<s>
The	O
Thompson	B-Application
shell	I-Application
was	O
the	O
first	O
Unix	B-Application
shell	I-Application
,	O
introduced	O
in	O
the	O
first	O
version	O
of	O
Unix	B-Application
in	O
1971	O
,	O
and	O
was	O
written	O
by	O
Ken	O
Thompson	O
.	O
</s>
<s>
It	O
was	O
a	O
simple	O
command	O
interpreter	O
,	O
not	O
designed	O
for	O
scripting	O
,	O
but	O
nonetheless	O
introduced	O
several	O
innovative	O
features	O
to	O
the	O
command-line	B-Application
interface	I-Application
and	O
led	O
to	O
the	O
development	O
of	O
the	O
later	O
Unix	B-Application
shells	I-Application
.	O
</s>
<s>
The	O
name	O
"	O
shell	O
"	O
for	O
a	O
command-line	B-Application
interpreter	O
and	O
the	O
concept	O
of	O
making	O
the	O
shell	O
a	O
user	O
program	O
outside	O
of	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
were	O
introduced	O
in	O
Unix	B-Application
's	O
precursor	O
Multics	B-Application
.	O
</s>
<s>
An	O
early	O
feature	O
of	O
the	O
Thompson	B-Application
shell	I-Application
was	O
a	O
compact	O
syntax	O
for	O
input/output	O
redirection	B-Device
.	O
</s>
<s>
In	O
Multics	B-Application
,	O
redirecting	O
the	O
input	O
or	O
output	O
of	O
a	O
command	O
required	O
separate	O
commands	O
to	O
start	O
and	O
stop	O
redirection	B-Device
;	O
in	O
Unix	B-Application
,	O
one	O
could	O
simply	O
add	O
an	O
argument	O
to	O
the	O
command	B-Application
line	I-Application
consisting	O
of	O
the	O
symbol	O
followed	O
by	O
a	O
filename	O
for	O
input	O
or	O
the	O
symbol	O
for	O
output	O
,	O
and	O
the	O
shell	O
would	O
redirect	O
I/O	O
for	O
the	O
duration	O
of	O
the	O
command	O
.	O
</s>
<s>
This	O
syntax	O
was	O
already	O
present	O
by	O
the	O
release	O
of	O
the	O
first	O
version	O
of	O
Unix	B-Application
in	O
1971	O
.	O
</s>
<s>
A	O
later	O
addition	O
was	O
the	O
concept	O
of	O
pipes	B-Operating_System
.	O
</s>
<s>
At	O
the	O
suggestion	O
of	O
Douglas	O
McIlroy	O
,	O
the	O
redirection	B-Device
syntax	O
was	O
expanded	O
so	O
that	O
the	O
output	O
of	O
one	O
command	O
could	O
be	O
passed	O
to	O
the	O
input	O
of	O
another	O
command	O
.	O
</s>
<s>
This	O
syntax	O
proved	O
too	O
ambiguous	O
and	O
was	O
easily	O
confused	O
with	O
redirection	B-Device
to	O
and	O
from	O
files	O
—	O
the	O
system	O
cannot	O
tell	O
if	O
"	O
command2	O
"	O
is	O
the	O
command	O
"	O
command2	O
"	O
or	O
the	O
file	O
"	O
command2	O
"	O
.	O
</s>
<s>
By	O
Version	O
4	O
,	O
the	O
syntax	O
had	O
changed	O
to	O
use	O
both	O
the	O
|	O
and	O
^	O
symbols	O
to	O
denote	O
pipes	B-Operating_System
:	O
</s>
<s>
The	O
Thompson	B-Application
shell	I-Application
syntax	O
for	O
redirection	B-Device
with	O
and	O
,	O
and	O
piping	O
with	O
|	O
,	O
has	O
proven	O
durable	O
and	O
has	O
been	O
adopted	O
by	O
most	O
other	O
Unix	B-Application
shells	I-Application
and	O
command	O
shells	O
of	O
several	O
other	O
operating	O
systems	O
,	O
most	O
notably	O
on	O
DOS	B-Device
,	O
OS/2	B-Application
and	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
As	O
a	O
result	O
,	O
by	O
the	O
1975	O
release	O
of	O
Version	B-Operating_System
6	I-Operating_System
Unix	I-Operating_System
,	O
it	O
was	O
becoming	O
clear	O
that	O
the	O
Thompson	B-Application
shell	I-Application
was	O
inadequate	O
for	O
most	O
serious	O
programming	O
tasks	O
.	O
</s>
<s>
At	O
this	O
time	O
,	O
the	O
developers	O
of	O
the	O
Programmer	B-Operating_System
's	I-Operating_System
Workbench	I-Operating_System
UNIX	I-Operating_System
distribution	O
,	O
most	O
notably	O
John	O
Mashey	O
,	O
began	O
modifying	O
the	O
Thompson	B-Application
shell	I-Application
to	O
make	O
it	O
more	O
suitable	O
for	O
programming	O
.	O
</s>
<s>
The	O
result	O
,	O
known	O
as	O
the	O
PWB	B-Application
shell	I-Application
or	O
the	O
Mashey	B-Application
shell	I-Application
,	O
included	O
more	O
advanced	O
flow-control	O
mechanisms	O
and	O
introduced	O
shell	O
variables	O
,	O
but	O
remained	O
limited	O
by	O
the	O
necessity	O
to	O
remain	O
compatible	O
with	O
the	O
Thompson	B-Application
shell	I-Application
.	O
</s>
<s>
Finally	O
,	O
the	O
Thompson	B-Application
shell	I-Application
was	O
replaced	O
as	O
the	O
main	O
Unix	B-Application
shell	I-Application
by	O
the	B-Device
Bourne	I-Device
shell	I-Device
in	O
Version	B-Operating_System
7	I-Operating_System
Unix	I-Operating_System
and	O
the	O
C	B-Operating_System
shell	I-Operating_System
in	O
2BSD	O
,	O
both	O
released	O
in	O
1979	O
.	O
</s>
<s>
Since	O
virtually	O
all	O
modern	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
systems	I-Operating_System
are	O
descended	O
from	O
V7	O
and	O
2BSD	O
,	O
the	O
Thompson	B-Application
shell	I-Application
is	O
generally	O
no	O
longer	O
used	O
.	O
</s>
<s>
It	O
is	O
,	O
however	O
,	O
available	O
as	O
open-source	B-Application
as	O
part	O
of	O
several	O
Ancient	B-Operating_System
Unix	I-Operating_System
source	O
distributions	O
,	O
and	O
has	O
been	O
ported	O
to	O
modern	O
Unices	B-Operating_System
as	O
a	O
historical	O
exhibit	O
.	O
</s>
