<s>
rc	B-Operating_System
(	O
for	O
"	O
run	B-Device
commands	I-Device
"	O
)	O
is	O
the	O
command	O
line	O
interpreter	O
for	O
Version	B-Operating_System
10	I-Operating_System
Unix	I-Operating_System
and	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
It	O
resembles	O
the	B-Device
Bourne	I-Device
shell	I-Device
,	O
but	O
its	O
syntax	O
is	O
somewhat	O
simpler	O
.	O
</s>
<s>
It	O
was	O
created	O
by	O
Tom	O
Duff	O
,	O
who	O
is	O
better	O
known	O
for	O
an	O
unusual	O
C	B-Language
programming	I-Language
language	I-Language
construct	O
(	O
"	O
Duff	O
's	O
device	O
"	O
)	O
.	O
</s>
<s>
A	O
port	O
of	O
the	O
original	O
rc	B-Operating_System
to	O
Unix	O
is	O
part	O
of	O
Plan	B-Application
9	I-Application
from	I-Application
User	I-Application
Space	I-Application
.	O
</s>
<s>
A	O
rewrite	O
of	O
rc	B-Operating_System
for	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
by	O
Byron	O
Rakitzis	O
is	O
also	O
available	O
but	O
includes	O
some	O
incompatible	O
changes	O
.	O
</s>
<s>
Rc	B-Operating_System
uses	O
C-like	O
control	O
structures	O
instead	O
of	O
the	O
original	O
Bourne	B-Device
shell	I-Device
's	O
ALGOL-like	B-Language
structures	O
,	O
except	O
that	O
it	O
uses	O
an	O
if	O
not	O
construct	O
instead	O
of	O
else	O
,	O
and	O
has	O
a	O
Bourne-like	O
for	O
loop	O
to	O
iterate	O
over	O
lists	O
.	O
</s>
<s>
In	O
rc	B-Operating_System
,	O
all	O
variables	O
are	O
lists	O
of	O
strings	O
,	O
which	O
eliminates	O
the	O
need	O
for	O
constructs	O
like	O
"	O
$@	O
"	O
.	O
</s>
<s>
es	B-Operating_System
(	O
for	O
"	O
extensible	O
shell	O
"	O
)	O
is	O
an	O
open	B-License
source	I-License
,	O
command	O
line	O
interpreter	O
developed	O
by	O
Rakitzis	O
and	O
Paul	O
Haahr	O
that	O
uses	O
a	O
scripting	B-Language
language	I-Language
syntax	O
influenced	O
by	O
the	O
rc	B-Operating_System
shell	I-Operating_System
.	O
</s>
<s>
It	O
was	O
originally	O
based	O
on	O
code	O
from	O
Byron	O
Rakitzis	O
's	O
clone	O
of	O
rc	B-Operating_System
for	O
Unix	O
.	O
</s>
<s>
Extensible	O
shell	O
is	O
intended	O
to	O
provide	O
a	O
fully	O
functional	B-Language
programming	I-Language
language	I-Language
as	O
a	O
Unix	B-Application
shell	I-Application
.	O
</s>
<s>
It	O
does	O
so	O
by	O
introducing	O
"	O
program	O
fragments	O
"	O
in	O
braces	O
as	O
a	O
new	O
datatype	O
,	O
lexical	O
scoping	O
via	O
let	B-Application
,	O
and	O
some	O
more	O
minor	O
improvements	O
.	O
</s>
<s>
The	O
bulk	O
of	O
es	B-Operating_System
development	O
occurred	O
in	O
the	O
early	O
1990s	O
,	O
after	O
the	O
shell	O
was	O
introduced	O
at	O
the	O
Winter	O
1993	O
USENIX	B-Operating_System
conference	O
in	O
San	O
Diego	O
,	O
Official	O
releases	O
appear	O
to	O
have	O
ceased	O
after	O
0.9-beta-1	O
in	O
1997	O
,	O
and	O
es	B-Operating_System
lacks	O
features	O
as	O
compared	O
to	O
more	O
popular	O
shells	O
,	O
such	O
as	O
zsh	B-Application
and	O
bash	B-Operating_System
.	O
</s>
<s>
The	B-Device
Bourne	I-Device
shell	I-Device
script	O
:	O
</s>
<s>
is	O
expressed	O
in	O
rc	B-Operating_System
as	O
:	O
</s>
<s>
Rc	B-Operating_System
also	O
supports	O
more	O
dynamic	O
piping	O
:	O
</s>
<s>
a	O
<{b}	O
<{c}	O
#	O
becomes	O
a	O
{	O
standard	O
output	O
of	O
b}	O
{	O
standard	O
output	O
of	O
c}	O
.	O
</s>
