Lookup logs with priority, date, grep and boot with journalctl

By priority:

journalctl -p err

-p possible option could be obtained from this command:

journalctl -p <TAB> <TAB>

Filter by time:

journalctl -S 02:00
journalctl -S '2024-05-05 02:04:55'
journalctl -S 02:00 -U 03:00

To grep:

journalctl -p info -g '^b'

See the logs for boot. Current boot:

journalctl -b 0

for previous boot:

journalctl -b -1