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This document mainly introduces automatic login under Ubuntu.
Check Settings→Users→Automatic Login
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For tty1-tty6
$ sudo sed -i "s/ExecStart=.*/ExecStart=-\/sbin\/agetty --noclear --autologin root \%I \$TERM/g" /lib/systemd/system/getty@.service
For ttyS0
$ sudo sed -i "s/ExecStart=.*/ExecStart=-\/sbin\/agetty --autologin root --keep-baud 115200,38400,9600 \%I \$TERM/g" /lib/systemd/system/serial-getty@.service
Reboot to take effect.