepts	O
of	O
Plan	B-Operating_System
9	I-Operating_System
were	O
supposed	O
to	O
be	O
most	O
useful	O
when	O
combined	O
.	O
</s>
<s>
For	O
example	O
,	O
to	O
implement	O
a	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
server	O
,	O
a	O
union	O
directory	O
can	O
be	O
created	O
,	O
overlaying	O
the	O
router	B-Protocol
's	O
directory	O
tree	O
with	O
its	O
own	O
.	O
</s>
<s>
Similarly	O
,	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
can	O
be	O
implemented	O
by	O
overlaying	O
in	O
a	O
union	O
directory	O
a	O
hierarchy	O
from	O
a	O
remote	O
gateway	B-Application
,	O
using	O
secured	O
9P	B-Operating_System
over	O
the	O
public	O
Internet	O
.	O
</s>
<s>
A	O
union	O
directory	O
with	O
the	O
hierarchy	O
and	O
filters	O
can	O
be	O
used	O
to	O
sandbox	O
an	O
untrusted	O
application	O
or	O
to	O
implement	O
a	O
firewall	B-Application
.	O
</s>
<s>
As	O
a	O
benefit	O
from	O
the	O
system	O
's	O
design	O
,	O
most	O
tasks	O
in	O
Plan	B-Operating_System
9	I-Operating_System
can	O
be	O
accomplished	O
by	O
using	O
ls	B-Application
,	O
cat	B-Device
,	O
grep	B-Application
,	O
cp	B-Application
and	O
rm	B-Application
utilities	O
in	O
combination	O
with	O
the	O
rc	B-Operating_System
shell	I-Operating_System
(	O
the	O
default	O
Plan	B-Operating_System
9	I-Operating_System
shell	B-Operating_System
)	O
.	O
</s>
<s>
Factotum	B-Application
is	O
an	O
authentication	B-Application
and	O
key	O
management	O
server	O
for	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
It	O
handles	O
authentication	B-Application
on	O
behalf	O
of	O
other	O
programs	O
such	O
that	O
both	O
secret	O
keys	O
and	O
implementation	O
details	O
need	O
only	O
be	O
known	O
to	O
Factotum	B-Application
.	O
</s>
<s>
Unlike	O
Unix	B-Application
,	O
Plan	B-Operating_System
9	I-Operating_System
was	O
designed	O
with	O
graphics	O
in	O
mind	O
.	O
</s>
<s>
After	O
booting	O
,	O
a	O
Plan	B-Operating_System
9	I-Operating_System
terminal	B-General_Concept
will	O
run	O
the	O
rio	B-Language
windowing	B-Application
system	I-Application
,	O
in	O
which	O
the	O
user	O
can	O
create	O
new	O
windows	O
displaying	O
rc	B-Operating_System
.	O
</s>
<s>
Graphical	O
programs	O
invoked	O
from	O
this	O
shell	B-Operating_System
replace	O
it	O
in	O
its	O
window	O
.	O
</s>
<s>
The	O
plumber	B-Application
provides	O
an	O
inter-process	B-Operating_System
communication	I-Operating_System
mechanism	O
which	O
allows	O
system-wide	O
hyperlinking	O
.	O
</s>
<s>
Sam	B-Application
and	O
acme	B-Application
are	O
Plan	B-Operating_System
9	I-Operating_System
's	O
text	O
editors	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
supports	O
the	O
Kfs	O
,	O
Paq	O
,	O
Cwfs	O
,	O
FAT	B-Application
,	O
and	O
Fossil	B-Application
file	I-Application
systems	I-Application
.	O
</s>
<s>
The	O
last	O
was	O
designed	O
at	O
Bell	O
Labs	O
specifically	O
for	O
Plan	B-Operating_System
9	I-Operating_System
and	O
provides	O
snapshot	O
storage	O
capability	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
directly	O
with	O
a	O
hard	O
drive	O
