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.
> 
> 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,

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
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW

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