<s>
To	O
shut	O
down	O
or	O
power	B-Device
off	I-Device
a	O
computer	O
is	O
to	O
remove	O
power	O
from	O
a	O
computer	O
's	O
main	O
components	O
in	O
a	O
controlled	O
way	O
.	O
</s>
<s>
After	O
a	O
computer	O
is	O
shut	O
down	O
,	O
main	O
components	O
such	O
as	O
CPUs	O
,	O
RAM	B-Architecture
modules	O
and	O
hard	B-Device
disk	I-Device
drives	I-Device
are	O
powered	O
down	O
,	O
although	O
some	O
internal	O
components	O
,	O
such	O
as	O
an	O
internal	O
clock	O
,	O
may	O
retain	O
power	O
.	O
</s>
<s>
The	O
shutdown	B-Device
feature	O
and	O
command	B-Application
is	O
available	O
in	O
Microsoft	B-Application
Windows	I-Application
,	O
ReactOS	B-Application
,	O
HP	B-Operating_System
MPE/iX	I-Operating_System
,	O
and	O
in	O
a	O
number	O
of	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
such	O
as	O
Apple	O
macOS	B-Application
.	O
</s>
<s>
In	O
Microsoft	B-Application
Windows	I-Application
and	O
ReactOS	B-Application
,	O
a	O
PC	O
or	O
server	O
is	O
shut	O
down	O
by	O
selecting	O
the	O
item	O
from	O
the	O
Start	O
menu	O
on	O
the	O
desktop	O
.	O
</s>
<s>
Just	O
like	O
other	O
operating	O
systems	O
,	O
Windows	B-Application
has	O
the	O
option	O
to	O
prohibit	O
selected	O
users	O
from	O
shutting	O
down	O
a	O
computer	O
.	O
</s>
<s>
On	O
a	O
home	O
PC	O
,	O
every	O
user	O
may	O
have	O
the	O
shutdown	B-Device
option	O
,	O
but	O
in	O
computers	O
on	O
large	O
networks	O
(	O
such	O
as	O
Active	B-Protocol
Directory	I-Protocol
)	O
,	O
an	O
administrator	O
can	O
revoke	O
the	O
access	O
rights	O
of	O
selected	O
users	O
to	O
shut	O
down	O
a	O
Windows	B-Application
computer	O
.	O
</s>
<s>
Nowadays	O
there	O
are	O
many	O
software	O
utilities	O
which	O
can	O
automate	O
the	O
task	O
of	O
shutting	O
down	O
a	O
Windows	B-Application
computer	O
,	O
enabling	O
automatic	O
computer	O
control	O
.	O
</s>
<s>
The	O
Windows	B-Application
Shutdown	B-Device
website	O
lists	O
various	O
software	O
utilities	O
to	O
automate	O
the	O
task	O
of	O
shutting	O
down	O
.	O
</s>
<s>
In	O
Windows	B-Application
,	O
a	O
program	O
can	O
shut	O
down	O
the	O
system	O
by	O
calling	O
the	O
ExitWindowsEx	O
or	O
NtShutdownSystem	O
function	O
.	O
</s>
<s>
There	O
is	O
also	O
a	O
shutdown	B-Device
command	B-Application
that	O
can	O
be	O
executed	O
within	O
a	O
command	B-Operating_System
shell	I-Operating_System
window	O
.	O
</s>
<s>
shutdown.exe	O
is	O
the	O
command-line	B-Application
shutdown	B-Device
application	O
(	O
located	O
in	O
%windir%	O
\System32\shutdown.exe	O
)	O
that	O
can	O
shut	O
down	O
the	O
user	O
's	O
computer	O
or	O
another	O
computer	O
on	O
the	O
user	O
's	O
network	O
.	O
</s>
<s>
More	O
than	O
one	O
parameter	O
can	O
be	O
used	O
at	O
a	O
time	O
for	O
this	O
command	B-Application
.	O
</s>
<s>
-aStops	O
shutdown.exe	O
.	O
</s>
<s>
-c	O
"	O
message	O
"	O
Allows	O
a	O
message	O
to	O
be	O
shown	O
in	O
the	O
System	O
Shutdown	B-Device
window	O
.	O
</s>
<s>
In	O
Apple	O
macOS	B-Application
the	O
computer	O
can	O
be	O
shut	O
down	O
by	O
choosing	O
"	O
Shut	O
Down	O
…	O
"	O
from	O
the	O
Apple	O
Menu	O
,	O
by	O
pressing	O
key/button	O
(	O
or	O
key	O
)	O
,	O
or	O
by	O
pressing	O
the	O
power	O
key	O
to	O
bring	O
up	O
the	O
power	O
management	O
dialog	O
box	O
and	O
selecting	O
button	O
"	O
Shut	O
down	O
"	O
.	O
</s>
<s>
An	O
administrator	O
may	O
also	O
use	O
the	O
Unix	B-Application
shutdown	B-Device
command	B-Application
as	O
well	O
.	O
</s>
<s>
On	O
newer	O
and	O
some	O
older	O
Apple	O
computers	O
,	O
starting	O
with	O
Mac	B-Operating_System
OS	I-Operating_System
9	I-Operating_System
,	O
the	O
user	O
is	O
given	O
a	O
time	O
limit	O
in	O
which	O
the	O
computer	O
will	O
automatically	O
shut	O
down	O
if	O
the	O
user	O
does	O
not	O
click	O
the	O
"	O
Shut	O
Down	O
"	O
button	O
.	O
</s>
<s>
In	O
Unix	B-Application
and	O
Linux	B-Application
,	O
the	O
shutdown	B-Device
command	B-Application
can	O
be	O
used	O
to	O
turn	O
off	O
or	O
reboot	B-Operating_System
a	O
computer	O
.	O
</s>
<s>
Only	O
the	O
superuser	B-Application
can	O
shut	O
the	O
system	O
down	O
.	O
</s>
<s>
One	O
commonly	O
issued	O
form	O
of	O
this	O
command	B-Application
is	O
shutdown	B-Device
-h	O
now	O
,	O
which	O
will	O
shut	O
down	O
a	O
system	O
immediately	O
.	O
</s>
<s>
Another	O
one	O
is	O
shutdown	B-Device
-r	O
now	O
to	O
reboot	B-Operating_System
.	O
</s>
<s>
Another	O
form	O
allows	O
the	O
user	O
to	O
specify	O
an	O
exact	O
time	O
or	O
a	O
delay	O
before	O
shutdown	B-Device
:	O
shutdown	B-Device
-h	O
20:00	O
will	O
turn	O
the	O
computer	O
off	O
at	O
8:00	O
PM	O
,	O
and	O
shutdown	B-Device
-r	O
+1	O
will	O
automatically	O
reboot	B-Operating_System
the	O
machine	O
in	O
one	O
minute	O
of	O
issuing	O
the	O
command	B-Application
.	O
</s>
