SC2198

ext=png
allowedExt=(jpg bmp png)
[ "$ext" = "${allowedExt[@]}" ] && echo "Extension is valid"

ext=png
allowedExt=(jpg bmp png)
for value in "${allowedExt[@]}"
do
  [ "$ext" = "$value" ] && echo "Extension is valid"
done

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[*]}".

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; [ "$@" ]