Currently bamboo is running the inline scripts with bash on Unix and with cmd.exe on Windows and there is no way to change this behaviour.
This also means that you cannot write a single script that would be executed on all platforms.
Most of the other CI systems, including Jenkins, do allow this by simply using the shebang line in the script in order to choose what executable to use.
We discovered this design bug while evaluating Bamboo as an alternative to Jenkins. So far that's a real deal breaker.