Irregularities when using -v, -z, -n

[eduardo-chibas]

I am getting confused with the following code

  local aa
  local bb=''
  local cc='cc'

  echo "local aa"

  [[ -v aa ]] && echo "[[ -v aa ]]: Variable exists."
  [[ ! -v aa ]] && echo "[[ ! -v aa ]]: Variable unknown."

  [[ -z aa ]] && echo "[[ -z aa ]]: Variable exists, size is zero."
  [[ ! -z aa ]] && echo "[[ ! -z aa ]]: Variable exists, size is positive 
(not-zero)."

  [[ -n aa ]] && echo "[[ -n aa ]]: Variable exists, size is positive 
(not-zero)."
  [[ ! -n aa ]] && echo "[[ ! -n aa ]]: Variable exists, size is zero."

  # -------------------------------------------------------------

  echo ""
  echo "local bb=''"

  [[ -v bb ]] && echo "[[ -v bb ]]: Variable exists."
  [[ ! -v bb ]] && echo "[[ ! -v bb ]]: Variable unknown."

  [[ -z bb ]] && echo "[[ -z bb ]]: Variable exists, size is zero."
  [[ ! -z bb ]] && echo "[[ ! -z bb ]]: Variable exists, size is positive 
(not-zero)."

  [[ -n bb ]] && echo "[[ -n bb ]]: Variable exists, size is positive 
(not-zero)."
  [[ ! -n bb ]] && echo "[[ ! -n bb ]]: Variable exists, size is zero."

  # -------------------------------------------------------------

  echo ""
  echo "local cc='cc'"

  [[ -v cc ]] && echo "[[ -v cc ]]: Variable exists."
  [[ ! -v cc ]] && echo "[[ ! -v cc ]]: Variable unknown."

  [[ -z cc ]] && echo "[[ -z cc ]]: Variable exists, size is zero."
  [[ ! -z cc ]] && echo "[[ ! -z cc ]]: Variable exists, size is positive 
(not-zero)."

  [[ -n cc ]] && echo "[[ -n cc ]]: Variable exists, size is positive 
(not-zero)."
  [[ ! -n cc ]] && echo "[[ ! -n cc ]]: Variable exists, size is zero."

  # -------------------------------------------------------------


Here are the results:

local aa
[[ ! -v aa ]]: Variable unknown.
[[ ! -z aa ]]: Variable exists, size is positive (not-zero).
[[ -n aa ]]: Variable exists, size is positive (not-zero).

local bb=''
[[ -v bb ]]: Variable exists.
[[ ! -z bb ]]: Variable exists, size is positive (not-zero).
[[ -n bb ]]: Variable exists, size is positive (not-zero).

local cc='cc'
[[ -v cc ]]: Variable exists.
[[ ! -z cc ]]: Variable exists, size is positive (not-zero).
[[ -n cc ]]: Variable exists, size is positive (not-zero).