Follow journal and show only entries with err priority

journalctl -p err -f

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

man 3 sd-daemon

fprintf(stderr, SD_NOTICE "Hello World!\n")

# runtime
sudo journalctl --vacuum-size=50M
 
# permanent
echo SystemMaxUse=50M | sudo tee -a /etc/systemd/journald.conf

journalctl --directory=/mnt/var/lib/journal
journalctl --file=/mnt/var/lib/journal/user-1000@*