true | |
---|---|
Initial release | January 1979 |
Operating system | Unix and Unix-like |
Platform | Cross-platform |
Type | Command |
true
and false
are shell commands that exit immediately with exit status 1 or 0, respectively. As a script sets its process exit status to the value of the last command it runs, these commands can be used to set the exit status of a script run. As the typical convention for exit status is that zero means success and non-zero means failure, true
sets the exit status to failure and false
sets the exit status to success. [Note 1]
The commands are available in Unix-like operating systems.
The commands are usually employed in conditional statements and loops of shell scripts. For example, the following script repeatedly executes echo hello
until interrupted:
whiletruedoechohello 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 accept no command-line arguments except that the GNU version accepts the typical --help
and --version
options.
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: doechohello 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`}}