How to debug a GLSL shader?

void main(){
  float bug=0.0;
  vec3 tile=texture2D(colMap, coords.st).xyz;
  vec4 col=vec4(tile, 1.0);

  if(something) bug=1.0;

  col.x+=bug;

  gl_FragColor=col;
}

You can't easily communicate back to the CPU from within GLSL. Using glslDevil or other tools is your best bet.

A printf would require trying to get back to the CPU from the GPU running the GLSL code. Instead, you can try pushing ahead to the display. Instead of trying to output text, output something visually distinctive to the screen. For example you can paint something a specific color only if you reach the point of your code where you want add a printf. If you need to printf a value you can set the color according to that value.

I have found Transform Feedback to be a useful tool for debugging vertex shaders. You can use this to capture the values of VS outputs, and read them back on the CPU side, without having to go through the rasterizer.

Here is another link to a tutorial on Transform Feedback.