<s>
A	O
runlevel	B-Device
is	O
a	O
mode	O
of	O
operation	O
in	O
the	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
that	O
implements	O
Unix	O
System	O
V-style	O
initialization	O
.	O
</s>
<s>
Conventionally	O
,	O
seven	O
runlevels	B-Device
exist	O
,	O
numbered	O
from	O
zero	O
to	O
six	O
.	O
</s>
<s>
Only	O
one	O
runlevel	B-Device
is	O
executed	O
on	O
startup	O
;	O
run	B-Device
levels	I-Device
are	O
not	O
executed	O
one	O
after	O
another	O
(	O
i.e.	O
</s>
<s>
only	O
runlevel	B-Device
2	O
,	O
3	O
,	O
or	O
4	O
is	O
executed	O
,	O
not	O
more	O
of	O
them	O
sequentially	O
or	O
in	O
any	O
other	O
order	O
)	O
.	O
</s>
<s>
A	O
runlevel	B-Device
defines	O
the	O
state	O
of	O
the	O
machine	O
after	O
boot	B-Operating_System
.	O
</s>
<s>
Different	O
runlevels	B-Device
are	O
typically	O
assigned	O
(	O
not	O
necessarily	O
in	O
any	O
particular	O
order	O
)	O
to	O
the	O
single-user	B-Operating_System
mode	I-Operating_System
,	O
multi-user	B-Operating_System
mode	O
without	O
network	O
services	O
started	O
,	O
multi-user	B-Operating_System
mode	O
with	O
network	O
services	O
started	O
,	O
system	O
shutdown	O
,	O
and	O
system	B-Operating_System
reboot	I-Operating_System
system	O
states	O
.	O
</s>
<s>
The	O
exact	O
setup	O
of	O
these	O
configurations	O
varies	O
between	O
operating	B-General_Concept
systems	I-General_Concept
and	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
runlevel	B-Device
4	O
might	O
be	O
a	O
multi-user	B-Operating_System
GUI	O
no-server	O
configuration	O
on	O
one	O
distribution	O
,	O
and	O
nothing	O
on	O
another	O
.	O
</s>
<s>
Runlevels	B-Device
commonly	O
follow	O
the	O
general	O
patterns	O
described	O
in	O
this	O
article	O
;	O
however	O
,	O
some	O
distributions	O
employ	O
certain	O
specific	O
configurations	O
.	O
</s>
<s>
In	O
standard	O
practice	O
,	O
when	O
a	O
computer	O
enters	O
runlevel	B-Device
zero	O
,	O
it	O
shuts	O
off	O
,	O
and	O
when	O
it	O
enters	O
runlevel	B-Device
six	O
,	O
it	O
reboots	B-Device
.	O
</s>
<s>
The	O
intermediate	O
runlevels	B-Device
(	O
1	O
–	O
5	O
)	O
differ	O
in	O
terms	O
of	O
which	O
drives	O
are	O
mounted	O
and	O
which	O
network	O
services	O
are	O
started	O
.	O
</s>
<s>
Default	O
runlevels	B-Device
are	O
typically	O
3	O
,	O
4	O
,	O
or	O
5	O
.	O
</s>
<s>
Lower	O
runlevels	B-Device
are	O
useful	O
for	O
maintenance	O
or	O
emergency	O
repairs	O
,	O
since	O
they	O
usually	O
offer	O
no	O
network	O
services	O
at	O
all	O
.	O
</s>
<s>
The	O
particular	O
details	O
of	O
runlevel	B-Device
configuration	O
differ	O
widely	O
among	O
operating	B-General_Concept
systems	I-General_Concept
,	O
and	O
also	O
among	O
system	O
administrators	O
.	O
</s>
<s>
In	O
various	O
Linux	B-Application
distributions	I-Application
,	O
the	O
traditional	O
script	O
used	O
in	O
the	O
Version	O
7	O
Unix	O
was	O
first	O
replaced	O
by	O
runlevels	B-Device
and	O
then	O
by	O
systemd	B-Application
states	O
on	O
most	O
major	O
distributions	O
.	O
</s>
<s>
+Standard	O
runlevelsIDNameDescription0OffTurns	O
off	O
the	O
device.1Single	O
user	O
modeDoes	O
not	O
configure	O
network	O
interfaces	O
or	O
start	O
daemons.6RebootReboots	O
the	O
device	O
.	O
</s>
<s>
Although	O
systemd	B-Application
is	O
,	O
,	O
used	O
by	O
default	O
in	O
most	O
major	O
Linux	B-Application
distributions	I-Application
,	O
runlevels	B-Device
can	O
still	O
be	O
used	O
through	O
the	O
means	O
provided	O
by	O
the	O
sysvinit	O
project	O
.	O
</s>
<s>
After	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
has	O
booted	O
,	O
the	O
program	O
reads	O
the	O
file	O
to	O
determine	O
the	O
behavior	O
for	O
each	O
runlevel	B-Device
.	O
</s>
<s>
Unless	O
the	O
