Check if array is empty in bash

As noted by @cheapner in the comments, you are not defining your arrays correctly.

key=()
key1=("2014" "kdjg")

key is here an empty array and key1 has 2 elements.

This then prints the number of elements in the arrays, which is 0 and 2 respectively:

echo "${#key[@]}"
echo "${#key1[@]}"

And this prints empty and not empty respectively:

if (( ${#key[@]} == 0 )); then
    echo empty
fi

if (( ${#key1[@]} != 0 )); then
    echo not empty
fi

For plain arrays (not associative arrays) empty check, I simply use:

if [[ -z ${array[@]} ]]; then
    # do something
fi

EDIT:

If (and only if) the array certainly will never contain an empty 1st element, testing for only the first element would suffice, which might be faster for large arrays, but would require verification:

[[ -z $array ]] && ...

NOTE: As already stated above, testing the array for size also works - which one's faster, would require verification, too.

BUT this test also works for arrays, which only contain one empty element and appears to be the safest method - for -z the test fails:

array=('')
[[ ${#array[@]} -eq 0 ]] && echo TRUE
[[ ${#array[@]} == 0 ]] && echo TRUE # as string compare
-> no output (OK)

[[ -z ${array[@]} ]] && echo TRUE
-> TRUE (FAIL)

A trick to work around the array single empty element issue with the -z test operator is to use @Q as expansion operator, since it outputs empty quotes - this also works for only checking the 1st array element:

[[ -z ${testA[@]@Q} ]] && echo TRUE
-> no output (OK)

[[ -z ${testA@Q} ]] && echo TRUE
-> no output (OK)