Getting "???" and no permissions when using 'adb devices' on Linux
On your device, Go to Settings > About Device and repeatedly tap "Build Number" until it says you're a developer. Then go back to Settings and into Developer Options. Turn it on and check the box for USB debugging. Then Revoke USB debugging authorisations.
Now, connect your device, install its drivers if you haven't and type any ADB command. A message will popup on your device asking for authorization. Click Allow. Once you have done that, when you type adb devices it should say &lt;device_id&gt; device