I have an ASUS ThinkPad, Android 4.4.2, Kernel 3.10.20. I've checked Settings->Developer Options->USB debugging. I've disabled Wi-Fi and BT (because I've seen some devices have some conflict there). Now here's the command line sequence from my linux desktop trying to connect to my ASUS:
joe@OptiPlex-755:~$ adb kill-server
joe@OptiPlex-755:~$ adb usb
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
already in USB mode
joe@OptiPlex-755:~$ adb devices -l
List of devices attached
E6OKCY917549 device usb:1-2 product:WW_K011 model:K011 device:K011
joe@OptiPlex-755:~$ adb get-state
device
joe@OptiPlex-755:~$ adb connect usb
unable to connect to usb:5555
joe@OptiPlex-755:~$
adb connect
is only needed if you are trying to connect to a device over TCP IP, i.e. wireless network. You don't need to issue any connect commands if adb already sees your device inadb devices
prompt. Just issue any other command, e.g.adb shell ls /sdcard/
and it should connect to the only available device.