Re: snippet of configure script - explain please

[Matthew Seaman]

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.



Anyway thanks for the clarification,

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?

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, otherwise 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; omission
  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

--
Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
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