<s>
A	O
Unix	B-Application
shell	I-Application
is	O
a	O
command-line	O
interpreter	B-Application
or	O
shell	B-Operating_System
that	O
provides	O
a	O
command	O
line	O
user	B-Application
interface	I-Application
for	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
The	O
shell	B-Operating_System
is	O
both	O
an	O
interactive	O
command	B-Language
language	I-Language
and	O
a	O
scripting	B-Language
language	I-Language
,	O
and	O
is	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
control	O
the	O
execution	O
of	O
the	O
system	O
using	O
shell	B-Language
scripts	I-Language
.	O
</s>
<s>
Users	O
typically	O
interact	O
with	O
a	O
Unix	B-Application
shell	I-Application
using	O
a	O
terminal	B-Application
emulator	I-Application
;	O
however	O
,	O
direct	O
operation	O
via	O
serial	O
hardware	O
connections	O
or	O
Secure	B-Protocol
Shell	I-Protocol
are	O
common	O
for	O
server	O
systems	O
.	O
</s>
<s>
All	O
Unix	B-Application
shells	I-Application
provide	O
filename	O
wildcarding	B-Algorithm
,	O
piping	B-Operating_System
,	O
here	O
documents	O
,	O
command	B-Device
substitution	I-Device
,	O
variables	O
and	O
control	O
structures	O
for	O
condition-testing	B-Language
and	O
iteration	B-Algorithm
.	O
</s>
<s>
Generally	O
,	O
a	O
shell	B-Operating_System
is	O
a	O
program	O
that	O
executes	O
other	O
programs	O
in	O
response	O
to	O
text	O
commands	O
.	O
</s>
<s>
A	O
sophisticated	O
shell	B-Operating_System
can	O
also	O
change	O
the	O
environment	O
in	O
which	O
other	O
programs	O
execute	O
by	O
passing	O
named	O
variables	O
,	O
a	O
parameter	O
list	O
,	O
or	O
an	O
input	O
source	O
.	O
</s>
<s>
In	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
users	O
typically	O
have	O
many	O
choices	O
of	O
command-line	O
interpreters	B-Application
for	O
interactive	O
sessions	O
.	O
</s>
<s>
When	O
a	O
user	O
logs	O
into	O
the	O
system	O
interactively	O
,	O
a	O
shell	B-Operating_System
program	O
is	O
automatically	O
executed	O
for	O
the	O
duration	O
of	O
the	O
session	O
.	O
</s>
<s>
The	O
type	O
of	O
shell	B-Operating_System
,	O
which	O
may	O
be	O
customized	O
for	O
each	O
user	O
,	O
is	O
typically	O
stored	O
in	O
the	O
user	O
's	O
profile	O
,	O
for	O
example	O
in	O
the	O
local	O
file	O
or	O
in	O
a	O
distributed	O
configuration	O
system	O
such	O
as	O
NIS	B-Operating_System
or	O
LDAP	B-Protocol
;	O
however	O
,	O
the	O
user	O
may	O
execute	O
any	O
other	O
available	O
shell	B-Operating_System
interactively	O
.	O
</s>
<s>
On	O
operating	B-General_Concept
systems	I-General_Concept
with	O
a	O
windowing	B-Application
system	I-Application
,	O
such	O
as	O
macOS	B-Application
and	O
desktop	O
Linux	B-Application
distributions	I-Application
,	O
some	O
users	O
may	O
never	O
use	O
the	O
shell	B-Operating_System
directly	O
.	O
</s>
<s>
On	O
Unix	O
systems	O
,	O
the	O
shell	B-Operating_System
has	O
historically	O
been	O
the	O
implementation	O
language	O
of	O
system	O
startup	O
scripts	O
,	O
including	O
the	O
program	O
that	O
starts	O
a	O
windowing	B-Application
system	I-Application
,	O
configures	O
networking	O
,	O
and	O
many	O
other	O
essential	O
functions	O
.	O
</s>
<s>
However	O
,	O
some	O
system	O
vendors	O
have	O
replaced	O
the	O
traditional	O
shell-based	O
startup	O
system	O
(	O
init	B-Operating_System
)	O
with	O
different	O
approaches	O
,	O
such	O
as	O
systemd	B-Application
.	O
</s>
<s>
The	O
first	O
Unix	B-Application
shell	I-Application