or	O
backed	O
with	O
Venti	B-Application
,	O
an	O
archival	O
file	B-Application
system	I-Application
and	O
permanent	O
data	O
storage	O
system	O
.	O
</s>
<s>
The	O
distribution	O
package	O
for	O
Plan	B-Operating_System
9	I-Operating_System
includes	O
special	O
compiler	O
variants	O
and	O
programming	O
languages	O
,	O
and	O
provides	O
a	O
tailored	O
set	O
of	O
libraries	O
along	O
with	O
a	O
windowing	O
user	B-Application
interface	I-Application
system	O
specific	O
to	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
The	O
bulk	O
of	O
the	O
system	O
is	O
written	O
in	O
a	O
dialect	O
of	O
C	B-Language
(	O
ANSI	O
C	B-Language
with	O
some	O
extensions	O
and	O
some	O
other	O
features	O
left	O
out	O
)	O
.	O
</s>
<s>
A	O
concurrent	O
programming	O
language	O
called	O
Alef	B-Application
was	O
available	O
in	O
the	O
first	O
two	O
editions	O
,	O
but	O
was	O
then	O
dropped	O
for	O
maintenance	O
reasons	O
and	O
replaced	O
by	O
a	O
threading	B-Operating_System
library	O
for	O
C	B-Language
.	O
</s>
<s>
Though	O
Plan	B-Operating_System
9	I-Operating_System
was	O
supposed	O
to	O
be	O
a	O
further	O
development	O
of	O
Unix	B-Application
concepts	O
,	O
compatibility	O
with	O
preexisting	O
Unix	B-Application
software	O
was	O
never	O
the	O
goal	O
for	O
the	O
project	O
.	O
</s>
<s>
Many	O
command-line	B-Application
utilities	O
of	O
Plan	B-Operating_System
9	I-Operating_System
share	O
the	O
names	O
of	O
Unix	B-Application
counterparts	O
,	O
but	O
work	O
differently	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
can	O
support	O
POSIX	O
applications	O
and	O
can	O
emulate	O
the	O
Berkeley	B-Protocol
socket	I-Protocol
interface	I-Protocol
through	O
the	O
ANSI/POSIX	O
Environment	O
(	O
APE	O
)	O
that	O
implements	O
an	O
interface	B-Device
close	O
to	O
ANSI	O
C	B-Language
and	O
POSIX	O
,	O
with	O
some	O
common	O
extensions	O
(	O
the	O
native	B-Language
Plan	B-Operating_System
9	I-Operating_System
C	B-Language
interfaces	O
conform	O
to	O
neither	O
standard	O
)	O
.	O
</s>
<s>
It	O
also	O
includes	O
a	O
POSIX-compatible	O
shell	B-Operating_System
.	O
</s>
<s>
APE	O
's	O
authors	O
claim	O
to	O
have	O
used	O
it	O
to	O
port	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
(	O
X11	B-Operating_System
)	O
to	O
Plan	B-Operating_System
9	I-Operating_System
,	O
although	O
they	O
do	O
not	O
ship	O
X11	B-Operating_System
"	O
because	O
supporting	O
it	O
properly	O
is	O
too	O
big	O
a	O
job	O
"	O
.	O
</s>
<s>
Some	O
Linux	B-Application
binaries	O
can	O
be	O
used	O
with	O
the	O
help	O
of	O
a	O
"	O
linuxemu	O
"	O
(	O
Linux	B-Application
emulator	O
)	O
application	O
;	O
however	O
,	O
it	O
is	O
still	O
a	O
work	O
in	O
progress	O
.	O
</s>
<s>
Vice	O
versa	O
,	O
the	O
vx32	B-Application
virtual	O
machine	O
allows	O
a	O
slightly	O
modified	O
Plan	B-Operating_System
9	I-Operating_System
kernel	I-Operating_System
to	O
run	O
as	O
a	O
user	O
process	B-Operating_System
in	O
Linux	B-Application
,	O
supporting	O
unmodified	O
Plan	B-Operating_System
9	I-Operating_System
programs	O
.	O
