Initial release | January 1979; 45 years ago (1979-01) |
---|---|
Operating system | Unix and Unix-like |
Platform | Cross-platform |
Type | Command |
InUnix-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.[Note 1]
The commands are usually employed in conditional statements and loopsofshell 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 shelltofalse, 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 --help
displays a usage summary and --version
displays the program version.
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`}}
true
: return true value – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Groupfalse
: return false value – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
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