true and false (commands)

In Unix-like operating systems, true and false are commands whose only function is to always return the value 0 or 1.

Overview

The shell regards 0 as the logical value true and 1 as false. It is usually employed in conditional statements and loops of shell scripts where Boolean conditions are given as the return value of a program. For example, the following Bourne shell script echos the string hello until interrupted:

while true
do
  echo hello
done

It can be used to make a sequence of otherwise useful commands fail, 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 some versions, the standard parameter --help displays a usage summary and --version displays the program version.

true may also be written as a single colon (:). In that form, it is generally built into the shell, and may therefore be more efficient. In its alias of :, true is often used (in POSIX-compatible shells such as the Bourne shell) as a dummy command when assigning default values to shell variables through the ${parameter:=word} parameter expansion form. For example, from bashbug, the bug-reporting script for bash:

: ${TMPDIR:=/tmp}
: ${EDITOR=$DEFEDITOR}
: ${USER=${LOGNAME-`whoami`}}

See also

• List of Unix programs

External links

• true: return true value – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
• false: return false value – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group

Manual pages

• true(1): Do nothing, successfully – GNU Coreutils reference
• false(1): Do nothing, unsuccessfully – GNU Coreutils reference
• true: Do nothing, successfully – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
• false: Do nothing, unsuccessfully – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group