<s>
In	O
computing	O
,	O
time	O
is	O
a	O
command	B-Application
in	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
It	O
is	O
used	O
to	O
determine	O
the	O
duration	O
of	O
execution	O
of	O
a	O
particular	O
command	B-Application
.	O
</s>
<s>
time(1 )	O
can	O
exist	O
as	O
a	O
standalone	O
program	O
(	O
such	O
as	O
GNU	B-Operating_System
time	O
)	O
or	O
as	O
a	O
shell	B-Application
builtin	O
in	O
most	O
case	O
(	O
e.g.	O
</s>
<s>
in	O
sh	B-Application
,	O
bash	B-Operating_System
,	O
tcsh	B-Language
or	O
in	O
zsh	B-Application
)	O
.	O
</s>
<s>
The	O
total	O
CPU	O
time	O
is	O
the	O
combination	O
of	O
the	O
amount	O
of	O
time	O
the	O
CPU	O
or	O
CPUs	O
spent	O
performing	O
some	O
action	O
for	O
a	O
program	O
and	O
the	O
amount	O
of	O
time	O
they	O
spent	O
performing	O
system	B-Operating_System
calls	I-Operating_System
for	O
the	O
kernel	B-Operating_System
on	O
the	O
program	O
's	O
behalf	O
.	O
</s>
<s>
Conversely	O
,	O
when	O
a	O
program	O
executes	O
a	O
system	B-Operating_System
call	I-Operating_System
such	O
as	O
exec	O
or	O
fork	O
,	O
it	O
is	O
accumulating	O
system	O
CPU	O
time	O
.	O
</s>
<s>
The	O
term	O
"	O
real	O
time	O
"	O
in	O
this	O
context	O
refers	O
to	O
elapsed	O
wall-clock	B-General_Concept
time	I-General_Concept
,	O
like	O
using	O
a	O
stop	O
watch	O
.	O
</s>
<s>
Because	O
a	O
program	O
may	O
fork	O
children	O
whose	O
CPU	O
times	O
(	O
both	O
user	O
and	O
sys	O
)	O
are	O
added	O
to	O
the	O
values	O
reported	O
by	O
the	O
time	O
command	B-Application
,	O
but	O
on	O
a	O
multicore	O
system	O
these	O
tasks	O
are	O
run	O
in	O
parallel	O
,	O
the	O
total	O
CPU	O
time	O
may	O
be	O
greater	O
than	O
the	O
real	O
time	O
.	O
</s>
<s>
To	O
use	O
the	O
command	B-Application
,	O
simply	O
precede	O
any	O
command	B-Application
by	O
the	O
word	O
time	O
,	O
such	O
as	O
:	O
</s>
<s>
When	O
the	O
command	B-Application
completes	O
,	O
time	O
will	O
report	O
how	O
long	O
it	O
took	O
to	O
execute	O
the	O
ls	B-Application
command	B-Application
in	O
terms	O
of	O
user	O
CPU	O
time	O
,	O
system	O
CPU	O
time	O
,	O
and	O
real	O
time	O
.	O
</s>
<s>
The	O
output	O
format	O
varies	O
between	O
different	O
versions	O
of	O
the	O
command	B-Application
,	O
and	O
some	O
give	O
additional	O
statistics	O
,	O
as	O
in	O
this	O
example	O
:	O
</s>
<s>
(	O
either	O
a	O
standalone	O
program	O
,	O
or	O
when	O
Bash	B-Operating_System
shell	I-Operating_System
is	O
running	O
in	O
POSIX	O
mode	O
AND	O
is	O
invoked	O
as	O
time	O
-p	O
)	O
reports	O
to	O
standard	O
error	O
output	O
.	O
</s>
<s>
Current	O
versions	O
of	O
GNU	B-Operating_System
time	O
,	O
report	O
more	O
than	O
just	O
a	O
time	O
by	O
default	O
:	O
</s>
<s>
Format	O
of	O
the	O
output	O
for	O
GNU	B-Operating_System
time	O
,	O
can	O
be	O
adjusted	O
using	O
TIME	O
environment	O
variable	O
,	O
and	O
it	O
can	O
include	O
information	O
other	O
than	O
the	O
execution	O
time	O
(	O
i.e.	O
</s>
<s>
According	O
to	O
the	O
source	O
code	O
of	O
the	O
GNU	B-Operating_System
implementation	O
of	O
time	O
,	O
most	O
information	O
shown	O
by	O
time	O
is	O
derived	O
from	O
the	O
wait3	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
On	O
systems	O
that	O
do	O
not	O
have	O
a	O
wait3	O
call	O
that	O
returns	O
status	O
information	O
,	O
the	O
times	O
system	B-Operating_System
call	I-Operating_System
is	O
used	O
instead	O
.	O
</s>
<s>
In	O
a	O
popular	O
Unix	B-Application
shell	I-Application
Bash	B-Operating_System
,	O
time	O
is	O
a	O
special	O
keyword	O
,	O
that	O
can	O
be	O
put	O
before	O
a	O
pipeline	B-Operating_System
(	O
or	O
single	O
command	B-Application
)	O
,	O
that	O
measures	O
time	O
of	O
entire	O
pipeline	B-Operating_System
,	O
not	O
just	O
a	O
singular	O
(	O
first	O
)	O
command	B-Application
,	O
and	O
uses	O
a	O
different	O
default	O
format	O
,	O
and	O
puts	O
empty	O
line	O
before	O
reporting	O
times	O
:	O
</s>
<s>
The	O
is	O
not	O
a	O
builtin	O
,	O
but	O
a	O
special	O
keyword	O
,	O
and	O
ca	O
n't	O
be	O
treated	O
as	O
a	O
function	O
or	O
command	B-Application
.	O
</s>
<s>
It	O
also	O
ignores	O
pipeline	B-Operating_System
redirections	O
(	O
even	O
when	O
executed	O
as	O
time	O
-p	O
,	O
unless	O
entire	O
Bash	B-Operating_System
is	O
run	O
in	O
"	O
POSIX	O
mode	O
"	O
)	O
.	O
</s>
<s>
Documentation	O
of	O
this	O
can	O
be	O
accessed	O
using	O
man	O
1	O
bash	B-Operating_System
,	O
or	O
within	O
bash	B-Operating_System
itself	O
using	O
help	O
time	O
.	O
</s>
