Plymouth scripting language manual

I have script and I need to know more about its scripting language specifications. I have tried specifications, but it is basic intro. There is no other Math object beside Image and Sprite. I need to know if there are more objects?

For example, because they said its code like JavaScript and C,

screen_width = Window.GetWidth();
screen_height = Window.GetHeight();
screen_x = Window.GetX();
screen_y = Window.GetY();
background_image = Image("BACKGROUND.PNG");
ratio = screen_height / screen_width;
background_ratio = background_image.GetHeight() / background_image.GetWidth();
factor = 0;

text_msg ="Said Bakr\nPlease Wait...";

However, returns #NULL I also need to know, is there any way to get bash commands output in variables of that script? such as osName = $(uname) the later returns error in the daemon.