</s>
<s>
In	O
1991	O
,	O
Plan	B-Operating_System
9	I-Operating_System
's	O
designers	O
compared	O
their	O
system	O
to	O
other	O
early	O
nineties	O
operating	O
systems	O
in	O
terms	O
of	O
size	O
,	O
showing	O
that	O
the	O
source	O
code	O
for	O
a	O
minimal	O
(	O
"	O
working	O
,	O
albeit	O
not	O
very	O
useful	O
"	O
)	O
version	O
was	O
less	O
than	O
one-fifth	O
the	O
size	O
of	O
a	O
Mach	B-Operating_System
microkernel	I-Operating_System
without	O
any	O
device	B-Application
drivers	I-Application
(	O
5899	O
or	O
4622	O
lines	O
of	O
code	O
for	O
Plan	B-Operating_System
9	I-Operating_System
,	O
depending	O
on	O
metric	O
,	O
vs.	O
25530	O
lines	O
)	O
.	O
</s>
<s>
(	O
According	O
to	O
a	O
2006	O
count	O
,	O
the	O
kernel	O
was	O
then	O
some	O
150,000	O
lines	O
,	O
but	O
this	O
was	O
compared	O
against	O
more	O
than	O
4.8million	O
in	O
Linux	B-Application
.	O
)	O
</s>
<s>
Within	O
the	O
operating	O
systems	O
research	O
community	O
,	O
as	O
well	O
as	O
the	O
commercial	O
Unix	B-Application
world	O
,	O
other	O
attempts	O
at	O
achieving	O
distributed	O
computing	O
and	O
remote	O
filesystem	B-Application
access	O
were	O
made	O
concurrently	O
with	O
the	O
Plan	B-Operating_System
9	I-Operating_System
design	O
effort	O
.	O
</s>
<s>
These	O
included	O
the	O
Network	B-Protocol
File	I-Protocol
System	I-Protocol
and	O
the	O
associated	O
vnode	B-Application
architecture	O
developed	O
at	O
Sun	O
Microsystems	O
,	O
and	O
more	O
radical	O
departures	O
from	O
the	O
Unix	B-Application
model	O
such	O
as	O
the	O
Sprite	B-Operating_System
OS	I-Operating_System
from	O
UC	O
Berkeley	O
.	O
</s>
<s>
Sprite	B-Operating_System
developer	O
Brent	O
Welch	O
points	O
out	O
that	O
the	O
SunOS	O
vnode	B-Application
architecture	O
is	O
limited	O
compared	O
to	O
Plan	B-Operating_System
9	I-Operating_System
's	O
capabilities	O
in	O
that	O
it	O
does	O
not	O
support	O
remote	O
device	O
access	O
and	O
remote	O
inter-process	B-Operating_System
communication	I-Operating_System
cleanly	O
,	O
even	O
though	O
it	O
could	O
have	O
,	O
had	O
the	O
preexisting	O
UNIX	B-Protocol
domain	I-Protocol
sockets	I-Protocol
(	O
which	O
"	O
can	O
essentially	O
be	O
used	O
to	O
name	O
user-level	O
servers	O
"	O
)	O
been	O
integrated	O
with	O
the	O
vnode	B-Application
architecture	O
.	O
</s>
<s>
One	O
critique	O
of	O
the	O
"	O
everything	B-Application
is	I-Application
a	I-Application
file	I-Application
"	O
,	O
communication-by-textual-message	O
design	O
of	O
Plan	B-Operating_System
9	I-Operating_System
pointed	O
out	O
limitations	O
of	O
this	O
paradigm	O
compared	O
to	O
the	O
typed	O
interfaces	O
of	O
Sun	O
's	O
object-oriented	B-Application
operating	I-Application
system	I-Application
,	O
Spring	B-Application
:	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
demonstrated	O
that	O
an	O
integral	O
concept	O
of	O
Unix	B-Application
—	O
that	O
every	O
system	O
interface	B-Device
could	O
be	O
represented	O
as	O
a	O
set	O
of	O
files	O
—	O
could	O
be	O
successfully	O
implemented	O
in	O
a	O
modern	O
distributed	O
