Run bash --version to check. You should see something like this:
$ bash --version
GNU bash, version 4.4.23(1)-release (x86_64-apple-darwin17.5.0)
...
On Linux boxes, this is now the default with recent distributions. On macOS
though, the default is a Bash version 3. In this case, you need to install a
more recent one whith brew install bash and ensure that /usr/local/bin in
front of your $PATH.