${var-$default} vs. ${var=$default} (was: Re: [PATCHES] Bootstrap: Allow user overriding of $AUTOCONF and $PERL.)

[Stefano Lattarini]

[Removing automake-patches from CC]

At Tuesday 27 July 2010, Eric Blake wrote:
> On 07/27/2010 03:37 PM, Stefano Lattarini wrote:
> >>> I'm used to this variant, with less typing:
> >>> : ${AUTOCONF=autoconf}
> >>> 
> >>> But your way works, too.
> > 
> > Can you confirm it's as portable as the ways suggested in the
> > autoconf manual?  If yes, do you think it could be an useful
> > addition to that manual?
> 
> The autoconf manual already states that ${var:=value} doesn't work,
> and that if value occupies more than one word, then you need to
> use quotes or an intermediate shell variable.  Then there is the
> Ultrix bug, where you need var=${var="$val"} to avoid 8-bit
> cleanliness issues, and the Solaris bug where $val containing }
> causes parse problems, leading to this recommended construct when
> you are defaulting to an unknown intermediate value:
> 
>       test "${var+set}" = set || var={value}
> 
> But by itself, ${var=value} is portable, where value is not a shell
> substitution.
But I was using ${var-value}, and I didn't find much about it in
the autoconf manual...  What I'd like to know is if `${var-$default}' 
shares the same problems of `${var=$default}'.

P.S. It seems that at least the "foo=${foo='}'}" bug of Solaris 
/bin/sh is also shared by "foo=${foo-'}'}... I have no access to 
Ultrix, QNX or Digital Unix to test the other mentioned bugs.

Regards,
   Stefano