true and false (commands)

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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 (exit status zero) or failure (non-zero) of the command. The true and false commands represent the logical values of command success, because true returns 0, and false returns 1.

Usage[edit]

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. (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 most Linux versions, the standard parameter --help displays a usage summary and --version displays the program version.

Null command[edit]

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`}}

See also[edit]

References[edit]

  1. ^ "Colon", The Open group base specifications, issue 7, IEEE std 1003.1-2008 
  2. ^ Cooper, Mendel (April 2011), "Null command", Advanced Bash-scripting guide, 6.3, The Linux documentation project, retrieved 2011-08-04 

External links[edit]

Manual pages[edit]