true and false (commands)

true
Initial release	January 1979; 45 years ago
Cross-platform
Type	Command

In

logical values of command success, because true returns 0, and false returns 1.^{[Note 1]}

Usage

The commands are usually employed in

loops of shell scripts

. For example, the following shell script repeats the echo hello loop until interrupted:

while true
do
  echo hello
done

The commands can be used to ignore the success or failure of a sequence of other commands, as in the example:

make … && false

Setting a user's

FTP

. (Although /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 --help displays a usage summary and --version displays the program version.

The true command is sometimes substituted with the very similar null command,[1] written as a single colon (:). The null command is 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 : instead of true:

while :
do
  echo hello
done

The null command may take parameters, which are ignored. It is also used as a

bashbug, the bug-reporting script for Bash

:

 : ${TMPDIR:=/tmp}
 : ${EDITOR=$DEFEDITOR}
 : ${USER=${LOGNAME-`whoami`}}

Notes

^ These are distinct from the truth values of classical logic and most general purpose programming languages: true (1 or T) and false (0 or ⊥).

References

^ "Colon", The Open group base specifications, issue 7, IEEE std 1003.1-2008
^ Cooper, Mendel (April 2011), "Null command", Advanced Bash-scripting guide, 6.3, The Linux documentation project, retrieved 2011-08-04

External links

true: return true value – Shell and Utilities Reference,
The Single UNIX Specification, Version 4 from The Open Group

false: return false value – Shell and Utilities Reference,
The Single UNIX Specification, Version 4 from The Open Group

Manual pages

true(1): Do nothing, successfully –
Coreutils
reference
false(1): Do nothing, unsuccessfully –
Coreutils
reference
true(1): Return true value – FreeBSD manual page
false(1): Return false value – FreeBSD manual page

[1] These are distinct from the truth values of classical logic and most general purpose programming languages: true (1 or T) and false (0 or ⊥).

[2] "Colon", The Open group base specifications, issue 7, IEEE std 1003.1-2008

[3] Cooper, Mendel (April 2011), "Null command", Advanced Bash-scripting guide, 6.3, The Linux documentation project, retrieved 2011-08-04

[Note 1]

v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

v t e GNU Core Utilities command-line interface programs
File system	chcon chmod chown chgrp cksum cp dd df dir dircolors install ln ls mkdir mkfifo mknod mktemp mv realpath rm rmdir shred sync touch truncate vdir
Text utilities	b2sum base32 base64 cat cksum comm csplit cut expand fmt fold head join md5sum nl numfmt od paste ptx pr sha1sum shuf sort split sum tac tail tr tsort unexpand uniq wc
Shell utilities	arch basename chroot date dirname du echo env expr factor false groups hostid id link logname nice nohup nproc pathchk pinky printenv printf pwd readlink runcon seq sleep stat stdbuf stty tee test timeout true tty uname unlink uptime users who whoami yes

Usage

Null command

See also

Notes

References

External links

Manual pages