"adb devices" command won't detect my 4.4 Android phone

This happens to me sometimes with certain devices. Here's what I find works:

1. Find the Vendor ID: You can do this by typing lsusb. My output is something like this:

   Bus 002 Device 097: ID abc1:1234 Fictional Company, Ltd. 
   

   In this case, abc1 is the Vendor ID.

2. Create an adb_usb.ini file: Run the following

   echo "0x<your device's Vendor ID>" > ~/.android/adb_usb.ini
   

3. Restart adb

   adb kill-server
   adb start-server
   

Since doing that, my devices have all shown up in adb devices.

Set mode from MTP to PTP. I had the same problem.

MTP means Media Transfer Protocol. PTP means Picture Transfer Protocol, which means that the phone appears to the computer as a digital camera.

To set the mode: Settings > Developer Options > scroll down to Select USB Configuration, tap this and select PTP or just click at notification in status bar and select PTP.