On Ubuntu 16.04:
sudo apt install libpam-fprintd
Optionally, also install fprintd-demo. You’ll have to run fprintd_demo as root for it to access the fingerprint scanning device.
To store your fingerprint and enable use of the fingerprint scanner:
Note, run fprintd-enroll as your user account, not root.
That’s it. The log-in screen will ask you to swipe your finger rather than use a password, but you’ll still have to click “log in” after you do so. Sudo, both command line and graphical, will also ask to swipe, but you won’t have to do anything else.