View source for True and false (commands)

{{short description|Standard Unix utility}}
{{lowercase title}}
{{Infobox software
| name                   = true
| logo                   =
| screenshot             =
| screenshot size        =
| caption                =
| author                 =
| developer              =
| released               = {{Start date and age|1979|1}}
| latest release version =
| latest release date    =
| operating system       = [[Unix]] and [[Unix-like]]
| platform               = [[Cross-platform]]
| genre                  = [[Command (computing)|Command]]
| license                =
| website                =
}}
In [[Unix-like]] [[operating system]]s, '''<code>true</code>''' and '''<code>false</code>''' are commands whose only function is to always return with a predetermined [[exit status]]. Programmers and scripts often use the exit status of a command to assess success (exit status zero) or failure (non-zero) of the command. The <code>true</code> and <code>false</code> commands represent the [[logical value]]s of command success, because ''true'' returns 0, and ''false'' returns 1.<ref group=Note>These are distinct from the [[truth value]]s of [[classical logic]] and most general purpose programming languages: [[Logical truth|true (1 or T)]] and [[False (logic)|false (0 or ⊥)]].</ref>
==Usage==
The commands are usually employed in [[Conditional (programming)|conditional statement]]s and [[program loop|loops]] of [[shell script]]s. For example, the following shell script repeats the ''echo hello'' loop until interrupted:
<syntaxhighlight lang="bash">
while true
do
echo hello
done
</syntaxhighlight>
The commands can be used to ignore the success or failure of a sequence of other commands, as in the example:
<syntaxhighlight lang="bash">make … && false</syntaxhighlight>
Setting a user's [[Unix shell|login shell]] to {{mono|'''false'''}}, in [[Passwd (file)|/etc/passwd]], effectively denies them access to an interactive shell, but their account may still be valid for other services, such as [[FTP]]. (Although {{mono|/sbin/nologin}}, if available, may be more fitting for this purpose, as it prints a notification before terminating the session.)
The programs take no "actual" parameters; in the GNU version, the standard parameter <code>--help</code> displays a usage summary and <code>--version</code> displays the program version.
==Null command==
The ''true'' command is sometimes substituted with the very similar null command,<ref>{{citation
|url=http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_16
|title=Colon |work=The Open group base specifications, issue 7
|id=IEEE std 1003.1-2008}}</ref> written as a single colon (<code>:</code>). The null command is [[shell builtin|built into the shell]], and may therefore be more efficient if ''true'' is an external program (''true'' is usually a shell built in function). We can rewrite the upper example using <code>:</code> instead of <code>true</code>:
<syntaxhighlight lang="bash">
while :
do
echo hello
done
</syntaxhighlight>
The null command may take parameters, which are ignored. It is also used as a [[no-op]] dummy command for side-effects such as assigning default values to [[shell variable]]s through the <code>${parameter:=word}</code> parameter expansion form.<ref>{{citation
|chapter-url=http://tldp.org/LDP/abs/html/special-chars.html#COLON0REF |access-date=2011-08-04
|chapter=Null command |title=Advanced Bash-scripting guide, 6.3
|first=Mendel |last=Cooper |date=April 2011
|publisher=The Linux documentation project}}</ref> For example, from ''[[bashbug]]'', the bug-reporting script for [[Bash (Unix shell)|Bash]]:
<syntaxhighlight lang="bash">
: ${TMPDIR:=/tmp}
: ${EDITOR=$DEFEDITOR}
: ${USER=${LOGNAME-`whoami`}}
</syntaxhighlight>
==See also==
*[[List of Unix commands]]
*[[Two-valued logic]]
*[[IEFBR14]]
==Notes==
{{Reflist|group=Note}}
==References==
{{Reflist}}
==External links==
{{Wikibooks|Guide to Unix|Commands}}
*{{man|cu|true|SUS|return true value}}
*{{man|cu|false|SUS|return false value}}
===Manual pages===
*[https://www.gnu.org/software/coreutils/manual/html_node/true-invocation.html true(1)]: Do nothing, successfully – [[GNU]] [[Coreutils]] reference
*[https://www.gnu.org/software/coreutils/manual/html_node/false-invocation.html false(1)]: Do nothing, unsuccessfully – [[GNU]] [[Coreutils]] reference
*[http://man.freebsd.org/true true(1)]: Return true value – [[FreeBSD]] manual page
*[http://man.freebsd.org/false false(1)]: Return false value – [[FreeBSD]] manual page
{{Unix commands}}
{{Core Utilities commands}}
[[Category:Standard Unix programs]]
[[Category:Unix SUS2008 utilities]]
[[Category:IBM i Qshell commands]]