#!/bin/sh
was 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.
Specify #!/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.