The cfheadless client doesn't have a UI, it's run directly from the command line and is suited for headless hosts like the Raspberry Pi.

The script is located in the bin directory in the crazyflie-clients-python repository and client. Here's how to use the script:

$ bin/cfheadless -h

usage: cfheadless [-h] [-u URI] [-i INPUT] [-d] [-c CONTROLLER]
                  [--controllers] [-x]

optional arguments:
  -h, --help            show this help message and exit
  -u URI, --uri URI     URI to use for connection to the Crazyradio dongle,
                        defaults to radio://0/10/250K
  -i INPUT, --input INPUT
                        Input mapping to use for the controller,defaults to
                        PS3_Mode_1
  -d, --debug           Enable debug output
  -c CONTROLLER, --controller CONTROLLER
                        Use controller with specified id, id defaults to 0
  --controllers         Only display available controllers and exit
  -x, --x-mode          Enable client-side X-mode

The client is exited either by taking out the Crazyradio USB dongle or pressing Ctrl+C

Connect to a Crazyflie at channel 100 and speed 250Kbit using input mapping PS3_Mode_1

crazyflie-clients-python$ bin/cfheadless -u radio://0/100/250K -PS1_Mode_1