Arrays concatenate in [[ ]] — use a loop

Problem

`[[ "$arr" == "value" ]]` doesn't compare the array — it concatenates the elements with the first character of IFS (usually a space) and compares the joined string. The "absent element" case looks identical to "matches", and adding elements silently changes behavior.

Bad

langs=(en de fr)
if [[ "${langs[@]}" == "fr" ]]; then ...   # checks "en de fr" == "fr"

Good

# Membership test with a loop:
for l in "${langs[@]}"; do
  [[ "$l" == "fr" ]] && found=1
done

# Or with a regex over the joined form:
if [[ " ${langs[*]} " == *" fr "* ]]; then ...
fi

Explanation

The padded-space pattern (`" ${arr[*]} "` matched against `*" item "*`) is a common idiom for "contains" without a loop. The loop version is clearer when the test grows complex.

When It Matters

Comparing "${arr[@]}" against a pattern inside [[ ]] does not test each element: the array is flattened into one string first, so the match succeeds if the pattern appears anywhere in the concatenation. A membership test written this way returns true for a value that spans two elements, which is a silent authorisation bug in permission checks. The surprise is that it works in casual testing — single-word elements usually produce the answer you expected.

Second Example

Exceptions

Joining an array into a string is fine when you want the string — building a display line or a command argument. The warning is about using that joined form as if it tested individual elements.

Faq