" how can I execute this commands saved (somewhere in the pc) to rebuild the saved sketches?"
Not too sure what you mean here.
Open the PDE, load the sketch and run it?
Perhaps use the new processing-java (.exe on Windows) coming with the latest betas?

Hi PhiLho, first of all, thanks for the reply.

I try to explain myself more clearly.

There is no easy way to do that...
Perhaps you should instead save information that can be played back by your sketch. Not executable code, but something telling to draw such and such shape, with this and that parameters.

I thought there was a way to eval the processing string from a file...

" eval the processing string from a file"
The Java version of Processing needs a compilation phase. It is harder than in JavaScript or PHP... (which are interpreted languages)

You can create classes drawing a shape, based on the same interface. Then store an instance of each class in a HashMap with the keyword as key and the instance as value.
When you read a line with the rect(a, b, c, d) command, you can extract the Rect drawing object with the rect key, and pass it the parameters to get it to draw it.

A quick proof of concept, hopefully clearer than my explanations above...

1. interface Shape
2. {
3.   void draw(int[] params); // Ensure all classes implementing the interface will have this method
4. }
5. class Rect implements Shape
6. {
7.   void draw(int[] params)
8.   {
9.     rect(params[0], params[1], params[2], params[3]);
10.   }
11. }
12. class Ellipse implements Shape
13. {
14.   void draw(int[] params)
15.   {
16.     ellipse(params[0], params[1], params[2], params[3]);
17.   }
18. }
19. class Fill implements Shape
20. {
21.   // Not a real drawing, but same effect
22.   void draw(int[] params)
23.   {
24.     fill(params[0], params[1], params[2]);
25.   }
26. }
27. // The catalog of shapes
28. HashMap<String, Shape> shapes = new HashMap<String, Shape>();
29. void setup()
30. {
31.   size(400, 400);
32.   noLoop();
33.   shapes.put("rect", new Rect());
34.   shapes.put("ellipse", new Ellipse());
35.   shapes.put("fill", new Fill());
36. }
37. // The drawing instructions, can be read from a file
38. String[] drawings =
39. {
40.   "ellipse(10,20,55,55);",
41.   "rect(100,30,55,55);",
42.   "ellipse(250,310,55,55);",
43.   "fill(55,110,220);",
44.   "rect(120,130,55,55);",
45. };
46. void draw()
47. {
48.   background(255);
49.   for (String line : drawings)
50.   {
51.     line = line.replaceAll("\\);", ""); // Remove tail
52.     String[] parts = split(line, "(");
53.     String shapeName = parts[0];
54.     println(parts[1]);
55.     String[] paramsStr = split(parts[1], ",");
56.     int[] paramsInt = int(paramsStr);
57.     println(paramsInt);
58.     Shape shape = shapes.get(shapeName);
59.     shape.draw(paramsInt);
60.   }
61. }