journalctl

Query the systemd journal. More information: https://manned.org/journalctl.

• Show all messages with priority level 3 (errors) from this [b]oot:
  journalctl -b --priority={{3}}
• Show all messages from last [b]oot:
  journalctl -b -1
• Delete journal logs which are older than 2 days:
  journalctl --vacuum-time={{2d}}
• [f]ollow new messages (like tail -f for traditional syslog):
  journalctl -f
• Show all messages by a specific [u]nit:
  journalctl -u {{unit}}
• Filter messages within a time range (either timestamp or placeholders like "yesterday"):
  journalctl --since {{now|today|yesterday|tomorrow}} --until {{YYYY-MM-DD HH:MM:SS}}
• Show all messages by a specific process:
  journalctl _PID={{pid}}
• Show all messages by a specific executable:
  journalctl {{path/to/executable}}