<s>
A	O
Unix-like	B-Operating_System
(	O
sometimes	O
referred	O
to	O
as	O
UN*X	B-Operating_System
or	O
*	B-Operating_System
nix	I-Operating_System
)	O
operating	B-General_Concept
system	I-General_Concept
is	O
one	O
that	O
behaves	O
in	O
a	O
manner	O
similar	O
to	O
a	O
Unix	B-Application
system	I-Application
,	O
although	O
not	O
necessarily	O
conforming	O
to	O
or	O
being	O
certified	O
to	O
any	O
version	O
of	O
the	O
Single	O
UNIX	B-Application
Specification	O
.	O
</s>
<s>
A	O
Unix-like	B-Operating_System
application	B-Application
is	O
one	O
that	O
behaves	O
like	O
the	O
corresponding	O
Unix	B-Application
command	I-Application
or	O
shell	B-Application
.	O
</s>
<s>
Although	O
there	O
are	O
general	O
philosophies	B-Operating_System
for	O
Unix	B-Application
design	O
,	O
there	O
is	O
no	O
technical	O
standard	O
defining	O
the	O
term	O
,	O
and	O
opinions	O
can	O
differ	O
about	O
the	O
degree	O
to	O
which	O
a	O
particular	O
operating	B-General_Concept
system	I-General_Concept
or	O
application	B-Application
is	O
Unix-like	B-Operating_System
.	O
</s>
<s>
Some	O
well-known	O
examples	O
of	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
include	O
Linux	B-Operating_System
and	O
BSD	B-Operating_System
.	O
</s>
<s>
Many	O
popular	O
applications	O
,	O
such	O
as	O
the	O
Apache	B-Application
web	I-Application
server	I-Application
and	O
the	O
Bash	B-Operating_System
shell	I-Operating_System
,	O
are	O
also	O
designed	O
to	O
be	O
used	O
on	O
Unix-like	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
One	O
of	O
the	O
key	O
features	O
of	O
Unix-like	B-Operating_System
systems	I-Operating_System
is	O
their	O
ability	O
to	O
support	O
multiple	O
users	O
and	O
processes	O
simultaneously	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
many	O
older	O
operating	B-General_Concept
systems	I-General_Concept
,	O
which	O
were	O
designed	O
to	O
only	O
support	O
a	O
single	O
user	O
or	O
process	O
at	O
a	O
time	O
.	O
</s>
<s>
Another	O
important	O
feature	O
of	O
Unix-like	B-Operating_System
systems	I-Operating_System
is	O
their	O
modularity	B-Architecture
.	O
</s>
<s>
This	O
means	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
made	O
up	O
of	O
many	O
small	O
,	O
interchangeable	O
components	O
that	O
can	O
be	O
added	O
or	O
removed	O
as	O
needed	O
.	O
</s>
<s>
This	O
makes	O
it	O
easy	O
to	O
customize	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
suit	O
the	O
needs	O
of	O
different	O
users	O
or	O
environments	O
.	O
</s>
<s>
The	B-Application
Open	I-Application
Group	I-Application
owns	O
the	O
UNIX	B-Application
trademark	O
and	O
administers	O
the	O
Single	O
UNIX	B-Application
Specification	O
,	O
with	O
the	O
"	O
UNIX	B-Application
"	O
name	O
being	O
used	O
as	O
a	O
certification	O
mark	O
.	O
</s>
<s>
They	O
do	O
not	O
approve	O
of	O
the	O
construction	O
"	O
Unix-like	B-Operating_System
"	O
,	O
and	O
consider	O
it	O
a	O
misuse	O
of	O
their	O
trademark	O
.	O
</s>
<s>
Their	O
guidelines	O
require	O
"	O
UNIX	B-Application
"	O
to	O
be	O
presented	O
in	O
uppercase	O
or	O
otherwise	O
distinguished	O
from	O
the	O
surrounding	O
text	O
,	O
strongly	O
encourage	O
using	O
it	O
as	O
a	O
branding	O
adjective	O
for	O
a	O
generic	O
word	O
such	O
as	O
"	O
system	O
"	O
,	O
and	O
discourage	O
its	O
use	O
in	O
hyphenated	O
phrases	O
