The following error occurs when executing “ADB shell”:

error: device unauthorized.
This adb server's $ADB_VENDOR_KEYS is not set
Try 'adb kill-server' if that seems wrong.
Otherwise check for a confirmation dialog on your device.

This is probably because “allow USB debugging” is not displayed on Android This dialog box, or no operation on this dialog box, results in the re plugging of the data cable and the pop-up of “allow USB debugging?” In the dialog box, click OK