adb devices "no permissions"

When you using hardware devices to debug android applications, sometimes your machine may cannot found or initialize the hardware.

$ adb devices
List of devices attached
emulator-5554    device
????????????    no permissions

What happened?

1. Your machine does not have udev rules.
2. adb does not have a permission

How to

1. See this page and create the rules.
2. Kill adb server then restart it as root permission.

$ adb kill-server
$ sudo adb start-server
$ adb devices
List of devices attached
353019043208217    device