Enabling a USB Serial Port Console: getty systemd

Source: https://www.rogerirwin.co.nz/linux-open-source/enabling-a-serial-port-console/
Update by Phillips Hsieh

Using the serial port to login is a good way to get access to a device that doesn't have a network connection. This could be a small embedded Linux computer.

Raspberry Pi

enable_uart=1
# after reboot
systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service

# Or
raspi-config
# select 3 Interface Options - 
# I6 Serial Port 
# Would you like a login shell to be accessible over serial? Yes
# Would you like the serial port hardware to be enabled?  Yes
# Reboot

On a Raspberry Pi you can edit /boot/firmware/config.txt and add this at the bottom. Then reboot. More details can be found here.

Linux devices with systemd

# For a USB serial adaptor
systemctl enable serial-getty@ttyUSB0.service

vim /etc/systemd/system/getty.target.wants/serial-getty@ttyUSB0.service
# Modify like below that allow auto login root then ask password
# ExecStart=-/sbin/agetty -o '-p -- \\u' --keep-baud 115200,57600,38400,9600 --autologin root %I $TERM

systemctl daemon-reload

# For a built in serial port /dev/ttyS0
systemctl start serial-getty@ttyUSB0.service

You can now log in using the serial console on /dev/ttyUSB0 or /dev/ttyS0

Connect another computer to it using a NULL modem cable.
Then open a terminal emulator such as minicom which support send [send break F] trigger with CTRL-A (key)
Set the baud rate and port.
Press enter a few times and you should then see a login prompt.