Re: 4.4 change in behavior from 4.3: how to catch unset when using ${#length}

[L. A. Walsh]

Chet Ramey wrote:
> On 10/21/16 5:41 PM, L. A. Walsh wrote:
>   
> > [previously] one could have the illusion of this working 
> > w/o complaint -- and returning 0 when the array was 0-len 
> > or unset, or the array length, otherwise:
> >  echo ${#array[@]:-0}
> > But I note it only seemed to work in arrays, and in 4.4 gets a
> > syntax error:
> >  echo ${#array[@]:-0}   bash: ${#array[@]:-0}: bad substitution
> >     
>
> Because it is a syntax error, and if it were not it would be ambiguous.
> The ${param:-word} word expansion takes a parameter, not another word
> expansion, as the object to be expanded.
>   
----
   I don't understand the ambiguity.  If param= "#obj[@]", then it
is clear to me that the user wants the length of an array named "obj".
If it doesn't have the [@|*], then the user wants the # of characters
in "obj".  It seems both cases have clearly defined meanings.

   In either case, if the variable isn't set, or if array is unset, then
the length cannot be set.  If the length is not set, the the text to the
right of the ':-' is substituted.

   Alternatively, neither ${#unset} nor ${#unset[@]} should generate
errors (i.e. if the length operator is defined on "unset" variables, then
no error would be generated).  However, if one has "undefined-warnings"
turned on, both of those trigger an undefined warning. 

   If one doesn't have the "undefined-warnings" turned on, then the
"#" operator says that the length of an undefined variable or array
is '0': a dubious result, unless one claims that the '#' "handles" an
undefined(unset) variable and returns its length as '0'.

   If you want '#' to return '0' for undefined/unset objects (vars or 
arrays),
then it shouldn't trigger the "-u: unset variable" exception -- in the same
way one can safely test those names with ${object:-} and not trigger
the "unset-variable exception".

   Either way would work for me -- returning 0 and not triggering the 
error,
or having "#" return "undef/unset" if used with an unset object which could
then be tested as part of a ${object:-} type expansion.

   It seems the ambiguity results from the length operator not returning
results consistent with triggered "unset" errors, no?