Re: [Help-bash] Distinguish between unset and empty variables in loop.

[Greg Wooledge]

On Wed, Nov 23, 2016 at 08:35:33PM +0100, Christof Warlich wrote:
> >The usual procedures to distinguish between unset and empty
> >variables   (i.e. [ -z ${var+x} ] or [[ -v var ]]) do not work
> >here, as the loop variable is always set.

Uh... yeah?

> Please revisit my example, taking the for-loop into account:
> 
>       xxx=hi:
>       yyy="";
>       for i in xxx yyy zzz; do
>               [ -z ${!i} ] && eval "$i=default"; echo $i=${!i};
>       done

Oh.  You're doing some bizarre indirection thing.  You want an associative
array instead.

> I can't see how to apply the ${var+x} (or ${x+hello}} pattern to this 
> situation.

declare -A map
map[xxx]=hi
map[yyy]=""
unset 'map[zzz]'

for i in xxx yyy zzz; do
  if test "${map[$i]+defined}"; then
    echo "element $i is defined in the map"
  fi
done

If you can't use an associative array because you're stuck with bash 3,
then my advice is to upgrade to bash 4, or switch to something that HAS
associative arrays, like awk, perl, python, tcl, ....