Date: Fri, 15 Jan 2016 08:09:46 +0000 From: Matthew Seaman <matthew@FreeBSD.org> To: freebsd-questions@freebsd.org Subject: Re: Sh-test, bug or not? Message-ID: <5698A94A.8040604@FreeBSD.org> In-Reply-To: <0LwGDy-1a6CqZ3TAs-0186MD@mail.gmx.com> References: <0LwGDy-1a6CqZ3TAs-0186MD@mail.gmx.com>
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On 15/01/2016 04:32, CK wrote:
> If you have authoritative knowledge on the subject,
> please state if this functionality is correct:
>=20
> $ [ ! "" -a "" ] && echo pass || echo fail
> pass
> $ [ ! 11 -a "" ] && echo pass || echo fail
> pass
>=20
> The "-a" operator binds stronger than the "!" operator.
> Intuition based on functionality in awk/C would suppose
> that the "!" operator would bind stronger than the "-a"
> operator, especially since "-a" does in fact have higher
> precedence than the "-o" operator, as in awk/C.
>=20
> In order to make it work as "expected", it gets ugly:
>=20
> $ [ ! "" -a "" ] && echo pass || echo fail
> pass
> $ [ \( ! "" \) -a "" ] && echo pass || echo fail
> fail
>=20
> $ [ ! 11 -a "" ] && echo pass || echo fail
> pass
> $ [ \( ! 11 \) -a "" ] && echo pass || echo fail
> fail
>=20
> I never noticed this in 20 years, so I don't know if it always
> worked this way, or if something changed in my upgrade from
> 4.11 to 9.3.
I think that while unexpected in comparison to other languages which
have inherited C-like operator precedence rules, this is according to
the POSIX standard for test(1). As the man page says:
The test grammar is inherently ambiguous. In order to assure a
degree of
consistency, the cases described in the IEEE Std 1003.2 (``POSIX.2''=
),
section D11.2/4.62.4, standard are evaluated consistently according =
to
the rules specified in the standards document. All other cases are
sub-
ject to the ambiguity in the command semantics.
and it notes that the ambiguous cases are those involving (), -a and -o.
Your test might be more clearly expressed as:
$ [ ! "" ] && [ "" ] && echo pass || echo fail
fail
(although I'd recommend the -z and -n operators for testing the
emptiness / undefinedness or not of strings.)
Cheers,
Matthew
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