system	O
.	O
</s>
<s>
Some	O
features	O
from	O
Plan	B-Operating_System
9	I-Operating_System
,	O
like	O
the	O
UTF-8	O
character	B-General_Concept
encoding	I-General_Concept
of	O
Unicode	O
,	O
have	O
been	O
implemented	O
in	O
other	O
operating	O
systems	O
.	O
</s>
<s>
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
such	O
as	O
Linux	B-Application
have	O
implemented	O
9P2000	B-Operating_System
,	O
Plan	B-Operating_System
9	I-Operating_System
's	O
protocol	O
for	O
accessing	O
remote	O
files	O
,	O
and	O
have	O
adopted	O
features	O
of	O
rfork	O
,	O
Plan	B-Operating_System
9	I-Operating_System
's	O
process	B-Operating_System
creation	O
mechanism	O
.	O
</s>
<s>
Additionally	O
,	O
in	O
Plan	B-Application
9	I-Application
from	I-Application
User	I-Application
Space	I-Application
,	O
several	O
of	O
Plan	B-Operating_System
9	I-Operating_System
's	O
applications	O
and	O
tools	O
,	O
including	O
the	O
sam	B-Application
and	O
acme	B-Application
editors	O
,	O
have	O
been	O
ported	O
to	O
Unix	B-Application
and	O
Linux	B-Application
systems	O
and	O
have	O
achieved	O
some	O
level	O
of	O
popularity	O
.	O
</s>
<s>
Several	O
projects	O
seek	O
to	O
replace	O
the	B-Operating_System
GNU	I-Operating_System
operating	I-Operating_System
system	I-Operating_System
programs	O
surrounding	O
the	O
Linux	B-Application
kernel	O
with	O
the	O
Plan	B-Operating_System
9	I-Operating_System
operating	O
system	O
programs	O
.	O
</s>
<s>
The	O
9wm	B-Language
window	B-Application
manager	I-Application
was	O
inspired	O
by	O
8½	B-Application
,	O
the	O
older	O
windowing	B-Application
system	I-Application
of	O
Plan	B-Operating_System
9	I-Operating_System
;	O
wmii	B-Language
is	O
also	O
heavily	O
influenced	O
by	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
In	O
computer	O
science	O
research	O
,	O
Plan	B-Operating_System
9	I-Operating_System
has	O
been	O
used	O
as	O
a	O
grid	B-Architecture
computing	I-Architecture
platform	O
and	O
as	O
a	O
vehicle	O
for	O
research	O
into	O
ubiquitous	B-Architecture
computing	I-Architecture
without	O
middleware	B-General_Concept
.	O
</s>
<s>
In	O
commerce	O
,	O
Plan	B-Operating_System
9	I-Operating_System
underlies	O
Coraid	O
storage	O
systems	O
.	O
</s>
<s>
However	O
,	O
Plan	B-Operating_System
9	I-Operating_System
has	O
never	O
approached	O
Unix	B-Application
in	O
popularity	O
,	O
and	O
has	O
been	O
primarily	O
a	O
research	O
tool	O
:	O
</s>
<s>
Other	O
factors	O
that	O
contributed	O
to	O
low	O
adoption	O
of	O
Plan	B-Operating_System
9	I-Operating_System
include	O
the	O
lack	O
of	O
commercial	O
backup	O
,	O
the	O
low	O
number	O
of	O
end-user	O
applications	O
,	O
and	O
the	O
lack	O
of	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
proponents	O
and	O
developers	O
claim	O
that	O
the	O
problems	O
hindering	O
its	O
adoption	O
have	O
been	O
solved	O
,	O
that	O
its	O
original	O
goals	O
as	O
a	O
distributed	O
system	O
,	O
development	O
environment	O
,	O
and	O
research	O
platform	O
have	O
been	O
met	O
,	O
and	O
that	O
it	O
enjoys	O
moderate	O
