Re: ${var:+foo "$empty"}

[Stephane Chazelas]

On Mon, Feb 07, 2005 at 05:21:44PM -0500, Chet Ramey wrote:
> > There's still one obscur thing:
> > 
> > $ a() { echo $#; }
> > $ a=
> > $ a ${a+"$a"}
> > 1
> > $ a ${a+ "$a" }
> > 0
> 
> That shouldn't be unexpected.  First, keep in mind that the characters
> between the operator and the closing brace are, in effect, quoted, since
> they are processed as a single unit (but that `quoted' is the wrong term
> to use -- clear? ;-) ).
> 
> Now, consider what happens to a double quoted variable expansion when
> it appears in the middle of a string: when expanded, if the expansion
> is empty, it simply disappears.  For example, x"$a"y expands to `xy' when
> x is unset or null.
> 
> When a variable expansion is wholly quoted, and the expansion is empty, the
> result is a null argument.
> 
> So, the first example expands to "" (quoting is preserved).  The second
> example expands to "  " (unquoted), which is removed by word splitting.
[...]

Ok, I'll take your word that there exists a sensible reason,
then ;). Note that I get opposite results with ksh93:

$ a() { echo $#; }
$ a=
$ a ${a+ "$a" }
1
$ a ${a+"$a"}
0

I get 1 1 with pdksh and 0 0 with zsh in sh emulation (but then,
zsh has yet another /logic/ for it).

I guess I'll have to classify it for my own use as non-portable
reliably then.

-- 
Stéphane