<s>
In	O
Unix-based	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
init	B-Device
(	O
short	O
for	O
initialization	O
)	O
is	O
the	O
first	O
process	B-Operating_System
started	O
during	O
booting	B-Operating_System
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Init	B-Device
is	O
a	O
daemon	B-Operating_System
process	I-Operating_System
that	O
continues	O
running	O
until	O
the	O
system	O
is	O
shut	O
down	O
.	O
</s>
<s>
It	O
is	O
the	O
direct	O
or	O
indirect	O
ancestor	B-Operating_System
of	O
all	O
other	O
processes	O
and	O
automatically	O
adopts	O
all	O
orphaned	B-Operating_System
processes	I-Operating_System
.	O
</s>
<s>
Init	B-Device
is	O
started	O
by	O
the	O
kernel	B-Operating_System
during	O
the	O
booting	B-Operating_System
process	B-Operating_System
;	O
a	O
kernel	B-Operating_System
panic	I-Operating_System
will	O
occur	O
if	O
the	O
kernel	B-Operating_System
is	O
unable	O
to	O
start	O
it	O
,	O
or	O
it	O
should	O
die	O
for	O
any	O
reason	O
.	O
</s>
<s>
Init	B-Device
is	O
typically	O
assigned	O
process	B-Operating_System
identifier	I-Operating_System
1	O
.	O
</s>
<s>
In	O
Unix	B-Application
systems	I-Application
such	O
as	O
System	B-Operating_System
III	I-Operating_System
and	O
System	B-Operating_System
V	I-Operating_System
,	O
the	O
design	O
of	O
init	B-Device
has	O
diverged	O
from	O
the	O
functionality	O
provided	O
by	O
the	O
init	B-Device
in	O
Research	B-Operating_System
Unix	I-Operating_System
and	O
its	O
BSD	B-Operating_System
derivatives	O
.	O
</s>
<s>
Up	O
until	O
recently	O
,	O
most	O
Linux	B-Application
distributions	I-Application
employed	O
a	O
traditional	O
init	B-Device
that	O
was	O
somewhat	O
compatible	O
with	O
SystemV	B-Operating_System
,	O
while	O
some	O
distributions	O
such	O
as	O
Slackware	B-Application
use	O
BSD-style	O
startup	O
scripts	O
,	O
and	O
others	O
such	O
as	O
Gentoo	B-Application
have	O
their	O
own	O
customized	O
versions	O
.	O
</s>
<s>
Since	O
then	O
,	O
several	O
additional	O
init	B-Device
implementations	O
have	O
been	O
created	O
,	O
attempting	O
to	O
address	O
design	O
limitations	O
in	O
the	O
traditional	O
versions	O
.	O
</s>
<s>
These	O
include	O
launchd	B-Application
,	O
the	O
Service	B-Operating_System
Management	I-Operating_System
Facility	I-Operating_System
,	O
systemd	B-Application
,	O
Runit	B-Device
and	O
OpenRC	B-Device
.	O
</s>
<s>
Research	B-Operating_System
Unix	I-Operating_System
init	B-Device
runs	O
the	O
initialization	O
shell	B-Language
script	I-Language
located	O
at	O
/etc/rc	B-Device
,	O
then	O
launches	O
getty	B-Device
on	O
terminals	O
under	O
the	O
control	O
of	O
/etc/ttys	O
.	O
</s>
<s>
There	O
are	O
no	O
runlevels	B-Device
;	O
the	O
/etc/rc	B-Device
file	O
determines	O
what	O
programs	O
are	O
run	O
by	O
init	B-Device
.	O
</s>
<s>
BSD	B-Operating_System
init	B-Device
was	O
,	O
prior	O
to	O
4.3BSD	B-Operating_System
,	O
the	O
same	O
as	O
Research	B-Operating_System
UNIX	I-Operating_System
's	O
init	B-Device
;	O
in	O
4.3BSD	B-Operating_System
,	O
it	O
added	O
support	O
for	O
running	O
a	O
windowing	B-Application
system	I-Application
such	O
as	O
X	B-Operating_System
on	O
graphical	O
terminals	O
under	O
the	O
control	O
of	O
/etc/ttys	O
.	O
</s>
<s>
To	O
remove	O
the	O
requirement	O
to	O