but	O
growing	O
popularity	O
.	O
</s>
<s>
Inferno	B-Operating_System
,	O
through	O
its	O
hosted	O
capabilities	O
,	O
has	O
been	O
a	O
vehicle	O
for	O
bringing	O
Plan	B-Operating_System
9	I-Operating_System
technologies	O
to	O
other	O
systems	O
as	O
a	O
hosted	O
part	O
of	O
heterogeneous	O
computing	B-Architecture
grids	I-Architecture
.	O
</s>
<s>
Several	O
projects	O
work	O
to	O
extend	O
Plan	B-Operating_System
9	I-Operating_System
,	O
including	O
9atom	O
and	O
9front	B-Operating_System
.	O
</s>
<s>
These	O
forks	B-Application
augment	O
Plan	B-Operating_System
9	I-Operating_System
with	O
additional	O
hardware	B-Application
drivers	I-Application
and	O
software	O
,	O
including	O
an	O
improved	O
version	O
of	O
the	O
Upas	O
e-mail	O
system	O
,	O
the	O
Go	B-Application
compiler	O
,	O
Mercurial	B-Protocol
version	B-Architecture
control	I-Architecture
system	I-Architecture
support	O
(	O
and	O
now	O
also	O
a	O
git	O
implementation	O
)	O
,	O
and	O
other	O
programs	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
was	O
ported	O
to	O
the	O
Raspberry	B-Operating_System
Pi	I-Operating_System
single-board	O
computer	O
.	O
</s>
<s>
The	O
Harvey	O
project	O
attempts	O
to	O
replace	O
the	O
custom	O
Plan	B-Operating_System
9	I-Operating_System
C	B-Language
compiler	O
with	O
GCC	B-Application
,	O
to	O
leverage	O
modern	O
development	O
tools	O
such	O
as	O
GitHub	B-Application
and	O
Coverity	O
,	O
and	O
speed	O
up	O
development	O
.	O
</s>
<s>
Since	O
Windows	O
10	O
version	O
1903	O
,	O
the	O
Windows	O
Subsystem	O
for	O
Linux	B-Application
implements	O
the	O
Plan	B-Operating_System
9	I-Operating_System
Filesystem	I-Operating_System
Protocol	I-Operating_System
as	O
a	O
server	O
and	O
the	O
host	O
Windows	O
operating	O
system	O
acts	O
as	O
a	O
client	B-Operating_System
.	O
</s>
<s>
Inferno	B-Operating_System
is	O
a	O
descendant	O
of	O
Plan	B-Operating_System
9	I-Operating_System
,	O
and	O
shares	O
many	O
design	O
concepts	O
and	O
even	O
source	O
code	O
in	O
the	O
kernel	O
,	O
particularly	O
around	O
devices	O
and	O
the	O
Styx/9P2000	O
protocol	O
.	O
</s>
<s>
Inferno	B-Operating_System
shares	O
with	O
Plan	B-Operating_System
9	I-Operating_System
the	O
Unix	B-Application
heritage	O
from	O
Bell	O
Labs	O
and	O
the	B-Operating_System
Unix	I-Operating_System
philosophy	I-Operating_System
.	O
</s>
<s>
Many	O
of	O
the	O
command	B-Application
line	I-Application
tools	I-Application
in	O
Inferno	B-Operating_System
were	O
Plan	B-Operating_System
9	I-Operating_System
tools	O
that	O
were	O
translated	O
to	O
Limbo	B-Application
.	O
</s>
<s>
9atom	O
augments	O
the	O
Plan	B-Operating_System
9	I-Operating_System
distribution	O
with	O
the	O
addition	O
of	O
a	O
386	O
PAE	B-General_Concept
kernel	O
,	O
an	O
amd64	B-Device
cpu	O
and	O
terminal	B-General_Concept
kernel	O
,	O
nupas	O
,	O
extra	O
pc	O
hardware	O
support	O
,	O
IL	O
and	O
Ken	O
's	O
fs	O
.	O
</s>
<s>
9front	B-Operating_System
is	O
a	O
fork	B-Application
of	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
It	O
