<s>
In	O
computing	O
,	O
a	O
shell	B-Application
is	O
a	O
computer	O
program	O
that	O
exposes	O
an	O
operating	B-General_Concept
system	I-General_Concept
's	O
services	O
to	O
a	O
human	O
user	O
or	O
other	O
programs	O
.	O
</s>
<s>
In	O
general	O
,	O
operating	B-Operating_System
system	I-Operating_System
shells	I-Operating_System
use	O
either	O
a	O
command-line	B-Application
interface	I-Application
(	O
CLI	B-Application
)	O
or	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
,	O
depending	O
on	O
a	O
computer	O
's	O
role	O
and	O
particular	O
operation	O
.	O
</s>
<s>
It	O
is	O
named	O
a	O
shell	B-Application
because	O
it	O
is	O
the	O
outermost	O
layer	O
around	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Command-line	B-Application
shells	O
require	O
the	O
user	O
to	O
be	O
familiar	O
with	O
commands	O
and	O
their	O
calling	O
syntax	O
,	O
and	O
to	O
understand	O
concepts	O
about	O
the	O
shell-specific	O
scripting	O
language	O
(	O
for	O
example	O
,	O
bash	B-Operating_System
)	O
,	O
while	O
graphical	O
shells	O
place	O
a	O
low	O
burden	O
on	O
beginning	O
computer	O
users	O
and	O
are	O
characterized	O
as	O
being	O
easy	O
to	O
use	O
,	O
yet	O
most	O
GUI-enabled	O
operating	B-General_Concept
systems	I-General_Concept
also	O
provide	O
CLI	B-Application
shells	O
,	O
normally	O
for	O
performing	O
advanced	O
tasks	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
provide	O
various	O
services	O
to	O
their	O
users	O
,	O
including	O
file	B-Application
management	I-Application
,	O
process	B-Operating_System
management	O
(	O
running	O
and	O
terminating	O
applications	B-Application
)	O
,	O
batch	B-General_Concept
processing	I-General_Concept
,	O
and	O
operating	B-General_Concept
system	I-General_Concept
monitoring	O
and	O
configuration	O
.	O
</s>
<s>
Most	O
operating	B-Operating_System
system	I-Operating_System
shells	I-Operating_System
are	O
not	O
direct	O
interfaces	O
to	O
the	O
underlying	O
kernel	O
,	O
even	O
if	O
a	O
shell	B-Application
communicates	O
with	O
the	O
user	O
via	O
peripheral	B-Device
devices	I-Device
attached	O
to	O
the	O
computer	O
directly	O
.	O
</s>
<s>
Shells	O
are	O
actually	O
special	O
applications	B-Application
that	O
use	O
the	O
kernel	O
API	B-Application
in	O
just	O
the	O
same	O
way	O
as	O
it	O
is	O
used	O
by	O
other	O
application	B-Application
programs	I-Application
.	O
</s>
<s>
A	O
shell	B-Application
manages	O
the	O
user	O
system	O
interaction	O
by	O
prompting	O
users	O
for	O
input	O
,	O
interpreting	O
their	O
input	O
,	O
and	O
then	O
handling	O
output	O
from	O
the	O
underlying	O
operating	B-General_Concept
system	I-General_Concept
(	O
much	O
like	O
a	O
read	B-Application
–	I-Application
eval	I-Application
–	I-Application
print	I-Application
loop	I-Application
,	I-Application
REPL	I-Application
)	O
.	O
</s>
<s>
Since	O
the	O
operating	B-Operating_System
system	I-Operating_System
shell	I-Operating_System
is	O
actually	O
an	O
application	O
,	O
it	O
may	O
easily	O
be	O
replaced	O
with	O
another	O
similar	O
application	O
,	O
for	O
most	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Initially	O
available	O
on	O
multi-user	B-Operating_System
mainframes	B-Architecture
,	O
which	O
provided	O
text-based	B-Application
UIs	O
for	O
each	O
active	O
user	O
