Re: Builtin test and parsing of conditionals

[Bart Schaefer <schaefer@brasslantern.com>]

On Sep 4,  5:31pm, Peter Stephenson wrote:
} Subject: Re: Builtin test and parsing of conditionals
}
} On Wed, 04 Sep 2013 09:15:03 -0700
} Bart Schaefer <schaefer@brasslantern.com> wrote:
} > According to some discussion on the austin-group (POSIX) mailing list,
} > the following:
} > 
} > 	test ! -a !
} > 	test ! -o !
} > 	test ! = !
} > 
} > should all be parsed as comparing the string "!" to the string "!", but
} > zsh gets this right only in the last case.
} 
} I don't understand that.  -a means "and" and -o means "or", unless
} they're being interpreted as strings, but I don't see how that could be
} if ! is supposed to be interpreted as a string.

Sorry, perhaps I should have been clearer:  "! -a !" means to compare the
truth value "!" to the truth value of "!".  But "truth value" in "test"
is the same as "non-empty string" (true) and "empty string" (false) [not
0 or 1 as numeric values], so the -a operator is still comparing two
strings.

The point being that "!" should be parsed as a string, not as a negation
operator, in the three-argument case.

} So where does the
} implicit comparison come from?  If -a were taken as high precedence you
} might read it as
} 
} test -n ! -a n !
} test -n ! -a n !

Presuming you mean -a -n there, this is exactly how the austin-group
thread claims it should be interpreted, and is reportedly how ksh and
bash and GNU /usr/bin/test all interpret it.

} (Outside our control, but I imagine people aren't daft enough to rely on
} this sort of behaviour in new scripts...?)

I would hope so, but ...