chroot and serial console to fix ubuntu distro upgrade gone wrong

I had to fix a do-distro-upgrade from 16.04 to 18.04 due to a severed ssh connection, and no screen running (apparently earlier distro upgrades used screen to prevent this kind of problem)

The machine as a PCengine apu2, so no video. Also, the root file system is sitting on a miniPCI ssd.

Eventually, my laptop and this chroot cheatsheet helped: https://aaronbonner.io/post/21103731114/chroot-into-a-broken-linux-install

1. Mount the root filesystem device

    mount -t ext4 /dev/<device> /mnt/
   

2. If there’s a different boot partition or anything else

    mount -t ext2 /dev/<device> /mnt/boot
   

3. Mount special devices

    mount -t proc none /mnt/proc
    mount -o bind /dev /mnt/dev
    mount -o bind /sys /mnt/sys
   

4. chroot

    chroot /mnt /bin/bash
    source /etc/profile
   

In order to help troubleshoot in the future, i followed this advice to get a systemd service unit for a constant shell on the serial port, but mine runs for some reason on S0: http://0pointer.de/blog/projects/serial-console.html

systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service

It won’t help if systemd does not start, but otherwise it is online really early.