1. Find the hidraw device corresponding to the unifying receiver:

# grep "Logitech USB Receiver" /sys/class/hidraw/hidraw*/device/uevent
/sys/class/hidraw/hidraw0/device/uevent:HID_NAME=Logitech USB Receiver
/sys/class/hidraw/hidraw1/device/uevent:HID_NAME=Logitech USB Receiver
/sys/class/hidraw/hidraw2/device/uevent:HID_NAME=Logitech USB Receiver

2. turn your device on

3. Start unifying with the following command
# sudo pairing_tool /dev/hidraw2

4. quickly turn your device off and on