Question

The closest solution I came to is:

declare -A foobar=([foo]=bar [bar]=foo)
(set -u; true ${foobar[foo]}) 2>/dev/null

Ideally I would like to use test -v to test if an array key is defined. But test -v foobar[bar] apparently always returns 1. Also I don’t want a global set +u and run the risk to access an undefined variable.

Answer 1

As of bash version “4.2.39(1)-release” I cannot see how to test for the presence of an array key with the test -v command. test -v foobar[foo] simply always fails. To me the best option looks like one should test with the ${foobar[foo]-bar} parameter substitution. In order to re-test Bash’s behavior with shopt’s nounset shell option and every new release I wrote the following Bash shell script:

#!/bin/bash

shopt -os nounset

declare -ri colors=1
if (( "$colors" )); then
  declare -r bold="\e[1m"
  declare -r red="\e[41m"
  declare -r green="\e[42m"
  declare -r end="\e[0;m"
else
  declare -r bold=
  declare -r red=
  declare -r green=
  declare -r end=
fi
declare -ri a="${1:-0}"
case "$a" in
  1)
    echo -e "$a. Accessing ${bold}\"\$foobar\"${end} when unset should result in an error."
    echo "$foobar"
    ;;
  2)
    echo -e "$a. Accessing ${bold}\"\${foobar-bar}\"${end} when unset should result in the \"bar\" default value."
    echo "${foobar-bar}"
    ;;
  3)
    echo -e "$a. Accessing ${bold}\"\${foobar:-bar}\"${end} when unset should result in the \"bar\" default value."
    echo "${foobar:-bar}"
    ;;
  4)
    echo -e "$a. Accessing ${bold}\"\$foobar\"${end} when set to the NULL string should result in it (whitespace)."
    declare -r foobar=
    echo "$foobar"
    ;;
  5)
    echo -e "$a. Accesssing ${bold}\"\${foobar-bar}\"${end} when set to the NULL string should result in it (whitespace)."
    declare -r foobar=
    echo "${foobar-bar}"
    ;;
  6)
    echo -e "$a. Accesssing ${bold}\"\${foobar:-bar}\"${end} when set to the NULL string should result in the \"bar\" default value."
    declare -r foobar=
    echo "${foobar:-bar}"
    ;;
  7)
    echo -e "$a. Accessing ${bold}\"\$foobar\"${end} when set to the \"foo\" string should result in it."
    declare -r foobar=foo
    echo "$foobar"
    ;;
  8)
    echo -e "$a. Accessing ${bold}\"\${foobar-bar}\"${end} when set to the \"foo\" string should result in it."
    declare -r foobar=foo
    echo "${foobar-bar}"
    ;;
  9)
    echo -e "$a. Accessing ${bold}\"\${foobar:-bar}\"${end} when set to the \"foo\" string should result in it."
    declare -r foobar=foo
    echo "${foobar:-bar}"
    ;;
  10)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]}\"${end} when unset should result in an error."
    declare -rA foobar=()
    echo "${foobar[foo]}"
    ;;
  11)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]-bar}\"${end} when unset should result in the \"bar\" default value."
    declare -rA foobar=()
    echo "${foobar[foo]-bar}"
    ;;
  12)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]:-bar}\"${end} when unset should result in the \"bar\" default value."
    declare -rA foobar=()
    echo "${foobar[foo]:-bar}"
    ;;
  13)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]}\"${end} when set to the NULL string should result in it (whitespace)."
    declare -rA foobar=([foo]=)
    echo "${foobar[foo]}"
    ;;
  14)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]-bar}\"${end} when set to the NULL string should result in it (whitespace)."
    declare -rA foobar=([foo]=)
    echo "${foobar[foo]-bar}"
    ;;
  15)
    echo -e "$a. Accessing ${bold}\"\${foobar[foo]:-bar}\"${end} when set to the NULL string should result in the \"bar\" default value."
    declare -rA foobar=([foo]=)
    echo "${foobar[foo]:-bar}"
    ;;
  16)
    echo -e "$a. Testing with ${bold}\"test -v foobar\"${end} when unset should fail."
    test -v foobar
    ;;
  17)
    echo -e "$a. Testing with ${bold}\"test -v foobar\"${end} when set to the NULL string should succeed."
    declare -r foobar=
    test -v foobar
    ;;
  18)
    echo -e "$a. Testing with ${bold}\"test -v foobar\"${end} when set to the \"bar\" string should succeed."
    declare -r foobar=
    test -v foobar
    ;;
  19)
    echo -e "$a. Testing with ${bold}\"test -v foobar[foo]\"${end} when unset should fail."
    declare -rA foobar=()
    test -v foobar[foo]
    ;;
  20)
    echo -e "$a. Testing with ${bold}\"test -v foobar[foo]\"${end} when set to the NULL string should fail."
    declare -rA foobar=([foo]=)
    test -v foobar[foo]
    ;;
  21)
    echo -e "$a. Testing with ${bold}\"test -v foobar[foo]\"${end} when set to the \"bar\" string should succeed."
    declare -rA foobar=([foo]=bar)
    test -v foobar[foo]
    ;;
  0)
    echo -e "Testing shopt's \"nounset\" shell option with Bash version \"$BASH_VERSION\"."
    echo
    declare -i b
    for ((b=1; b<=21; b+=1)); do
      "$0" "$b"
      if (( "$?" )); then
        echo -e "${red}Failure.${end}"
      else
        echo -e "${green}Success.${end}"
      fi
      echo
    done
    ;;
esac