#!/bin/shwas specified, so ____ is not supported, even when sh is actually bash.
The shebang indicates that the script works with
/bin/sh, but you are using non-standard features that may not work with
/bin/sh, even if /bin/sh is actually bash. Bash behaves differently when invoked as
sh, and disabling support for the highlighted feature is one part of that.
#!/usr/bin/env bash to ensure that bash (or your shell of choice) will be used, or rewrite the script to be more portable.
The Ubuntu wiki has a list of portability issues and suggestions on how to rewrite them.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.