simultaneously	O
by	O
means	O
of	O
a	O
text	O
terminal	B-General_Concept
connected	O
to	O
the	O
mainframe	B-Architecture
via	O
serial	O
line	O
or	O
modem	B-Application
,	O
remote	O
access	O
has	O
extended	O
to	O
Unix-like	B-Operating_System
systems	I-Operating_System
and	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
Secure	O
Shell	B-Application
protocol	O
is	O
usually	O
used	O
for	O
text-based	B-Application
shells	O
,	O
while	O
SSH	O
tunneling	O
can	O
be	O
used	O
for	O
X	B-Operating_System
Window	I-Operating_System
Systembased	O
graphical	B-Application
user	I-Application
interfaces	I-Application
(	O
GUIs	B-Application
)	O
.	O
</s>
<s>
On	O
Microsoft	B-Application
Windows	I-Application
,	O
Remote	O
Desktop	O
Protocol	O
can	O
be	O
used	O
to	O
provide	O
GUI	B-Application
remote	O
access	O
,	O
and	O
since	O
Windows	B-Application
Vista	I-Application
,	O
PowerShell	B-Application
Remote	I-Application
can	O
be	O
used	O
for	O
text-based	B-Application
remote	O
access	O
via	O
WMI	O
,	O
RPC	O
,	O
and	O
WS-Management	O
.	O
</s>
<s>
Most	O
operating	B-Operating_System
system	I-Operating_System
shells	I-Operating_System
fall	O
into	O
one	O
of	O
two	O
categories	O
command-line	B-Application
and	O
graphical	O
.	O
</s>
<s>
Command-line	B-Application
shells	O
provide	O
a	O
command-line	B-Application
interface	I-Application
(	O
CLI	B-Application
)	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
while	O
graphical	O
shells	O
provide	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
.	O
</s>
<s>
Other	O
possibilities	O
,	O
although	O
not	O
so	O
common	O
,	O
include	O
a	O
voice	O
user	O
interface	O
and	O
various	O
implementations	O
of	O
a	O
text-based	B-Application
user	I-Application
interface	I-Application
(	O
TUI	B-Application
)	O
that	O
are	O
not	O
CLI	B-Application
,	O
such	O
as	O
text-based	B-Application
menu	O
systems	O
.	O
</s>
<s>
The	O
relative	O
merits	O
of	O
CLI	B-Application
-	O
and	O
GUI-based	O
shells	O
are	O
often	O
debated	O
.	O
</s>
<s>
Early	O
interactive	O
systems	O
provided	O
a	O
simple	O
command-line	B-Application
interpreter	O
as	O
part	O
of	O
the	O
resident	B-Application
monitor	I-Application
.	O
</s>
<s>
This	O
interpreter	O
might	O
be	O
called	O
by	O
different	O
names	O
,	O
such	O
as	O
COMCON	O
on	O
DEC	O
TOPS-10	B-Operating_System
systems	O
.	O
</s>
<s>
Common	O
commands	O
would	O
log	O
the	O
user	O
on	O
and	O
off	O
the	O
system	O
,	O
allocate	O
,	O
free	O
,	O
and	O
manipulate	O
devices	O
and	O
files	O
,	O
and	O
query	O
various	O
pieces	O
of	O
information	O
about	O
the	O
system	O
or	O
a	O
user	O
process	B-Operating_System
.	O
</s>
<s>
In	O
1964	O
,	O
for	O
the	O
Multics	B-Application
operating	I-Application
system	I-Application
,	O
Louis	O
Pouzin	O
conceived	O
the	O
idea	O
of	O
"	O
using	O
commands	O
somehow	O
like	O
a	O
programming	O
language	O
,	O
"	O
and	O
coined	O
the	O
term	O
shell	B-Application
to	O
describe	O
it	O
.	O
</s>
<s>
In	O
a	O
1965	O
document	O
,	O
the	O
shell	B-Application
is	O
defined	O
as	O
"	O
a	O
common	O
procedure	O
called	O
automatically	O
by	O
the	O
supervisor	O
whenever	O
a	O
user	O
types	O
in	O
some	O
message	O
at	O
his	O
console	O
,	O
at	O
a	O
time	O
when	O
he	O
has	O
no	O
other	O
process	B-Operating_System
in	O
active	O
execution	O
