====== journald ======


Follow journal and show only entries with **err** priority
<code bash>journalctl -p err -f</code>
Valid priorities are (syslog):
  * 0 	emerg 	(panic / System is unusable)
  * 1 	alert   (Action must be taken immediately / A condition that should be corrected immediately, such as a corrupted system database)
  * 2 	crit    (Critical conditions / Hard device errors)
  * 3 	err 	(Error conditions)
  * 4 	warning  (Warning conditions 	)
  * 5 	notice   (Normal but significant conditions / Conditions that are not error conditions, but that may require special handling)
  * 6 	info 	 (Informational messages)
  * 7 	debug

====== Assigning stderr priorities ======

<code bash>man 3 sd-daemon</code>

<code c>
fprintf(stderr, SD_NOTICE "Hello World!\n")
</code>

====== reduce journal size ======

<code bash>
# runtime
sudo journalctl --vacuum-size=50M

# permanent
echo SystemMaxUse=50M | sudo tee -a /etc/systemd/journald.conf
</code>

====== read journal from custom location ======

<code bash>
journalctl --directory=/mnt/var/lib/journal
journalctl --file=/mnt/var/lib/journal/user-1000@*
</code>