You're right, i tried both and only ```pidof minerd```gave me the perfect answer.
I'm actually trying again to use ```start-stop-daemon``` after reading this post : http://unix.stackexchange.com/a/78077
```start-stop-daemon``` need the main process to stay alive. As I ask minerd to run on background, he forks his childs processes then kill his main parent process. As we still can ask ```start-stop-daemon``` to run background, it may not be a problem. 
stay tuned ;)