.	O
</s>
<s>
Other	O
parties	O
frequently	O
treat	O
"	O
Unix	B-Application
"	O
as	O
a	O
genericized	O
trademark	O
.	O
</s>
<s>
Some	O
add	O
a	O
wildcard	B-Algorithm
character	I-Algorithm
to	O
the	O
name	O
to	O
make	O
an	O
abbreviation	O
like	O
"	O
Un*x	B-Operating_System
"	O
or	O
"	O
*	B-Operating_System
nix	I-Operating_System
"	O
,	O
since	O
Unix-like	B-Operating_System
systems	I-Operating_System
often	O
have	O
Unix-like	B-Operating_System
names	O
such	O
as	O
AIX	B-Application
,	O
A/UX	B-Operating_System
,	O
HP-UX	B-Application
,	O
IRIX	B-Operating_System
,	O
Linux	B-Operating_System
,	O
Minix	B-Operating_System
,	O
Ultrix	B-Operating_System
,	O
Xenix	B-Application
,	O
and	O
XNU	B-Operating_System
.	O
</s>
<s>
These	O
patterns	O
do	O
not	O
literally	O
match	O
many	O
system	O
names	O
,	O
but	O
are	O
still	O
generally	O
recognized	O
to	O
refer	O
to	O
any	O
UNIX	B-Application
system	I-Application
,	O
descendant	O
,	O
or	O
work-alike	O
,	O
even	O
those	O
with	O
completely	O
dissimilar	O
names	O
such	O
as	O
Darwin/macOS	O
,	O
illumos/Solaris	O
or	O
FreeBSD	B-Operating_System
.	O
</s>
<s>
In	O
2007	O
,	O
Wayne	O
R	O
.	O
Gray	O
sued	O
to	O
dispute	O
the	O
status	O
of	O
UNIX	B-Application
as	O
a	O
trademark	O
,	O
but	O
lost	O
his	O
case	O
,	O
and	O
lost	O
again	O
on	O
appeal	O
,	O
with	O
the	O
court	O
upholding	O
the	O
trademark	O
and	O
its	O
ownership	O
.	O
</s>
<s>
"	O
Unix-like	B-Operating_System
"	O
systems	O
started	O
to	O
appear	O
in	O
the	O
late	O
1970s	O
and	O
early	O
1980s	O
.	O
</s>
<s>
Many	O
proprietary	B-Application
versions	O
,	O
such	O
as	O
Idris	B-Operating_System
(	O
1978	O
)	O
,	O
UNOS	B-Architecture
(	O
1982	O
)	O
,	O
Coherent	B-Operating_System
(	O
1983	O
)	O
,	O
and	O
UniFlex	B-Operating_System
(	O
1985	O
)	O
,	O
aimed	O
to	O
provide	O
businesses	O
with	O
the	O
functionality	O
available	O
to	O
academic	O
users	O
of	O
UNIX	B-Application
.	O
</s>
<s>
When	O
AT&T	O
allowed	O
relatively	O
inexpensive	O
commercial	O
binary	O
sub-licensing	O
of	O
UNIX	B-Application
in	O
1979	O
,	O
a	O
variety	O
of	O
proprietary	B-Application
systems	O
were	O
developed	O
based	O
on	O
it	O
,	O
including	O
AIX	B-Application
,	O
HP-UX	B-Application
,	O
IRIX	B-Operating_System
,	O
SunOS	B-Operating_System
,	O
Tru64	B-Operating_System
,	O
Ultrix	B-Operating_System
,	O
and	O
Xenix	B-Application
.	O
</s>
<s>
These	O
largely	O
displaced	O
the	O
proprietary	B-Application
clones	O
.	O
</s>
<s>
Growing	O
incompatibility	O
among	O
these	O
systems	O
led	O
to	O
the	O
creation	O
of	O
interoperability	O
standards	O
,	O
including	O
POSIX	O
and	O
the	O
Single	O
UNIX	B-Application
Specification	O
.	O
</s>
<s>
Various	O
free	O
,	O
low-cost	O
,	O
and	O
unrestricted	O
substitutes	O
for	O
UNIX	B-Application
emerged	O
in	O
the	O
1980s	O
and	O
1990s	O
,	O
including	O
4.4BSD	B-Operating_System
,	O
Linux	B-Operating_System
,	O
and	O
Minix	B-Operating_System
.	O
</s>
<s>
Some	O
of	O
these	O
have	O
in	O
turn	O
been	O
the	O
basis	O
for	O
commercial	O
"	O
Unix-like	B-Operating_System
"	O
systems	O
,	O
such	O
as	O
BSD/OS	B-Operating_System
and	O
macOS	B-Application
.	O
</s>
<s>
Several	O
versions	O
of	O
(	O
Mac	O
)	O
OS	O
X/macOS	O
running	O
on	O
Intel-based	O
Mac	O
computers	O
have	O
been	O
certified	O
under	O
the	O
Single	O
UNIX	B-Application
