Check if an associative array exists

Question

Since bash 4.2, the -v conditional expression has been added. It's used to check if a variable is set (has been assigned a value). It's a useful tool when writing scripts that run under set -o nounset, because attempting to use a non-initialized variable causes an error.

I have one issue with it, see the sample in a POSIX bash (set -o posix):

$ declare testvar=0
$ [ -v testvar ] && echo "It works!"
It works!
$ declare -A fizz=( [buzz]=jazz )
$ [ -v fizz[buzz] ] && echo "It works!"
It works!
$ [ -v fizz ] && echo "It doesn't work :("
$ 

As you can see, using -v on a regular variable works as expected. Using it to check the presence of a field within an associative array works as expected as well. However, when checking the presence of the associative array itself, it doesn't work.

Why is that, and is there a workaround?

Answer 1

If you want to know if it has any entries, you can check ${#fizz[@]}, but that doesn't tell you if it has been created empty. You can use a case statement or other pattern matching in conjunction with error checking.

tst() { local tst;   
  (( $# )) || return                             # *minimal* error handling...
  if tst="$(declare -p $1 2>&1)"                 # does the subshell error?
  then case "$tst" in                            # if not, check the output
       *\)*) (( $( eval echo \${#$1[@]} ) )) &&  # test number of entries
                echo "Has Args"              ||  # some implementations show '()'
                echo "Empty but defined";;       # some don't; this is for the *do*
       *-A*) echo "Empty but defined";;          # this is for the don't
          *) echo ERROR;;                        # shouldn't get here
       esac
  else echo Undefined                            # if doesn't exist
  fi
}


$: unset fizz
$: tst fizz
Undefined
$: declare -A fizz
$: tst fizz
Empty but defined
$: fizz[buzz]=jazz
$: tst fizz
Has Args

Dunno how useful that will be in your context though. Note that I wrote this one explicitly for associative arrays with minimal error checking not directly related to the point. For any sort of real production use this should be expanded a lot.

I don't like the eval, but my bash version doesn't have namerefs. :o/