Not sure I understand what you mean. /bin/sh is the system shell, I don't believ...

Not sure I understand what you mean. /bin/sh is the system shell, I don't believe it's possible to make it fish unless you change the symlink itself.

On ubuntu, /bin/sh is always dash and it's not possible to change without changing the symlink:

So, if you use chsh to make your shell fish, it would have no effect on os.system or the system function in C.