<s>
In	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
true	B-Device
and	I-Device
false	I-Device
are	O
commands	O
whose	O
only	O
function	O
is	O
to	O
always	O
return	O
with	O
a	O
predetermined	O
exit	B-Operating_System
status	I-Operating_System
.	O
</s>
<s>
Programmers	O
and	O
scripts	O
often	O
use	O
the	O
exit	B-Operating_System
status	I-Operating_System
of	O
a	O
command	B-Application
to	O
assess	O
success	O
(	O
exit	B-Operating_System
status	I-Operating_System
zero	O
)	O
or	O
failure	O
(	O
non-zero	O
)	O
of	O
the	O
command	B-Application
.	O
</s>
<s>
The	O
true	B-Device
and	I-Device
false	I-Device
commands	O
represent	O
the	O
logical	O
values	O
of	O
command	B-Application
success	O
,	O
because	O
true	O
returns	O
0	O
,	O
and	O
false	O
returns	O
1	O
.	O
</s>
<s>
The	O
commands	O
are	O
usually	O
employed	O
in	O
conditional	B-Language
statements	O
and	O
loops	O
of	O
shell	B-Language
scripts	I-Language
.	O
</s>
<s>
For	O
example	O
,	O
the	O
following	O
shell	B-Language
script	I-Language
repeats	O
the	O
echo	O
hello	O
loop	O
until	O
interrupted	O
:	O
</s>
<s>
Setting	O
a	O
user	O
's	O
login	B-Application
shell	I-Application
to	O
,	O
in	O
/etc/passwd	O
,	O
effectively	O
denies	O
them	O
access	O
to	O
an	O
interactive	O
shell	O
,	O
but	O
their	O
account	O
may	O
still	O
be	O
valid	O
for	O
other	O
services	O
,	O
such	O
as	O
FTP	B-Protocol
.	O
</s>
<s>
The	O
programs	O
take	O
no	O
"	O
actual	O
"	O
parameters	O
;	O
in	O
the	O
GNU	B-Operating_System
version	O
,	O
the	O
standard	O
parameter	O
--help	O
displays	O
a	O
usage	O
summary	O
and	O
--version	O
displays	O
the	O
program	O
version	O
.	O
</s>
<s>
The	O
true	O
command	B-Application
is	O
sometimes	O
substituted	O
with	O
the	O
very	O
similar	O
null	O
command	B-Application
,	O
written	O
as	O
a	O
single	O
colon	O
(	O
:	O
)	O
.	O
</s>
<s>
The	O
null	O
command	B-Application
is	O
built	O
into	O
the	O
shell	O
,	O
and	O
may	O
therefore	O
be	O
more	O
efficient	O
if	O
true	O
is	O
an	O
external	O
program	O
(	O
true	O
is	O
usually	O
a	O
shell	O
built	O
in	O
function	O
)	O
.	O
</s>
<s>
The	O
null	O
command	B-Application
may	O
take	O
parameters	O
,	O
which	O
are	O
ignored	O
.	O
</s>
<s>
It	O
is	O
also	O
used	O
as	O
a	O
no-op	B-Language
dummy	O
command	B-Application
for	O
side-effects	O
such	O
as	O
assigning	O
default	O
values	O
to	O
shell	O
variables	O
through	O
the	O
${parameter:=word}	O
parameter	O
expansion	O
form	O
.	O
</s>
<s>
For	O
example	O
,	O
from	O
bashbug	O
,	O
the	O
bug-reporting	O
script	O
for	O
Bash	B-Operating_System
:	O
</s>
