<s>
The	O
standard	O
Unix	B-Application
command	I-Application
who	O
displays	O
a	O
list	O
of	O
users	O
who	O
are	O
currently	O
logged	O
into	O
the	O
computer	O
.	O
</s>
<s>
The	O
who	O
command	B-Application
is	O
related	O
to	O
the	O
command	B-Application
,	O
which	O
provides	O
the	O
same	O
information	O
but	O
also	O
displays	O
additional	O
data	O
and	O
statistics	O
.	O
</s>
<s>
A	O
command	B-Application
that	O
displays	O
the	O
names	O
of	O
users	O
logged	O
in	O
was	O
first	O
implemented	O
within	O
Multics	B-Application
.	O
</s>
<s>
Later	O
,	O
it	O
appeared	O
in	O
Version	B-Operating_System
1	I-Operating_System
Unix	I-Operating_System
and	O
became	O
part	O
of	O
the	O
X/Open	O
Portability	O
Guide	O
since	O
issue	O
2	O
of	O
1987	O
.	O
</s>
<s>
It	O
was	O
inherited	O
into	O
the	O
first	O
version	O
of	O
POSIX.1	O
and	O
the	O
Single	O
Unix	B-Application
Specification	O
.	O
</s>
<s>
The	O
version	O
of	O
who	O
bundled	O
in	O
GNU	B-Application
coreutils	I-Application
was	O
written	O
by	O
Joseph	O
Arceneaux	O
,	O
David	O
MacKenzie	O
,	O
and	O
Michael	O
Stone	O
.	O
</s>
<s>
The	O
Single	O
UNIX	B-Application
Specification	O
(	O
SUS	O
)	O
specifies	O
that	O
who	O
should	O
list	O
information	O
about	O
accessible	O
users	O
.	O
</s>
<s>
The	O
XSI	O
extension	O
also	O
specifies	O
that	O
the	O
data	O
of	O
the	O
username	O
,	O
terminal	O
,	O
login	O
time	O
,	O
process	O
ID	B-Application
,	O
and	O
time	O
since	O
last	O
activity	O
occurred	O
on	O
the	O
terminal	O
,	O
furthermore	O
,	O
an	O
alternate	O
system	O
database	O
used	O
for	O
user	O
information	O
can	O
be	O
specified	O
as	O
an	O
optional	O
argument	O
to	O
.	O
</s>
<s>
The	O
command	B-Application
can	O
be	O
invoked	O
with	O
the	O
arguments	O
am	O
i	O
or	O
am	O
I	O
(	O
so	O
it	O
is	O
invoked	O
as	O
who	O
am	O
i	O
or	O
who	O
am	O
I	O
)	O
,	O
showing	O
information	O
about	O
the	O
current	O
terminal	O
only	O
(	O
see	O
the	O
command	B-Application
and	O
the	O
-m	O
option	O
below	O
,	O
of	O
which	O
this	O
invocation	O
is	O
equivalent	O
)	O
.	O
</s>
<s>
Other	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
may	O
add	O
extra	O
options	O
.	O
</s>
<s>
GNU	B-Operating_System
includes	O
a	O
-i	O
option	O
behaving	O
similarly	O
to	O
-u	O
and	O
a	O
-w	O
option	O
displaying	O
whether	O
the	O
user	O
listed	O
accepts	O
messages	O
(	O
the	O
SUS	O
displays	O
this	O
when	O
-T	O
is	O
specified	O
)	O
,	O
yet	O
GNU	B-Operating_System
who	O
and	O
BSD	B-Operating_System
who	O
both	O
omit	O
a	O
number	O
of	O
the	O
above	O
options	O
(	O
such	O
as	O
-a	O
,	O
-b	O
,	O
-d	O
,	O
and	O
others	O
)	O
;	O
GNU	B-Operating_System
who	O
instead	O
uses	O
-l	O
to	O
perform	O
DNS	O
lookups	O
on	O
hostnames	O
listed	O
.	O
</s>
<s>
Thus	O
,	O
the	O
format	O
of	O
the	O
output	O
differs	O
considerably	O
among	O
Unix	B-Application
implementations	O
.	O
</s>