user	O
specifies	O
another	O
value	O
as	O
a	O
kernel	B-Operating_System
boot	B-Operating_System
parameter	O
,	O
the	O
system	O
will	O
attempt	O
to	O
enter	O
(	O
start	O
)	O
the	O
default	O
runlevel	B-Device
.	O
</s>
<s>
Systems	O
conforming	O
to	O
the	O
Linux	B-Application
Standard	I-Application
Base	I-Application
(	O
LSB	O
)	O
need	O
not	O
provide	O
the	O
exact	O
run	B-Device
levels	I-Device
given	O
here	O
or	O
give	O
them	O
the	O
meanings	O
described	O
here	O
,	O
and	O
may	O
map	O
any	O
level	O
described	O
here	O
to	O
a	O
different	O
level	O
which	O
provides	O
the	O
equivalent	O
functionality	O
.	O
</s>
<s>
+LSB	O
4.1.0IDNameDescription0OffTurns	O
off	O
the	O
device.1Single-user	O
modeMode	O
for	O
administrative	O
tasks.2Multi-user	O
modeDoes	O
not	O
configure	O
network	O
interfaces	O
and	O
does	O
not	O
export	O
networks	O
services.3Multi-user	O
mode	O
with	O
networkingStarts	O
the	O
system	O
normally.4Not	O
used/user	O
-definableFor	O
special	O
purposes.5Full	O
modeSame	O
as	O
runlevel	B-Device
3	O
+	O
display	O
manager.6RebootReboots	O
the	O
device	O
.	O
</s>
<s>
Slackware	B-Application
Linux	I-Application
uses	O
runlevel	B-Device
1	O
for	O
maintenance	O
,	O
as	O
on	O
other	O
Linux	B-Application
distributions	I-Application
;	O
runlevels	B-Device
2	O
,	O
3	O
and	O
5	O
identically	O
configured	O
for	O
a	O
console	O
(	O
with	O
all	O
services	O
active	O
)	O
;	O
and	O
runlevel	B-Device
4	O
adds	O
the	O
X	O
Window	O
System	O
.	O
</s>
<s>
Starting	O
from	O
Solaris	B-Application
10	I-Application
,	O
SMF	O
(	O
Service	B-Operating_System
Management	I-Operating_System
Facility	I-Operating_System
)	O
is	O
used	O
instead	O
of	O
SVR4	O
run	B-Device
levels	I-Device
.	O
</s>
<s>
+Solaris	O
runlevelsIDDescription0On	O
SPARC	O
systems	O
,	O
it	O
returns	O
to	O
firmware	O
,	O
while	O
on	O
x86	O
systems	O
,	O
it	O
shuts	O
off	O
the	O
machine	O
.	O
</s>
<s>
If	O
the	O
user	O
presses	O
Enter	O
,	O
the	O
system	O
reboots.SSingle-user	O
mode	O
with	O
only	O
root	B-Application
filesystem	O
mounted	O
(	O
as	O
read-only	O
)	O
--	O
Solaris	O
10+	O
:	O
svc:/milestone/single	O
-user1Single-user	O
mode	O
with	O
all	O
local	O
filesystems	O
mounted	O
(	O
read-write	O
)	O
2Multi-user	O
mode	O
with	O
most	O
daemons	B-Operating_System
started	O
–	O
Solaris	O
10+	O
:	O
svc:/milestone/multi	O
-user3Multi-user	O
mode	O
;	O
identical	O
to	O
2	O
(	O
runlevel	B-Device
3	O
runs	O
both	O
and	O
)	O
,	O
with	O
filesystems	O
exported	O
,	O
plus	O
some	O
other	O
network	O
services	O
started	O
.	O
</s>
<s>
AIX	B-Application
does	O
not	O
follow	O
the	O
System	B-Operating_System
V	I-Operating_System
R4	O
(	O
SVR4	O
)	O
runlevel	B-Device
specification	O
,	O
with	O
runlevels	B-Device
from	O
0	O
to	O
9	O
available	O
,	O
as	O
well	O
as	O
from	O
a	O
to	O
c	O
(	O
or	O
h	O
)	O
.	O
</s>
<s>
0	O
and	O
1	O
are	O
reserved	O
,	O
2	O
is	O
the	O
default	O
normal	O
multi-user	B-Operating_System
mode	O
and	O
runlevels	B-Device
from	O
3	O
to	O
9	O
are	O
free	O
to	O
be	O
defined	O
by	O
the	O
administrator	O
.	O
</s>
<s>
Runlevels	B-Device
from	O
a	O
to	O
c	O
(	O
or	O
h	O
)	O
allow	O
the	O
execution	O
of	O
processes	O
in	O
that	O
runlevel	B-Device
without	O
killing	O
processes	O
started	O
in	O
another	O
.	O
</s>
<s>
The	O
S	O
,	O
s	O
,	O
M	O
and	O
m	O
runlevels	B-Device
are	O
not	O
true	O
runlevels	B-Device
,	O
but	O
are	O
used	O
to	O
tell	O
the	O
init	B-Operating_System
command	O
to	O
enter	O
maintenance	O
mode	O
.	O
</s>
<s>
When	O
the	O
system	O
enters	O
maintenance	O
mode	O
from	O
another	O
runlevel	B-Device
,	O
only	O
the	O
system	O
console	O
is	O
used	O
as	O
the	O
terminal	O
.	O
</s>