under	O
console	O
control	O
.	O
</s>
<s>
Multics	B-Application
also	O
introduced	O
the	O
active	O
function	O
,	O
a	O
key	O
concept	O
in	O
all	O
later	O
shells	O
.	O
</s>
<s>
In	O
1971	O
,	O
Ken	O
Thompson	O
developed	O
the	O
Thompson	B-Application
shell	I-Application
in	O
the	O
first	O
version	O
of	O
Unix	O
.	O
</s>
<s>
While	O
simpler	O
than	O
the	O
Multics	B-Application
shell	B-Application
,	O
it	O
contained	O
some	O
innovative	O
features	O
,	O
which	O
have	O
been	O
carried	O
forward	O
in	O
modern	O
shells	O
,	O
including	O
the	O
use	O
of	O
and	O
for	O
input	O
and	O
output	O
redirection	B-Device
.	O
</s>
<s>
The	O
graphical	O
shell	B-Application
first	O
appeared	O
in	O
Douglas	O
Engelbart	O
’s	O
NLS	O
system	O
,	O
demonstrated	O
in	O
December	O
,	O
1968	O
at	O
the	O
Fall	O
Joint	O
Computer	O
Conference	O
in	O
San	O
Francisco	O
,	O
in	O
what	O
has	O
been	O
called	O
The	O
Mother	O
of	O
All	O
Demos	O
.	O
</s>
<s>
Engelbart	O
’s	O
colleagues	O
at	O
Stanford	O
Research	O
Institute	O
brought	O
the	O
concept	O
to	O
the	O
Xerox	O
Palo	O
Alto	B-Device
Research	O
Center	O
(	O
PARC	O
)	O
,	O
where	O
it	O
appeared	O
on	O
the	O
Alto	B-Device
,	O
introduced	O
in	O
1973	O
.	O
</s>
<s>
From	O
there	O
the	O
idea	O
spread	O
to	O
Niklaus	O
Wirth	O
’s	O
Lilith	B-Device
in	O
1980	O
,	O
and	O
the	O
Apple	B-Device
Lisa	I-Device
in	O
1983	O
,	O
then	O
became	O
ubiquitous	O
.	O
</s>
<s>
A	O
command-line	B-Application
interface	I-Application
(	O
CLI	B-Application
)	O
is	O
an	O
operating	B-Operating_System
system	I-Operating_System
shell	I-Operating_System
that	O
uses	O
alphanumeric	O
characters	O
typed	O
on	O
a	O
keyboard	O
to	O
provide	O
instructions	O
and	O
data	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
interactively	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
teletypewriter	O
can	O
send	O
codes	O
representing	O
keystrokes	O
to	O
a	O
command	O
interpreter	O
program	O
running	O
on	O
the	O
computer	O
;	O
the	O
command	O
interpreter	O
parses	O
the	O
sequence	O
of	O
keystrokes	O
and	O
responds	O
with	O
an	O
error	O
message	O
if	O
it	O
cannot	O
recognize	O
the	O
sequence	O
of	O
characters	O
,	O
or	O
it	O
may	O
carry	O
out	O
some	O
other	O
program	O
action	O
such	O
as	O
loading	O
an	O
application	B-Application
program	I-Application
,	O
listing	O
files	O
,	O
logging	O
in	O
a	O
user	O
and	O
many	O
others	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
UNIX	O
have	O
a	O
large	O
variety	O
of	O
shell	B-Application
programs	O
with	O
different	O
commands	O
,	O
syntax	O
and	O
capabilities	O
,	O
with	O
the	O
POSIX	O
shell	B-Application
being	O
a	O
baseline	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
had	O
only	O
a	O
single	O
style	O
of	O
command	O
interface	O
;	O
commodity	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
such	O
as	O
MS-DOS	B-Application
came	O
with	O
a	O
standard	O
command	O
interface	O
(	O
COMMAND.COM	B-Operating_System
)	O
but	O
third-party	O
interfaces	O
were	O
also	O
often	O
available	O
,	O
providing	O
additional	O
features	O
or	O
functions	O
such	O
as	O
menuing	O
or	O
remote	O
program	O
execution	O
.	O
</s>
<s>
Application	B-Application
programs	I-Application
may	O
also	O
implement	O
a	O
command-line	B-Application
interface	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