was	O
the	O
Thompson	B-Application
shell	I-Application
,	O
sh	B-Device
,	O
written	O
by	O
Ken	O
Thompson	O
at	O
Bell	O
Labs	O
and	O
distributed	O
with	O
Versions	O
1	O
through	O
6	O
of	O
Unix	O
,	O
from	O
1971	O
to	O
1975	O
.	O
</s>
<s>
Though	O
rudimentary	O
by	O
modern	O
standards	O
,	O
it	O
introduced	O
many	O
of	O
the	O
basic	O
features	O
common	O
to	O
all	O
later	O
Unix	B-Application
shells	I-Application
,	O
including	O
piping	B-Operating_System
,	O
simple	O
control	O
structures	O
using	O
if	O
and	O
goto	O
,	O
and	O
filename	O
wildcarding	B-Algorithm
.	O
</s>
<s>
Though	O
not	O
in	O
current	O
use	O
,	O
it	O
is	O
still	O
available	O
as	O
part	O
of	O
some	O
Ancient	B-Operating_System
UNIX	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
It	O
was	O
modeled	O
after	O
the	O
Multics	B-Application
shell	B-Operating_System
,	O
developed	O
in	O
1965	O
by	O
American	O
software	O
engineer	O
Glenda	O
Schroeder	O
.	O
</s>
<s>
Schroeder	O
's	O
Multics	B-Application
shell	B-Operating_System
was	O
itself	O
modeled	O
after	O
the	O
RUNCOM	B-Device
program	O
Louis	O
Pouzin	O
showed	O
to	O
the	O
Multics	B-Application
Team	O
.	O
</s>
<s>
The	O
"	O
rc	B-Operating_System
"	O
suffix	O
on	O
some	O
Unix	O
configuration	O
files	O
(	O
for	O
example	O
,	O
"	O
.vimrc	O
"	O
)	O
,	O
is	O
a	O
remnant	O
of	O
the	O
RUNCOM	B-Device
ancestry	O
of	O
Unix	B-Application
shells	I-Application
.	O
</s>
<s>
The	O
PWB	B-Application
shell	I-Application
or	O
Mashey	B-Application
shell	I-Application
,	O
sh	B-Device
,	O
was	O
an	O
upward-compatible	O
version	O
of	O
the	O
Thompson	B-Application
shell	I-Application
,	O
augmented	O
by	O
John	O
Mashey	O
and	O
others	O
and	O
distributed	O
with	O
the	O
Programmer	B-Operating_System
's	I-Operating_System
Workbench	I-Operating_System
UNIX	I-Operating_System
,	O
circa	O
1975	O
–	O
1977	O
.	O
</s>
<s>
It	O
focused	O
on	O
making	O
shell	B-Operating_System
programming	O
practical	O
,	O
especially	O
in	O
large	O
shared	O
computing	O
centers	O
.	O
</s>
<s>
It	O
added	O
shell	B-Operating_System
variables	O
(	O
precursors	O
of	O
environment	O
variables	O
,	O
including	O
the	O
search	B-Application
path	I-Application
mechanism	O
that	O
evolved	O
into	O
$PATH	B-Application
)	O
,	O
user-executable	O
shell	B-Language
scripts	I-Language
,	O
and	O
interrupt-handling	O
.	O
</s>
<s>
As	O
shell	B-Operating_System
programming	O
became	O
widespread	O
,	O
these	O
external	O
commands	O
were	O
incorporated	O
into	O
the	O
shell	B-Operating_System
itself	O
for	O
performance	O
.	O
</s>
<s>
But	O
the	O
most	O
widely	O
distributed	O
and	O
influential	O
of	O
the	O
early	O
Unix	B-Application
shells	I-Application
were	O
the	B-Device
Bourne	I-Device
shell	I-Device
and	O
the	O
C	B-Operating_System
shell	I-Operating_System
.	O
</s>
<s>
The	B-Device
Bourne	I-Device
shell	I-Device
,	O
sh	B-Device
,	O
was	O
a	O
new	O
Unix	B-Application
shell	I-Application
by	O
Stephen	O
Bourne	O
at	O
Bell	O
Labs	O
.	O
</s>
<s>
Distributed	O
as	O
the	O
shell	B-Operating_System
for	O
UNIX	O
Version	O
7	O
in	O
1979	O
,	O
it	O
introduced	O
the	O
rest	O
of	O
the	O
basic	O
features	O
considered	O
common	O
to	O
all	O
the	O
later	O
Unix	B-Application
shells	I-Application
,	O
including	O
here	O
documents	O
,	O
command	B-Device
substitution	I-Device
,	O
more	O
generic	O
variables	O
and	O
more	O
extensive	O
builtin	O
control	O
structures	O
.	O
</s>
<s>
The	O
