True and false (commands) explained

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. All Unix shells interpret an exit status of zero as success and non-zero (usually) as failure, so sets success and sets failure.^[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 until interrupted:

while truedo echo hellodone

The commands can be used to ignore the success or failure of a sequence of other commands, as in the example:make … && falseSetting a user's login shell to, in, effectively denies them access to an interactive shell, but their account may still be valid for other services, such as FTP. (Although, 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, 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 hellodone

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. For example, from bashbug, the bug-reporting script for Bash:

: $ : $ : $

Null smileys

Either true or : can be used as a replacement for [[cat (Unix)|cat]] [[null device|/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.

External links

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

Notes and References

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 ⊥). Note that a built-in sets the exit status, while the program returns it, with the same effect.