Re: Empty array referenced by indirection reports unbound variable

[Chet Ramey]

On 4/7/21 10:39 AM, konsolebox wrote:
> On Wed, Apr 7, 2021 at 9:25 PM Chet Ramey <chet.ramey@case.edu> wrote:
> > On 4/5/21 4:45 PM, konsolebox wrote:
> > > set -u
> > > array=()
> > > __ref=array[@]
> > > : "${array[@]}" # Reports nothing
> >
> > This is a special case, mirroring the special case that POSIX carved out
> > for $@ and $* in 2009.
> >
> > > : "${!__ref}" # Unbound variable
> >
> > But this is not. Bash looks for __ref, finds it with a value of array[@],
> > and attempts to expand that. It expands to null (internally), since there
> > are no array elements, which becomes the value of the parameter expansion.
> >
> > Since the expansion (${!__ref}) resulted in the null string, `set -u' kicks
> > in and bash reports a fatal error.
>
> But that doesn't seem consistent since `set -- "${!__ref}"; echo "$#"`
> shows 0, meaning it didn't expand to a null string but practically
> nothing.  

The null string *is* nothing. Literally. 0x0. It's an ex-parrot.

> Does that mean internally ${!__ref} expands to null string
> but checks for a flag if it's an array expansion before the actual
> resulting expansion? 

No. It just expands to a null string.

> If that's the case shouldn't it be the same when
> reporting unbound variable?

Indirection does not check whether or not the variable it's indirecting
is $@/$* or ${array[@/*]}. It simply goes by the return value.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/