language	O
,	O
including	O
the	O
use	O
of	O
a	O
reversed	O
keyword	O
to	O
mark	O
the	O
end	O
of	O
a	O
block	O
,	O
was	O
influenced	O
by	O
ALGOL	B-Language
68	I-Language
.	O
</s>
<s>
Traditionally	O
,	O
the	B-Device
Bourne	I-Device
shell	I-Device
program	O
name	O
is	O
and	O
its	O
path	O
in	O
the	O
Unix	O
file	O
system	O
hierarchy	O
is	O
.	O
</s>
<s>
On	O
many	O
systems	O
,	O
sh	B-Device
may	O
be	O
a	O
symbolic	B-Application
link	I-Application
or	O
hard	B-Application
link	I-Application
to	O
one	O
of	O
these	O
alternatives	O
:	O
</s>
<s>
Almquist	B-Operating_System
shell	I-Operating_System
(	O
ash	B-Operating_System
)	O
:	O
written	O
as	O
a	O
BSD-licensed	O
replacement	O
for	O
the	B-Device
Bourne	I-Device
Shell	I-Device
;	O
often	O
used	O
in	O
resource-constrained	O
environments	O
.	O
</s>
<s>
The	O
sh	B-Device
of	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
(	O
and	O
their	O
derivatives	O
)	O
are	O
based	O
on	O
ash	B-Operating_System
that	O
has	O
been	O
enhanced	O
to	O
be	O
POSIX	O
conformant	O
.	O
</s>
<s>
Busybox	B-Application
:	O
a	O
set	O
of	O
Unix	B-Application
utilities	I-Application
for	O
small	O
and	O
embedded	O
systems	O
,	O
which	O
includes	O
2	O
shells	O
:	O
ash	B-Operating_System
,	O
a	O
derivative	O
of	O
the	O
Almquist	B-Operating_System
shell	I-Operating_System
;	O
and	O
hush	O
,	O
an	O
independent	O
implementation	O
of	O
a	O
Bourne	B-Device
shell	I-Device
.	O
</s>
<s>
Bourne-Again	B-Operating_System
shell	I-Operating_System
(	O
bash	B-Operating_System
)	O
:	O
written	O
as	O
part	O
of	O
the	B-Application
GNU	I-Application
Project	I-Application
to	O
provide	O
a	O
superset	O
of	O
Bourne	B-Device
Shell	I-Device
functionality	O
.	O
</s>
<s>
This	O
shell	B-Operating_System
can	O
be	O
found	O
installed	O
and	O
is	O
the	O
default	O
interactive	O
shell	B-Operating_System
for	O
users	O
on	O
most	O
Linux	B-Application
systems	O
.	O
</s>
<s>
Z	B-Application
shell	I-Application
(	O
zsh	B-Application
)	O
:	O
a	O
relatively	O
modern	O
shell	B-Operating_System
that	O
is	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
bash	B-Operating_System
.	O
</s>
<s>
It	O
's	O
the	O
default	O
shell	B-Operating_System
in	O
Kali	O
Linux	B-Application
since	O
2020.4	O
and	O
macOS	B-Application
since	O
10.15	O
Catalina	B-Device
.	O
</s>
<s>
The	O
POSIX	O
standard	O
specifies	O
its	O
standard	O
shell	B-Operating_System
as	O
a	O
strict	O
subset	O
of	O
the	O
Korn	B-Language
shell	I-Language
,	O
an	O
enhanced	O
version	O
of	O
the	B-Device
Bourne	I-Device
shell	I-Device
.	O
</s>
<s>
From	O
a	O
user	O
's	O
perspective	O
the	B-Device
Bourne	I-Device
shell	I-Device
was	O
immediately	O
recognized	O
when	O
active	O
by	O
its	O
characteristic	O
default	O
command	O
line	O
prompt	O
character	O
,	O
the	O
dollar	O
sign	O
(	O
)	O
.	O
</s>
<s>
The	O
C	B-Operating_System
shell	I-Operating_System
,	O
csh	B-Operating_System
,	O
was	O
modeled	O
on	O
the	O
C	O
programming	O
language	O
,	O
including	O
the	O
control	O
structures	O
and	O
the	O
expression	O
grammar	O
.	O
</s>
<s>
It	O
was	O
written	O
by	O
Bill	O
Joy	O
as	O
a	O
graduate	O
student	O
at	O
University	O
of	O
California	O
,	O
Berkeley	O
,	O
and	O
was	O
widely	O
distributed	O
with	O
BSD	B-Operating_System
Unix	I-Operating_System
.	O
</s>
<s>
The	O
C	B-Operating_System
shell	I-Operating_System
also	O
introduced	O
many	O
features	O
for	O
interactive	O
work	O
,	O
including	O
the	O
