Date: Thu, 10 Jul 2008 18:49:17 +0300 From: Giorgos Keramidas <keramida@ceid.upatras.gr> 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: <87lk09sps2.fsf@kobe.laptop> In-Reply-To: <200807110018.43081.malcolm.kay@internode.on.net> (Malcolm Kay's message of "Fri, 11 Jul 2008 00:18:42 %2B0930") 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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On Fri, 11 Jul 2008 00:18:42 +0930, Malcolm Kay <malcolm.kay@internode.on.net> 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=$as_ac_var; eval "test \"\${$as_var+set}\" = 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 "", otherwise you
>> get "set":
>> $ echo ${foo+set}
>>
>> $ echo ${HOME+set}
>> set
>
> So it appears; but is it stated anywhere that this shorthand is
> legitimate? I find it quite frequently arising from the GNU
> configuring tools but haven't found it elsewhere.
It's legitimate.
> Is it a deliberate shorthand or just a consequence of the way sh and
> bash happen to have been programmed? In other words is it a safe
> shorthand?
The shorthand version would work too:
if "${foo+set}" = 'set' ; then
bar
fi
The interesting bits in the Autoconf generated code are, however, the
side-effects of the expression:
* ${ac_var} is set to the name of the variable to check.
* The `ac_var' variable is expanded *twice* in the check. It is
expanded once before eval runs, to get the _name_ of the variable to
check, and then eval sees something like "${foo+set}".
* The assignment to `ac_var' is done inside a { ... } pair of
brackets; not in parentheses. This means that the rest of the
script can keep using ${ac_var} to find the name of the `autoconf
variable' this part of the script has set (the assignment to ac_var
happens in the current shell, and not in a sub-process).
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