True and false (commands)

true
Initial release	January 1979;46 years ago (1979-01)
Operating system	Unix and Unix-like
Platform	Cross-platform
Type	Command

true and false are shell commands that exit immediately with exit status 0 or 1, respectively. As a script sets its process exit status to the value of the last command it runs, these commands can be used to set the exit status of a script run. As the typical convention for exit status is that zero means success and non-zero means failure, true sets the exit status to failure and false sets the exit status to success. ^{[Note 1]}

The commands are available in Unix-like operating systems.

Use

The commands are usually employed in conditional statements and loops of shell scripts. For example, the following script repeatedly executes echo hello until interrupted:

whiletruedoechohello 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 accept no command-line arguments except that the GNU version accepts the typical --help and --version options.

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: doechohello 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`}}

Null smileys

Either true or : can be used as a replacement for cat /dev/null , so we have 3 "null smileys":

:> - create a file or empty it if it already exists;

:>> - create a file if it doesn't exist, unlike touch it does not change the timestamp of existing file;

:| - can be used instead of < /dev/null

Such usage is similar to the IEFBR14's standard usage.

See also

• IEFBR14  – "Do nothing" program on IBM mainframes
• List of POSIX commands
• Two-valued logic  – Classical logic of two values, either true or falsePages displaying short descriptions of redirect targets

Notes

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 ⊥).

References

1. "Colon", The Open group base specifications, issue 7, IEEE std 1003.1-2008
2. 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 5 from The Open Group
• false : return false value – Shell and Utilities Reference, The Single UNIX Specification , Version 5 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