in	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
the	O
telnet	B-Protocol
program	O
has	O
a	O
number	O
of	O
commands	O
for	O
controlling	O
a	O
link	O
to	O
a	O
remote	O
computer	O
system	O
.	O
</s>
<s>
A	O
feature	O
of	O
many	O
command-line	B-Application
shells	O
is	O
the	O
ability	O
to	O
save	O
sequences	O
of	O
commands	O
for	O
re-use	O
.	O
</s>
<s>
A	O
data	O
file	O
can	O
contain	O
sequences	O
of	O
commands	O
which	O
the	O
CLI	B-Application
can	O
be	O
made	O
to	O
follow	O
as	O
if	O
typed	O
in	O
by	O
a	O
user	O
.	O
</s>
<s>
Special	O
features	O
in	O
the	O
CLI	B-Application
may	O
apply	O
when	O
it	O
is	O
carrying	O
out	O
these	O
stored	O
instructions	O
.	O
</s>
<s>
Batch	B-General_Concept
mode	I-General_Concept
use	O
of	O
shells	O
usually	O
involves	O
structures	O
,	O
conditionals	O
,	O
variables	O
,	O
and	O
other	O
elements	O
of	O
programming	O
languages	O
;	O
some	O
have	O
the	O
bare	O
essentials	O
needed	O
for	O
such	O
a	O
purpose	O
,	O
others	O
are	O
very	O
sophisticated	O
programming	O
languages	O
in	O
and	O
of	O
themselves	O
.	O
</s>
<s>
Conversely	O
,	O
some	O
programming	O
languages	O
can	O
be	O
used	O
interactively	O
from	O
an	O
operating	B-Operating_System
system	I-Operating_System
shell	I-Operating_System
or	O
in	O
a	O
purpose-built	O
program	O
.	O
</s>
<s>
The	O
command-line	B-Application
shell	B-Application
may	O
offer	O
features	O
such	O
as	O
command-line	B-Application
completion	O
,	O
where	O
the	O
interpreter	O
expands	O
commands	O
based	O
on	O
a	O
few	O
characters	O
input	O
by	O
the	O
user	O
.	O
</s>
<s>
A	O
command-line	B-Application
interpreter	O
may	O
offer	O
a	O
history	O
function	O
,	O
so	O
that	O
the	O
user	O
can	O
recall	O
earlier	O
commands	O
issued	O
to	O
the	O
system	O
and	O
repeat	O
them	O
,	O
possibly	O
with	O
some	O
editing	O
.	O
</s>
<s>
Since	O
all	O
commands	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
had	O
to	O
be	O
typed	O
by	O
the	O
user	O
,	O
short	O
command	O
names	O
and	O
compact	O
systems	O
for	O
representing	O
program	O
options	O
were	O
common	O
.	O
</s>
<s>
A	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
provides	O
means	O
for	O
manipulating	O
programs	O
graphically	O
,	O
by	O
allowing	O
for	O
operations	O
such	O
as	O
opening	O
,	O
closing	O
,	O
moving	O
and	O
resizing	O
windows	O
,	O
as	O
well	O
as	O
switching	O
focus	B-Application
between	O
windows	O
.	O
</s>
<s>
Most	O
graphical	B-Application
user	I-Application
interfaces	I-Application
develop	O
the	O
metaphor	B-Application
of	I-Application
an	I-Application
"	I-Application
electronic	I-Application
desktop	I-Application
"	I-Application
,	O
where	O
data	O
files	O
are	O
represented	O
as	O
if	O
they	O
were	O
paper	O
documents	O
on	O
a	O
desk	O
,	O
and	O
application	B-Application
programs	I-Application
similarly	O
have	O
graphical	O
representations	O
instead	O
of	O
being	O
invoked	O
by	O
command	O
names	O
.	O
</s>
<s>
Graphical	O
shells	O
typically	O
build	O
on	O
top	O
of	O
a	O
windowing	B-Application
system	I-Application
.	O
</s>
<s>
In	O
the	O
case	O
of	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
or	O
Wayland	B-Language
,	O
the	O
shell	B-Application
consists	O
of	O
an	O
X	B-Application
window	I-Application
manager	I-Application
