Enable the serial console on Debian 9

Today I had two (planned) power outages that I forgot about, and when I came home my home server would not boot up anymore. I could not ssh into it, and it is located in the server room without a display. ​

I wanted to connect the home server with a serial cable to my laptop and see what happened, but the serial console was not enabled :/ ​

Enabling​

Here is what I had to do to activate the serial console on Debian 9: ​

1. Edit /etc/default/grub and change the GRUB_CMDLINE_LINUX_DEFAULT line to: ​

   GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0 console=tty0"​

   It enables the serial console in the linux kernel. Note that the default speed is 9600 baud. ​

   ​
2. Run update-grub. /boot/grub/grub.cfg will contain the console kernel parameters now. ​
3. Reboot. The serial console is activated. ​

Connect​

Before rebooting, I connected my laptop via a USB-to-serial adapter and a serial cable to the home server. Then I started minicom: ​

$ minicom -D /dev/ttyUSB0​

Once started, I changed the speed to 9600: ​

1. Ctrl + A​
2. p (connection settings)​
3. c (selects 9600)​
4. Return (close dialog)​

(Yes, I tried setting the speed with a cli parameter, but that did not work.) ​

Then I rebooted the home server and saw the kernel messages flowing into minicom. Then I could login as root and examine my system. ​

$ systemctl enable serial-getty@ttyS0.service​