Specification	O
.	O
</s>
<s>
The	O
BSD	B-Operating_System
variants	O
are	O
descendants	O
of	O
UNIX	B-Application
developed	O
by	O
the	O
University	O
of	O
California	O
at	O
Berkeley	O
with	O
UNIX	B-Application
source	O
code	O
from	O
Bell	O
Labs	O
.	O
</s>
<s>
However	O
,	O
the	O
BSD	B-Operating_System
code	O
base	O
has	O
evolved	O
since	O
then	O
,	O
replacing	O
all	O
of	O
the	O
AT&T	O
code	O
.	O
</s>
<s>
Since	O
the	O
BSD	B-Operating_System
variants	O
are	O
not	O
certified	O
as	O
compliant	O
with	O
the	O
Single	O
UNIX	B-Application
Specification	O
,	O
they	O
are	O
referred	O
to	O
as	O
"	O
UNIX-like	B-Operating_System
"	O
rather	O
than	O
"	O
UNIX	B-Application
"	O
.	O
</s>
<s>
Dennis	O
Ritchie	O
,	O
one	O
of	O
the	O
original	O
creators	O
of	O
Unix	B-Application
,	O
expressed	O
his	O
opinion	O
that	O
Unix-like	B-Operating_System
systems	I-Operating_System
such	O
as	O
Linux	B-Operating_System
are	O
de	O
facto	O
Unix	B-Application
systems	I-Application
.	O
</s>
<s>
Eric	O
S	O
.	O
Raymond	O
and	O
Rob	O
Landley	O
have	O
suggested	O
that	O
there	O
are	O
three	O
kinds	O
of	O
Unix-like	B-Operating_System
systems	I-Operating_System
:	O
</s>
<s>
Most	O
commercial	O
UNIX	B-Application
systems	I-Application
fall	O
into	O
this	O
category	O
.	O
</s>
<s>
So	O
do	O
the	O
BSD	B-Operating_System
systems	O
,	O
which	O
are	O
descendants	O
of	O
work	O
done	O
at	O
the	O
University	O
of	O
California	O
,	O
Berkeley	O
in	O
the	O
late	O
1970s	O
and	O
early	O
1980s	O
.	O
</s>
<s>
These	O
systemslargely	O
commercial	O
in	O
naturehave	O
been	O
determined	O
by	O
the	B-Application
Open	I-Application
Group	I-Application
to	O
meet	O
the	O
Single	O
UNIX	B-Application
Specification	O
and	O
are	O
allowed	O
to	O
carry	O
the	O
UNIX	B-Application
name	O
.	O
</s>
<s>
Most	O
such	O
systems	O
are	O
commercial	O
derivatives	O
of	O
the	O
System	B-Operating_System
V	I-Operating_System
code	O
base	O
in	O
one	O
form	O
or	O
another	O
,	O
although	O
Apple	O
macOS	B-Application
10.5	I-Application
and	O
later	O
is	O
a	O
BSD	B-Operating_System
variant	O
that	O
has	O
been	O
certified	O
,	O
EulerOS	O
and	O
Inspur	O
K-UX	O
are	O
Linux	B-Application
distributions	I-Application
that	O
have	O
been	O
certified	O
,	O
and	O
a	O
few	O
other	O
systems	O
(	O
such	O
as	O
IBM	B-Application
z/OS	I-Application
)	O
earned	O
the	O
trademark	O
through	O
a	O
POSIX	O
compatibility	B-General_Concept
layer	I-General_Concept
and	O
are	O
not	O
otherwise	O
inherently	O
Unix	B-Application
systems	I-Application
.	O
</s>
<s>
Many	O
ancient	B-Operating_System
UNIX	I-Operating_System
systems	I-Operating_System
no	O
longer	O
meet	O
this	O
definition	O
.	O
</s>
<s>
Broadly	O
,	O
any	O
Unix-like	B-Operating_System
system	I-Operating_System
that	O
behaves	O
in	O
a	O
manner	O
roughly	O
consistent	O
with	O
the	O
UNIX	B-Application
specification	O
,	O
including	O
having	O
a	O
"	O
program	B-Operating_System
which	O
manages	O
your	O
login	O
and	O
command	O
line	O
sessions	O
"	O
;	O
more	O
specifically	O
,	O
this	O
can	O
refer	O
to	O
systems	O
such	O
as	O
Linux	B-Operating_System
or	O
Minix	B-Operating_System
that	O
behave	O
similarly	O
to	O
a	O
UNIX	B-Application
system	I-Application
but	O
have	O
no	O
genetic	O
or	O
trademark	O
connection	O
to	O
the	O
AT&T	O
code	O
base	O
.	O
</s>
<s>
Most	O
free/open	O
