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+## Arrays don't work as operands in [ ]. Use a loop (or concatenate with * instead of @).
+
+### Problematic code:
+
+```sh
+ext=png
+allowedExt=(jpg bmp png)
+[ "$ext" = "${allowedExt[@]}" ] && echo "Extension is valid"
+```
+
+### Correct code:
+
+```sh
+ext=png
+allowedExt=(jpg bmp png)
+for value in "${allowedExt[@]}"
+do
+  [ "$ext" = "$value" ] && echo "Extension is valid"
+done
+```
+### Rationale:
+
+Array expansions become a series of words in `[ .. ]`. Operators expect single words only.
+
+The problematic code is equivalent to `[ "$ext" = jpg bmp png ]`, which is invalid syntax. A typical error message is `bash: [: too many arguments` or `dash: somefile: unexpected operator`. 
+
+Instead, use a `for` loop to iterate over values, and apply your condition to each.
+
+Alternatively, if you want to concatenate all the values in the array into a single string for your test, use `"$*"` or `"${array[*]}"`.
+
+### Exceptions:
+
+If you are dynamically building an a test expression, make your array the only thing in the test expression. ShellCheck will not emit a warning for: `set -- 1 -lt 2; [ "$@" ]`
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