Why it makes sense for true and false to ignore their arguments

March 7, 2016

It's standard when writing Unix command line programs to make them check their arguments and complain if the usage is incorrect. It's reasonably common to do this even for programs that don't take options or positional arguments. After all, if your command is supposed to take no arguments, it's really an error if someone runs it and gives it arguments.

(Not all scripts, programs, and so on actually check this, because you usually have to go at least a little bit out of your way to look at the argument count. But it's the kind of minor nit you might get code review comments about, or an issue report.)

true and false are an exception to this, in that they more or less completely ignore any arguments given to them. Part of this behavior is historical; the V7 /bin/true and /bin/false were extremely minimal, and when you're being minimal it's easiest to not even look at the arguments. But beyond the history, I think that this is perfectly sensible behavior for true and false because it makes them universal substitutes for other commands, for when you want to null out a command so that it does nothing.

Want to make a command do nothing but always succeed? Simple: 'mv command command.real; ln -s /bin/true command'. Want to do the same thing but have the command always fail? Use false instead of true. Sure, you can do the same thing with shell scripts that deliberately ignore the arguments and just do 'exit 0' or 'exit 1', but this is a little bit simpler and matches the historical behavior.

(You can also do this in shell scripts as a way of creating a 'don't actually do anything' mode, but there are probably better patterns there.)

On that note, it's interesting to note that although GNU true and false have command line options that will cause them to produce output, there is no way to get them to return the wrong exit status. And while they respond to --help and --version, they silently ignore other options (as opposed to, say, reporting a syntax error).

(This entry was sparked by Zev Weiss's mention of true in his comment on this entry.)

Sidebar: true and false in V7

In V7 Unix, true is an empty file and false is a file that is literally just 'exit 1'. Neither has a #! line at the start of the file, because that came in later. That true is empty instead of 'exit 0' saves V7 a disk block, which probably mattered back then.

Written on 07 March 2016.
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Last modified: Mon Mar 7 23:13:13 2016
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