My question may looks trivial... But it's because despite of my long experience of linux/unix/osx systems, I found myself lacking some basic notions.

I created a little bluetooth/arduino switch to control some appliances at home. It's a simple program like shown here <http://www.tinkernut.com/2014/03/arduino-bluetooth-basics/>

To enable or disable my switch, I do :

```
screen /dev/tty.HC-06-DevB

```

Then I type 0 or 1 in the console and my switch goes On or Off.

It works. Really, it's nothing difficult... Except that I don't know the principles behind the communication with the ressources located in /dev/ and I don't know how to start learning. 

How can I have a simple commande to send 1 or 0 to this particular device? Something like:

```
echo 1 > screen /dev/tty.HC-06-DevB

```

I tried the command below, it doesn't work:

```
echo 1 > /dev/tty.HC-06-DevB
zsh: resource busy: /dev/tty.HC-06-DevB

```

Where to learn what are these "things" in /dev ?

PS : I am using OS X El Capitan.