Software:true and false (commands)
Initial release | January 1979 |
---|---|
Operating system | Unix and Unix-like |
Platform | Cross-platform |
Type | Command |
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.[Note 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. (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.
Null command
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
- List of Unix commands
- Two-valued logic
- IEFBR14
Notes
- ↑ These are distinct from the truth values of classical logic and most general purpose programming languages: true (1 or T) and false (0 or ⊥).
References
- ↑ "Colon", The Open group base specifications, issue 7, IEEE std 1003.1-2008, http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_16
- ↑ Cooper, Mendel (April 2011), "Null command", Advanced Bash-scripting guide, 6.3, The Linux documentation project, http://tldp.org/LDP/abs/html/special-chars.html#COLON0REF, retrieved 2011-08-04
External links
- The Single UNIX Specification, Issue 7 from The Open Group : return true value – Commands & Utilities Reference,
- The Single UNIX Specification, Issue 7 from The Open Group : return false value – Commands & Utilities Reference,
Manual pages
- true(1): Do nothing, successfully – GNU Coreutils reference
- false(1): Do nothing, unsuccessfully – GNU Coreutils reference
- true(1): Return true value – FreeBSD manual page
- false(1): Return false value – FreeBSD manual page
Original source: https://en.wikipedia.org/wiki/True and false (commands).
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