True and false (commands)

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]}

The commands are available in Unix-like operating systems.

The commands are usually employed in conditional statements and loops of shell scripts. For example, the following script repeatedly executes echo hello 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 accept no command-line arguments except that the GNU version accepts the typical --help and --version options.

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`}}

• 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

The Wikibook Guide to Unix has a page on the topic of: Commands

• 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

• 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