edit	O
/etc/rc	B-Device
,	O
BSD	B-Operating_System
variants	O
have	O
long	O
supported	O
a	O
site-specific	O
/etc/rc.local	O
file	O
that	O
is	O
run	O
in	O
a	O
sub-shell	O
near	O
the	O
end	O
of	O
the	O
boot	B-Operating_System
sequence	I-Operating_System
.	O
</s>
<s>
A	O
fully	O
modular	O
system	O
was	O
introduced	O
with	O
NetBSD	B-Device
1.5	O
and	O
ported	O
to	O
FreeBSD	B-Operating_System
5.0	O
and	O
successors	O
.	O
</s>
<s>
This	O
system	O
executes	O
scripts	O
in	O
the	O
/etc/rc.d	O
directory	O
.	O
</s>
<s>
Unlike	O
System	B-Operating_System
V	I-Operating_System
's	O
script	O
ordering	O
,	O
which	O
is	O
derived	O
from	O
the	O
filename	O
of	O
each	O
script	O
,	O
this	O
system	O
uses	O
explicit	O
dependency	O
tags	O
placed	O
within	O
each	O
script	O
.	O
</s>
<s>
When	O
compared	O
to	O
its	O
predecessors	O
,	O
T	O
's	O
UNIX	B-Operating_System
System	I-Operating_System
III	I-Operating_System
introduced	O
a	O
new	O
style	O
of	O
system	B-Operating_System
startup	I-Operating_System
configuration	O
,	O
which	O
survived	O
(	O
with	O
modifications	O
)	O
into	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
and	O
is	O
therefore	O
called	O
the	O
"	O
SysV-style	O
init	B-Device
"	O
.	O
</s>
<s>
At	O
any	O
moment	O
,	O
a	O
running	O
System	B-Operating_System
V	I-Operating_System
is	O
in	O
one	O
of	O
the	O
predetermined	O
number	O
of	O
states	O
,	O
called	O
runlevels	B-Device
.	O
</s>
<s>
At	O
least	O
one	O
runlevel	B-Device
is	O
the	O
normal	O
operating	O
state	O
of	O
the	O
system	O
;	O
typically	O
,	O
other	O
runlevels	B-Device
represent	O
single-user	B-Operating_System
mode	I-Operating_System
(	O
used	O
for	O
repairing	O
a	O
faulty	O
system	O
)	O
,	O
system	O
shutdown	B-Device
,	O
and	O
various	O
other	O
states	O
.	O
</s>
<s>
Switching	O
from	O
one	O
runlevel	B-Device
to	O
another	O
causes	O
a	O
per-runlevel	O
set	O
of	O
scripts	O
to	O
be	O
run	O
,	O
which	O
typically	O
mount	O
filesystems	O
,	O
start	O
or	O
stop	O
daemons	B-Operating_System
,	O
start	O
or	O
stop	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
shutdown	B-Device
the	O
machine	O
,	O
etc	O
.	O
</s>
<s>
The	O
runlevels	B-Device
in	O
SystemV	B-Operating_System
describe	O
certain	O
states	O
of	O
a	O
machine	O
,	O
characterized	O
by	O
the	O
processes	O
and	O
daemons	B-Operating_System
running	O
in	O
each	O
of	O
them	O
.	O
</s>
<s>
In	O
general	O
,	O
there	O
are	O
seven	O
runlevels	B-Device
,	O
out	O
of	O
which	O
three	O
runlevels	B-Device
are	O
considered	O
"	O
standard	O
"	O
as	O
they	O
are	O
essential	O
to	O
the	O
operation	O
of	O
a	O
system	O
:	O
</s>
<s>
Aside	O
from	O
these	O
standard	O
ones	O
,	O
Unix	B-Application
and	O
Unix-like	O
systems	O
treat	O
runlevels	B-Device
somewhat	O
differently	O
.	O
</s>
<s>
The	O
common	O
denominator	O
,	O
the	O
/etc/inittab	O
file	O
,	O
defines	O
what	O
each	O
configured	O
runlevel	B-Device
does	O
in	O
a	O
given	O
system	O
.	O
</s>
<s>
On	O
Linux	B-Application
distributions	I-Application
defaulting	O
to	O
runlevel	B-Device
5	O
in	O
the	O
table	O
on	O
the	O
right	O
,	O
runlevel	B-Device
5	O
invokes	O
a	O
multiuser	O
graphical	O
environment	O
running	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
usually	O
with	O
a	O
display	B-Device
manager	I-Device
