How to create a service without any /system/etc/init.d directory I would to get a boot daemon service. 

My android 6 device is rooted and I installed busybox pack on it. After remount my /system partition in read/write, I make /system/etc/init.d directory  and I put my shell script inside. I rebooted my android device and the script didn't start alone. I checked the executable permission as well.

I browse the web around the question and I should modify my /init.rc file trough modify the boot.img file and this is a few complicate and init.rc seems target one file not a directory.

Is there an other procedure?
Thank for your help.
I found the solution here :
How to Enable Init.d Support on most Android

That's worked for me