If you need to force an application on your Mac (and probably Linux too?) to use a specific shell on your system, it can be accomplished by setting the SHELL environment variable.

This can be done either within a script:

#!/bin/bash
export SHELL=/bin/bash
open -a "/Applications/SuperCoolApp.app"

Or can be done right at the command line:

SHELL=/bin/bash open -a "/Applications/SuperCoolApp.app"

Bam! My thanks to my good friend Eli for the tip on this.

But… Why?

So why on earth would you want to do this? Let's say you've got an application that runs shell commands as a part of its operation. Ideally, the app would explicitly target a specific shell to ensure compatibility across systems, but it also might just default to whatever your system's login shell is. I ran into the latter case, and the commands being run by the application were failing because they were not compatible with my preferred shell, fish. So now I just open the application using the shell command above, and voilà! All works as it should.