True and false (commands)


In Unix-like operating systems, true and false 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 or failure of the command. The true and false 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:

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.
The programs take no "actual" parameters; in most Linux versions, the standard parameter --help displays a usage summary and --version 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. 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 expansion form. For example, from bashbug, the bug-reporting script for Bash:

: $
: $
: $

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