ADB Troubleshooting

Android Debugger Bridge (ADB) 

Detect your connected devices/emulator instances from command line console/terminal with 'adb devices' (of course without those single quotes).


That should list them. If you don't find any, it's troubleshoot time!

Troubleshooting

'adb' and 'android' are platform-tools or tools inside your SDK folder downloaded from Google. Update to the latest by launching the SDK manager 'android'.

Tip: It helps to have these tools folders listed in environment path variables. Otherwise, to invoke, you'll need to go in to that folder each time.


Use 'adb kill-server'
to recover broken adb sessions
At times, 'sudo adb kill-server' might help better on Linux. This followed by 'adb devices' will automatically run 'adb start-server'.

Sometimes explicit 'adb start-server' or 'sudo adb start-server' also helps.

If this isn't an emulator but a device over USB, check the USB cable.

Easily forgotten: Try with another cable.

Update USB drivers in Android SDK manager.
Update USB drivers for your OS if any. Disconnect the device. From control panel's device manager (on Windows) remove and add drivers manually. Android composite device will show up on pointing it to drivers downloaded as part of 'android' SDK update.

Enable USB debugging in device's Settings> Developer options> ...
If 'Developer options' don't show up, enable that by tapping Settings> About> Build version 6 times!

First time, unlock the device and accept the dialog asking for your permission for USB debugging.