includes	O
a	O
set	O
of	O
patches	O
based	O
on	O
the	O
current	O
Plan	B-Operating_System
9	I-Operating_System
distribution	O
.	O
</s>
<s>
Harvey	O
OS	O
is	O
an	O
effort	O
to	O
get	O
the	O
Plan	B-Operating_System
9	I-Operating_System
code	O
working	O
with	O
gcc	B-Application
and	O
clang	O
.	O
</s>
<s>
JehanneOS	O
is	O
an	O
experimental	O
OS	O
derived	O
from	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
Its	O
userland	O
and	O
modules	O
are	O
mostly	O
derived	O
from	O
9front	B-Operating_System
,	O
its	O
build	O
system	O
from	O
Harvey	O
OS	O
,	O
and	O
its	O
kernel	O
is	O
a	O
fork	B-Application
of	O
the	O
.	O
</s>
<s>
NIX	O
is	O
a	O
fork	B-Application
of	O
Plan9	O
aimed	O
at	O
multicore	O
systems	O
and	O
cloud	O
computing	O
.	O
</s>
<s>
node9	O
is	O
a	O
scripted	O
derivative	O
of	O
Plan9/Inferno	O
that	O
replaces	O
the	O
Limbo	B-Application
programming	I-Application
language	I-Application
and	O
virtual	O
machine	O
with	O
the	O
Lua	B-Language
language	O
and	O
virtual	O
machine	O
.	O
</s>
<s>
It	O
also	O
replaces	O
the	O
Inferno	B-Operating_System
per-platform	O
hosted	O
I/O	B-General_Concept
with	O
eventing	O
and	O
I/O	B-General_Concept
for	O
consistent	O
,	O
cross-platform	O
hosting	O
.	O
</s>
<s>
It	O
's	O
a	O
proof-of-concept	O
that	O
demonstrates	O
that	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
can	O
be	O
constructed	O
from	O
per-process	O
namespaces	O
and	O
generic	O
cloud	O
elements	O
to	O
construct	O
a	O
single-system-image	O
of	O
arbitrary	O
size	O
.	O
</s>
<s>
Starting	O
with	O
the	O
release	O
of	O
Fourth	O
edition	O
in	O
April	O
2002	O
,	O
the	O
full	O
source	O
code	O
of	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
is	O
freely	O
available	O
under	O
Lucent	B-License
Public	I-License
License	I-License
1.02	O
,	O
which	O
is	O
considered	O
to	O
be	O
an	O
open-source	B-Application
license	O
by	O
the	O
Open	B-Application
Source	I-Application
Initiative	O
(	O
OSI	O
)	O
,	O
free	B-Application
software	I-Application
license	O
by	O
the	B-Operating_System
Free	I-Operating_System
Software	I-Operating_System
Foundation	I-Operating_System
,	O
and	O
it	O
passes	O
the	O
Debian	B-License
Free	I-License
Software	I-License
Guidelines	I-License
.	O
</s>
<s>
In	O
February	O
2014	O
,	O
the	O
University	O
of	O
California	O
,	O
Berkeley	O
,	O
was	O
authorized	O
by	O
the	O
current	O
Plan	B-Operating_System
9	I-Operating_System
copyright	O
holder	O
–	O
Alcatel-Lucent	O
–	O
to	O
release	O
all	O
Plan	B-Operating_System
9	I-Operating_System
software	O
previously	O
governed	O
by	O
the	O
Lucent	B-License
Public	I-License
License	I-License
,	O
Version	O
1.02	O
under	O
the	O
GPL-2.0-only	B-License
.	O
</s>
<s>
On	O
March	O
23	O
,	O
2021	O
,	O
ownership	O
of	O
Plan	B-Operating_System
9	I-Operating_System
transferred	O
from	O
Bell	O
Labs	O
to	O
the	O
Plan	B-Operating_System
9	I-Operating_System
Foundation	I-Operating_System
,	O
and	O
all	O
previous	O
releases	O
have	O
been	O
relicensed	O
to	O
the	B-License
MIT	I-License
License	I-License
.	O
</s>