like	O
GDM	B-Language
or	O
KDM	B-Language
.	O
</s>
<s>
However	O
,	O
the	O
Solaris	B-Application
and	O
illumos	B-Application
operating	B-General_Concept
systems	I-General_Concept
typically	O
reserve	O
runlevel	B-Device
5	O
to	O
shut	O
down	O
and	O
automatically	O
power	B-Device
off	I-Device
the	O
machine	O
.	O
</s>
<s>
On	O
most	O
systems	O
,	O
all	O
users	O
can	O
check	O
the	O
current	O
runlevel	B-Device
with	O
either	O
the	O
runlevel	B-Device
or	O
who	B-Application
-r	O
command	O
.	O
</s>
<s>
The	O
root	B-Application
user	I-Application
typically	O
changes	O
the	O
current	O
runlevel	B-Device
by	O
running	O
the	O
telinit	B-Device
or	O
init	B-Device
commands	O
.	O
</s>
<s>
The	O
/etc/inittab	O
file	O
sets	O
the	O
default	O
runlevel	B-Device
with	O
the	O
:initdefault	O
:	O
entry	O
.	O
</s>
<s>
On	O
Unix	B-Application
systems	I-Application
,	O
changing	O
the	O
runlevel	B-Device
is	O
achieved	O
by	O
starting	O
only	O
the	O
missing	O
services	O
(	O
as	O
each	O
level	O
defines	O
only	O
those	O
that	O
are	O
started	O
/	O
stopped	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
changing	O
a	O
system	O
from	O
runlevel	B-Device
3	O
to	O
4	O
might	O
only	O
start	O
the	O
local	O
X	B-Operating_System
server	O
.	O
</s>
<s>
Going	O
back	O
to	O
runlevel	B-Device
3	O
,	O
it	O
would	O
be	O
stopped	O
again	O
.	O
</s>
<s>
Traditionally	O
,	O
one	O
of	O
the	O
major	O
drawbacks	O
of	O
init	B-Device
is	O
that	O
it	O
starts	O
tasks	O
serially	O
,	O
waiting	O
for	O
each	O
to	O
finish	O
loading	O
before	O
moving	O
on	O
to	O
the	O
next	O
.	O
</s>
<s>
When	O
startup	O
processes	O
end	O
up	O
Input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
blocked	O
,	O
this	O
can	O
result	O
in	O
long	O
delays	O
during	O
boot	B-Operating_System
.	O
</s>
<s>
Speeding	O
up	O
I/O	B-General_Concept
,	O
e.g.	O
</s>
<s>
by	O
using	O
SSDs	O
,	O
may	O
shorten	O
the	O
delays	O
but	O
it	O
does	O
not	O
address	O
the	O
root	B-Application
cause	O
.	O
</s>
<s>
Various	O
efforts	O
have	O
been	O
made	O
to	O
replace	O
the	O
traditional	O
init	B-Device
daemons	I-Device
to	O
address	O
this	O
and	O
other	O
design	O
problems	O
,	O
including	O
:	O
</s>
<s>
Dinit	O
,	O
a	O
service	B-Operating_System
manager	I-Operating_System
and	O
init	B-Device
system	I-Device
.	O
</s>
<s>
s6	O
,	O
a	O
software	O
suite	O
that	O
includes	O
an	O
init	B-Device
system	I-Device
.	O
</s>
<s>
systemd	B-Application
,	O
a	O
software	O
suite	O
,	O
full	O
replacement	O
for	O
init	B-Device
in	O
Linux	O
that	O
includes	O
an	O
init	B-Device
daemon	I-Device
,	O
with	O
concurrent	O
starting	O
of	O
services	O
,	O
service	B-Operating_System
manager	I-Operating_System
,	O
and	O
other	O
features	O
.	O
</s>
<s>
Upstart	B-Device
,	O
a	O
full	O
replacement	O
of	O
init	B-Device
designed	O
to	O
start	O
processes	O
asynchronously	O
.	O
</s>
<s>
Initiated	O
by	O
Ubuntu	B-Operating_System
and	O
used	O
by	O
them	O
until	O
2014	O
.	O
</s>
<s>
It	O
was	O
also	O
used	O
in	O
Fedora	O
9	O
,	O
Red	O
Hat	O
Enterprise	O
Linux	O
6	O
and	O
Google	B-Application
's	I-Application
Chrome	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
,	O
systemd	B-Application
has	O
been	O
adopted	O
by	O
most	O
major	O
Linux	B-Application
distributions	I-Application
.	O
</s>
