journalctl cheat sheet (Ubuntu)
View systemd journal logs.
Basics
journalctl -b
journalctl -f
journalctl --since "1 hour ago"
Unit & priority
journalctl -u nginx.service
journalctl -u nginx.service -f
journalctl -p err..alert
Disk usage
journalctl --disk-usage
sudo journalctl --vacuum-time=7d