true and false (commands)

In Unix-like operating systems, true and false 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 true and false commands represent the logical values of command success, because true returns 0, and false returns 1.

Usage

The commands are usually employed in conditional statements and 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 login shell to false, in /etc/passwd, effectively denies them access to an interactive shell, but their account may still be valid for other services, such as 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 most Linux versions, the standard parameter --help displays a usage summary and --version displays the program version.

Null command

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 no-op dummy command for side-effects such as assigning default values to shell variables through the ${parameter:=word} parameter expansion form.^[2] For example, from bashbug, the bug-reporting script for Bash:

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

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 – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group
false: return false value – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group

Manual pages

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

Unix command-line interface programs and shell builtins

File system	cat chmod chown chgrp cksum cmp cp dd du df file fsck fuser ln ls mkdir mount mv pax pwd rm rmdir size split tee touch type umask

Processes	at bg chroot cron fg kill killall nice pgrep pkill ps pstree time top

User environment	clear env exit finger history id logname mesg passwd su sudo uptime talk tput uname w wall who whoami write

Text processing	awk banner basename comm csplit cut diff dirname ed ex fmt fold head iconv join less more nl paste printf sed sort spell strings tail tr uniq vi wc xargs yes

Shell builtins	alias cd echo test unset wait

Networking	dig host ifconfig inetd netcat netstat nslookup ping rdate rlogin route ssh traceroute

Searching	find grep locate whatis whereis

Documentation	apropos help man

Miscellaneous	bc dc cal expr lp od sleep true and false

List of Unix utilities

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