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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.

Hey! I'll happily receive your comments via email

Andreas Wagner
Freelance System Administrator from Tallinn, Estonia.