True and false (commands)

Last updated
true
Initial releaseJanuary 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 1 or 0, 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]

Contents

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`}}

See also

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

Manual pages