snippet of configure script - explain please

[Giorgos Keramidas]

On Fri, 11 Jul 2008 00:18:42 +0930, Malcolm Kay <malcolm.kay at 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).