Except by default, bash from the Ports tree gets but into /usr/local/bin/bash. An easy fix for users, but a better hashbang would be #!/usr/bin/env bash
Assuming `bash` is in `/bin` is definitely a linuxism. It won't be there on BSDs, and even when there's a bash there it may not be up to date (e.g. on OSX where up-to-date software is installed via homebrew or macports and the binaries are not in /bin)
#!/bin/bash and then not using any features not available in sh (or using bash features that could be written trivially in sh) is definitely a linuxism.
#!/bin/bash just means your port has to depend on shells/bash, which is pretty reasonable.