True and false (commands)

In Unix-like operating systems,   and   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  and   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:

The commands can be used to ignore the success or failure of a sequence of other commands, as in the example: 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 the GNU version, the standard parameter  displays a usage summary and   displays the program version.

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  :

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

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