or	O
a	O
Wayland	B-Language
compositor	O
,	O
respectively	O
,	O
as	O
well	O
as	O
of	O
one	O
or	O
multiple	O
programs	O
providing	O
the	O
functionality	O
to	O
start	O
installed	O
applications	B-Application
,	O
to	O
manage	O
open	O
windows	O
and	O
virtual	O
desktops	O
,	O
and	O
often	O
to	O
support	O
a	O
widget	O
engine	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
macOS	B-Application
,	O
Quartz	O
Compositor	O
acts	O
as	O
the	O
windowing	B-Application
system	I-Application
,	O
and	O
the	O
shell	B-Application
consists	O
of	O
the	O
Finder	B-Application
,	O
the	O
Dock	O
,	O
SystemUIServer	O
,	O
and	O
Mission	O
Control	O
.	O
</s>
<s>
Modern	O
versions	O
of	O
the	O
Microsoft	B-Application
Windows	I-Application
operating	I-Application
system	I-Application
use	O
the	O
Windows	B-Application
shell	I-Application
as	O
their	O
shell	B-Application
.	O
</s>
<s>
Windows	B-Application
Shell	I-Application
provides	O
desktop	O
environment	O
,	O
start	O
menu	O
,	O
and	O
task	O
bar	O
,	O
as	O
well	O
as	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
for	O
accessing	O
the	O
file	B-Application
management	I-Application
functions	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Older	O
versions	O
also	O
include	O
Program	O
Manager	O
,	O
which	O
was	O
the	O
shell	B-Application
for	O
the	O
3.x	O
series	O
of	O
Microsoft	B-Application
Windows	I-Application
,	O
and	O
which	O
in	O
fact	O
shipped	O
with	O
later	O
versions	O
of	O
Windows	O
of	O
both	O
the	O
95	O
and	O
NT	O
types	O
at	O
least	O
through	O
Windows	O
XP	O
.	O
</s>
<s>
Desktop	B-Application
applications	I-Application
are	O
also	O
considered	O
shells	O
,	O
as	O
long	O
as	O
they	O
use	O
a	O
third-party	O
engine	O
.	O
</s>
<s>
Likewise	O
,	O
many	O
individuals	O
and	O
developers	O
dissatisfied	O
with	O
the	O
interface	O
of	O
Windows	O
Explorer	O
have	O
developed	O
software	O
that	O
either	O
alters	O
the	O
functioning	O
and	O
appearance	O
of	O
the	O
shell	B-Application
or	O
replaces	O
it	O
entirely	O
.	O
</s>
<s>
WindowBlinds	B-Application
by	O
StarDock	O
is	O
a	O
good	O
example	O
of	O
the	O
former	O
sort	O
of	O
application	O
.	O
</s>
<s>
LiteStep	O
and	O
Emerge	B-Application
Desktop	I-Application
are	O
good	O
examples	O
of	O
the	O
latter	O
.	O
</s>
<s>
Interoperability	O
programmes	O
and	O
purpose-designed	O
software	O
lets	O
Windows	O
users	O
use	O
equivalents	O
of	O
many	O
of	O
the	O
various	O
Unix-based	O
GUIs	B-Application
discussed	O
below	O
,	O
as	O
well	O
as	O
Macintosh	O
.	O
</s>
<s>
"	O
Shell	B-Application
"	O
is	O
also	O
used	O
loosely	O
to	O
describe	O
application	B-Application
software	I-Application
that	O
is	O
"	O
built	O
around	O
"	O
a	O
particular	O
component	O
,	O
such	O
as	O
web	O
browsers	O
and	O
email	O
clients	O
,	O
in	O
analogy	O
to	O
the	O
shells	O
found	O
in	O
nature	O
.	O
</s>
<s>
Indeed	O
,	O
the	O
(	O
command-line	B-Application
)	O
shell	B-Application
encapsulates	O
the	O
operating	B-General_Concept
system	I-General_Concept
kernel	O
.	O
</s>
<s>
In	O
expert	B-Application
systems	I-Application
,	O
a	O
shell	B-Application
is	O
a	O
piece	O
of	O
software	O
that	O
is	O
an	O
"	O
empty	O
"	O
expert	B-Application
system	I-Application
without	O
the	O
knowledge	O
base	O
for	O
any	O
particular	O
application	O
.	O
</s>
