adb device always unauthorized

Question

I'm running Ubuntu 16.04

adb version
Android Debug Bridge version 1.0.32

However, when I plug in my devices for testing via ADB, I always get an unauthorized error.

adb devices

List of devices attached 
LGD855a1098057  unauthorized

adb works flawlessly on Windows 7 x64 SP1 with Koushik Douttas Universal ADB driver and my friend's Macbook, this singles out the Ubuntu 16.04 as the problem.

Things I have tried:

• I tried revoking previous USB authorizations: nothing happened.

• Switching adb debugging on and off: however, the CLI just keeps telling me with the unauthorized error.

• Switching ports: each port on my 16-port development rig has been tried.

• Updating adb:

  android-tools-adb is already the newest version (5.1.1r36+git20160322-0ubuntu3).
  android-tools-fastboot is already the newest version (5.1.1r36+git20160322-0ubuntu3).
  

Answer 1

Same problem here with exact same Linux Ubuntu version. The solution for me that works well is

1. adb kill-server
2. sudo adb usb

After this, your mobile will ask for authorization and work well. So, adb needs root privilege.

Answer 2

Kill the adb server if running:

adb kill-server

Remove ~/.android directory with all its content:

rm -rf ~/.android

Don't forget to Revoke USB debugging authorizations in developer options on your Android device.

Start the adb server:

adb start-server

Search for devices:

adb devices

Answer 3

This worked for me:

• I revoked previous USB authorizations

• I killed and restarted the adb server using these commands:

  adb kill-server
  adb start-server
  

• I restarted the phone

• And ran adb devices

And the device didn't appear as unauthorized!