history	O
and	O
editing	O
mechanisms	O
,	O
aliases	O
,	O
directory	O
stacks	O
,	O
tilde	O
notation	O
,	O
cdpath	O
,	O
job	O
control	O
and	O
path	O
hashing	O
.	O
</s>
<s>
On	O
many	O
systems	O
,	O
csh	B-Operating_System
may	O
be	O
a	O
symbolic	B-Application
link	I-Application
or	O
hard	B-Application
link	I-Application
to	O
TENEX	B-Language
C	I-Language
shell	I-Language
(	O
tcsh	B-Language
)	O
,	O
an	O
improved	O
version	O
of	O
Joy	O
's	O
original	O
version	O
.	O
</s>
<s>
Although	O
the	O
interactive	O
features	O
of	O
csh	B-Operating_System
have	O
been	O
copied	O
to	O
most	O
other	O
shells	O
,	O
the	O
language	O
structure	O
has	O
not	O
been	O
widely	O
copied	O
.	O
</s>
<s>
The	O
only	O
work-alike	O
is	O
Hamilton	B-Device
C	I-Device
shell	I-Device
,	O
written	O
by	O
Nicole	B-Device
Hamilton	I-Device
,	O
first	O
distributed	O
on	O
OS/2	B-Application
in	O
1988	O
and	O
on	O
Windows	O
since	O
1992	O
.	O
</s>
<s>
These	O
files	O
usually	O
contain	O
commands	O
for	O
the	O
shell	B-Operating_System
and	O
are	O
executed	O
when	O
loaded	O
;	O
they	O
are	O
usually	O
used	O
to	O
set	O
important	O
variables	O
used	O
to	O
find	O
executables	O
,	O
like	O
$PATH	B-Application
,	O
and	O
others	O
that	O
control	O
the	O
behavior	O
and	O
appearance	O
of	O
the	O
shell	B-Operating_System
.	O
</s>
<s>
blank	O
means	O
a	O
file	O
is	O
not	O
read	O
by	O
a	O
shell	B-Operating_System
at	O
all	O
.	O
</s>
<s>
"	O
yes	O
"	O
means	O
a	O
file	O
is	O
always	O
read	O
by	O
a	O
shell	B-Operating_System
upon	O
startup	O
.	O
</s>
<s>
"	O
login	O
"	O
means	O
a	O
file	O
is	O
read	O
if	O
the	O
shell	B-Operating_System
is	O
a	O
login	O
shell	B-Operating_System
.	O
</s>
<s>
"	O
n/login	O
"	O
means	O
a	O
file	O
is	O
read	O
if	O
the	O
shell	B-Operating_System
is	O
not	O
a	O
login	O
shell	B-Operating_System
.	O
</s>
<s>
means	O
a	O
file	O
is	O
read	O
if	O
the	O
shell	B-Operating_System
is	O
interactive	O
.	O
</s>
<s>
Variations	O
on	O
the	O
Unix	B-Application
shell	I-Application
concept	O
that	O
do	O
n't	O
derive	O
from	O
Bourne	B-Device
shell	I-Device
or	O
C	B-Operating_System
shell	I-Operating_System
include	O
the	O
following	O
:	O
</s>
<s>
es	B-Operating_System
–	O
A	O
functional	B-Language
programming	I-Language
rc-compatible	O
shell	B-Operating_System
written	O
in	O
the	O
mid-1990s	O
.	O
</s>
<s>
Friendly	B-Language
interactive	I-Language
shell	I-Language
(	O
fish	O
)	O
–	O
First	O
released	O
in	O
2005	O
.	O
</s>
<s>
PowerShell	B-Application
–	O
An	O
object-oriented	B-Language
shell	B-Operating_System
developed	O
originally	O
for	O
Windows	O
OS	O
and	O
now	O
available	O
to	O
macOS	B-Application
and	O
Linux	B-Application
.	O
</s>
<s>
Qshell	B-Operating_System
–	O
A	O
shell	B-Operating_System
on	O
the	O
IBM	B-Application
i	I-Application
operating	B-General_Concept
system	I-General_Concept
based	O
on	O
POSIX	O
and	O
X/Open	O
standards	O
.	O
</s>
<s>
rc	B-Operating_System
–	O
The	O
default	O
shell	B-Operating_System
on	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
and	O
Version	B-Operating_System
10	I-Operating_System
Unix	I-Operating_System
written	O
by	O
Tom	O
Duff	O
.	O
</s>
<s>
Ports	O
have	O
been	O
made	O
to	O
various	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
scsh	B-Language
–	O
A	O
Scheme	B-Language
Shell	I-Language
.	O
</s>
<s>
wish	B-Language
–	O
A	O
windowing	O
shell	B-Operating_System
for	O
Tcl/Tk	B-Application
.	O
</s>
