Return the default shell to use on this system

def shell():
    '''
    Return the default shell to use on this system
    '''
    # Provides:
    #   shell
    if salt.utils.platform.is_windows():
        env_var = 'COMSPEC'
        default = r'C:\Windows\system32\cmd.exe'
    else:
        env_var = 'SHELL'
        default = '/bin/sh'

    return {'shell': os.environ.get(env_var, default)}