Date: Fri, 11 Jul 2008 01:33:45 +0930 From: Malcolm Kay <malcolm.kay@internode.on.net> To: freebsd-questions@freebsd.org Subject: Re: snippet of configure script - explain please Message-ID: <200807110133.45331.malcolm.kay@internode.on.net> In-Reply-To: <200807101704.40038.fbsd.questions@rachie.is-a-geek.net> References: <20080709172513.GA51206@mech-cluster238.men.bris.ac.uk> <200807110018.43081.malcolm.kay@internode.on.net> <200807101704.40038.fbsd.questions@rachie.is-a-geek.net>
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On Fri, 11 Jul 2008 12:34 am, Mel wrote:
> On Thursday 10 July 2008 16:48:42 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=$as_ac_var; eval "test \"\${$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.
> >
> > 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?
> >
> > Anyway thanks for the clarification,
>
> Hmm, I'm not sure if the colon syntax came first.
Looks like your supposition is correct. I dug out an old DEC Ultrix manual
which ducuments an 'sh' shell and a 'sh5' shell the 'sh' being the "normal"
bsd version of the Bourne shell and 'sh5' being a compatibility version
for system V scripts. The former (bsd version) does not use the ':' in
parameter substitutions. But the system V version does.
> Autotools claims to create
> portable shell code, though they
> also claim to make software developer's
> lives easier.
Malcolm
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