GLSL multiple lights

[Jacke]

Hi, does anyone know how to implement multiple lights in GLSL?

I have implemented the following code:
vertex shader:

varying vec3 normal, lightDir;
varying vec4 diffuse;
void main()
{
  gl_Position = gl_ProjectionMatrix * gl_ModelViewMatrix * gl_Vertex;
 
  vec3 transformedNormal = gl_NormalMatrix * gl_Normal;
 
  normal = gl_NormalMatrix * gl_Normal;
 
  if(vec3(gl_LightSource[0].position) != vec3(0.0, 0.0, 0.0)) {
    lightDir = vec3(gl_LightSource[0].position);
    diffuse = gl_FrontMaterial.diffuse * gl_LightSource[0].diffuse;
  }
 
}

fragment shader:

varying vec3 normal, lightDir;
varying vec4 diffuse;
void main()
{ 
  vec4 color;
  float dotProduct;  
 
  dotProduct = dot(normal, lightDir);
 
  color = dotProduct * diffuse;
 
  gl_FragColor = color;
}

So now I want to add 3 other lights, gl_LightSource[1], gl_LightSource[2] and gl_LightSource[3] which comes from different directions and have different colors and are defined in the .c file. How do I do this? You turn the lights on and off with the F1, F2, F3 and F4 keys and my if statement is supposed to check if the light is activated or not. I managed to get all the lights to work with a for statement before the if statement but the lights couldn't be activated at the same time with that solution, just one at a time. I want to be able to have blue light on one side of an object and red light on the other side, for an example.