Date: Thu, 10 Jul 2008 18:09:26 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Malcolm Kay <malcolm.kay@internode.on.net> Cc: Anton Shterenlikht <mexas@bristol.ac.uk>, Mel <fbsd.questions@rachie.is-a-geek.net>, freebsd-questions@freebsd.org Subject: Re: snippet of configure script - explain please Message-ID: <48764246.8080904@infracaninophile.co.uk> In-Reply-To: <200807110018.43081.malcolm.kay@internode.on.net> References: <20080709172513.GA51206@mech-cluster238.men.bris.ac.uk> <200807101354.46321.malcolm.kay@internode.on.net> <200807101415.51455.fbsd.questions@rachie.is-a-geek.net> <200807110018.43081.malcolm.kay@internode.on.net>
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Malcolm Kay wrote:
> On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
>> On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
>>
>>>> 9255 if { as_var=3D$as_ac_var; eval "test \"\${$as_var+set}\" =3D=
set"; };
>>>> then
>>> I find this line somewhat strange as I've not been able
>>> to find documentation for the expansion of ${parameter+set} under the=
>>> Bourne shell. (nor bash, nor ksh)
>>> *****************************************************
>>> Presumably someone out there knows where to find it?
>>> *****************************************************
>> It's shorthand for ${paramter:+set}, so if unset, you get "", otherwis=
e you=20
>> get "set":
>> $ echo ${foo+set}
>>
>> $ echo ${HOME+set}
>> set
>=20
> So it appears; but is it stated anywhere that this shorthand is legitim=
ate?
> I find it quite frequently arising from the GNU configuring tools but
> haven't found it elsewhere.
>=20
> Is it a deliberate shorthand or just a consequence of the way sh and ba=
sh=20
> happen to have been programmed? In other words is it a safe shorthand?
>=20
> Anyway thanks for the clarification,
This syntax is certainly legitimate, and it is covered in the sh(1)
man page. However, the relevant section talks mostly about the related
form:
${parameter:+word}
which means 'evaluate to null unless ${parameter} is unset or null, other=
wise evaluate to "word"'. Then there's a very small and easily
missed note to the effect:
"In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omissi=
on
of the colon results in a test for a parameter that is only unset."
So ${parameter+word} means 'evaluate to null unless ${parameter} is unset=
,
otherwise evaluate to "word"'
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW
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