How to read systemd journal file on system that has no systemd?
Solution 1:
chroot into the Arch Linux system:
cd /location/of/arch/root
chroot /location/of/arch/root /bin/bash
journalctl looks for the journal files in /var/log/journal/machine-id/. If you boot up with a rescue OS that has a different machine ID or a different root it hence won't find any data there.
Hence,
a) use journalctl --directory=/var/log/journal/<machine-id>
b) or journalctl -m
The "-m" swicth merges the journals of all directories in /var/log/journal/*, hence makes it unnecessary to figure out the machine id of the host.
Source