InUnix-likeoperating systems,true andfalse are commands whose only function is to always return with a predeterminedexit 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. Thetrue andfalse commands represent thelogical values of command success, becausetrue returns 0, andfalse returns 1.^{[Note 1]}

The commands are usually employed inconditional statements andloops ofshell scripts. For example, the following shell script repeats theecho hello loop until interrupted:

whiletruedoechohellodone

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'slogin shell tofalse, in/etc/passwd, effectively denies them access to an interactive shell, but their account may still be valid for other services, such asFTP. (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.

Thetrue command is sometimes substituted with the very similar null command,^[1] written as a single colon (:). The null command isbuilt into the shell, and may therefore be more efficient iftrue is an external program (true is usually a shell built in function). We can rewrite the upper example using: instead oftrue:

while:doechohellodone

The null command may take parameters, which are ignored. It is also used as ano-op dummy command for side-effects such as assigning default values toshell variables through the${parameter:=word} parameter expansion form.^[2] For example, frombashbug, the bug-reporting script forBash:

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

• List of Unix commands
• Two-valued logic
• IEFBR14

The WikibookGuide to Unix has a page on the topic of:Commands

• true: return true value – Shell and Utilities Reference,The Single UNIX Specification, Version 5 fromThe Open Group
• false: return false value – Shell and Utilities Reference,The Single UNIX Specification, Version 5 fromThe Open Group

• true(1): Do nothing, successfully –GNUCoreutils reference
• false(1): Do nothing, unsuccessfully –GNUCoreutils reference
• true(1): Return true value –FreeBSD manual page
• false(1): Return false value –FreeBSD manual page

• Standard Unix programs
• Unix SUS2008 utilities
• IBM i Qshell commands

• Articles with short description
• Short description is different from Wikidata