-source	O
implementations	O
of	O
the	O
UNIX	B-Application
design	O
,	O
whether	O
genetic	O
UNIX	B-Application
or	O
not	O
,	O
fall	O
into	O
the	O
restricted	O
definition	O
of	O
this	O
third	O
category	O
due	O
to	O
the	O
expense	O
of	O
obtaining	O
Open	B-Application
Group	I-Application
certification	O
,	O
which	O
costs	O
thousands	O
of	O
dollars	O
.	O
</s>
<s>
Around	O
2001	O
,	O
Linux	B-Operating_System
was	O
given	O
the	O
opportunity	O
to	O
get	O
a	O
certification	O
including	O
free	O
help	O
from	O
the	O
POSIX	O
chair	O
Andrew	O
Josey	O
for	O
the	O
symbolic	O
price	O
of	O
one	O
dollar	O
.	O
</s>
<s>
There	O
have	O
been	O
some	O
activities	O
to	O
make	O
Linux	B-Operating_System
POSIX-compliant	O
,	O
with	O
Josey	O
having	O
prepared	O
a	O
list	O
of	O
differences	O
between	O
the	O
POSIX	O
standard	O
and	O
the	O
Linux	B-Application
Standard	I-Application
Base	I-Application
specification	O
,	O
but	O
in	O
August	O
2005	O
,	O
this	O
project	O
was	O
shut	O
down	O
because	O
of	O
missing	O
interest	O
at	O
the	O
LSB	O
work	O
group	O
.	O
</s>
<s>
Some	O
non-Unix-like	O
operating	O
systems	O
provide	O
a	O
Unix-like	B-Operating_System
compatibility	B-General_Concept
layer	I-General_Concept
,	O
with	O
varying	O
degrees	O
of	O
Unix-like	B-Operating_System
functionality	O
.	O
</s>
<s>
IBM	B-Application
z/OS	I-Application
'	O
s	O
UNIX	B-Operating_System
System	I-Operating_System
Services	I-Operating_System
is	O
sufficiently	O
complete	O
as	O
to	O
be	O
certified	O
as	O
trademark	O
UNIX	B-Application
.	O
</s>
<s>
Cygwin	B-Language
,	O
MSYS	B-Application
,	O
and	O
MSYS2	O
each	O
provide	O
a	O
GNU	B-Operating_System
environment	O
on	O
top	O
of	O
the	O
Microsoft	B-Application
Windows	I-Application
user	B-Library
API	I-Library
,	O
sufficient	O
for	O
most	O
common	O
open	B-Application
source	I-Application
software	I-Application
to	O
be	O
compiled	O
and	O
run	O
.	O
</s>
<s>
The	O
MKS	B-Application
Toolkit	I-Application
and	O
UWIN	B-Application
are	O
comprehensive	O
interoperability	O
tools	O
which	O
allow	O
the	O
porting	O
of	O
Unix	B-Application
programs	O
to	O
Windows	O
.	O
</s>
<s>
Windows	O
NT-type	O
systems	O
have	O
a	O
POSIX	O
environmental	B-General_Concept
subsystem	I-General_Concept
.	O
</s>
<s>
Subsystem	B-Operating_System
for	I-Operating_System
Unix-based	I-Operating_System
Applications	I-Operating_System
(	O
previously	O
Interix	B-Operating_System
)	O
provides	O
Unix-like	B-Operating_System
functionality	O
as	O
a	O
Windows	B-Device
NT	I-Device
subsystem	O
(	O
discontinued	O
)	O
.	O
</s>
<s>
Windows	O
Subsystem	O
for	O
Linux	B-Operating_System
provides	O
a	O
Linux-compatible	O
kernel	B-Operating_System
interface	O
developed	O
by	O
Microsoft	O
and	O
containing	O
no	O
Linux	B-Operating_System
code	O
,	O
with	O
Ubuntu	B-Operating_System
user-mode	O
binaries	O
running	O
on	O
top	O
of	O
it	O
.	O
</s>
<s>
Windows	O
Subsystem	O
for	O
Linux	B-Operating_System
version	O
2	O
(	O
WSL2	O
)	O
provides	O
a	O
fully	O
functional	O
Linux	B-Operating_System
environment	O
running	O
in	O
a	O
virtual	O
machine	O
.	O
</s>
<s>
Other	O
means	O
of	O
Windows-Unix	O
interoperability	O
include	O
:	O
</s>
<s>
Hamilton	B-Device
C	I-Device
shell	I-Device
is	O
a	O
version	O
of	O
csh	B-Operating_System
written	O
specifically	O
for	O
Windows	O
.	O
</s>
