Cheatsheet Pages / systemd Cheatsheet
How to read logs from a previous boot — journalctl -b
The machine crashed or rebooted and the evidence is in last boot's journal.
Most management commands need sudo, and systemctl only works on a system actually booted with systemd — inside containers it fails with “System has not been booted with systemd as init system”. Every command was checked against systemd 255, and the example units pass systemd-analyze verify.
journalctl -b # this boot only
journalctl -b -1 # the PREVIOUS boot — where you look after a crash or a hang
journalctl --list-boots # list recorded boots with IDs and time ranges, then pick with -b <id>