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True and false (commands)
true and false are shell commands that exit immediately with exit status 0 or 1, 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.
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:
The commands can be used to ignore the success or failure of a sequence of other commands, as in the example:
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, 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:
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. For example, from bashbug, the bug-reporting script for Bash:
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True and false (commands)
true and false are shell commands that exit immediately with exit status 0 or 1, 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.
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:
The commands can be used to ignore the success or failure of a sequence of other commands, as in the example:
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, 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:
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. For example, from bashbug, the bug-reporting script for Bash: