show all messages

journalctl

show all messages from boot

journalctl -b

show messages with explanations

journalctl -x

show kernel ring buffer

journalctl -k

show messages of limitted time span

journalctl --since "2020-06-11"
journalctl --since "20 min ago"
journalctl --since "2020-06-11 00:00:00" --untill "2020-06-11 23:59:59"

show specific messages by unit, executable or process

journalctl -u networking
journalctl /usr/bin/sudo
journalctl _PID=1

show specific messages by priority or syslog facility

journalctl -p 5
journalctl -p 0..5
journalctl SYSLOG_FACILITY=10

follow new messages

journalctl -f

don't pipe output to a pager

journalctl --no-pager

Basically journal messages exist under /run/log/journal/ which is volatile directory. Below commands move journal messages directory to directory under /var/log/journal/.

echo "Storage=persistent" | sudo tee -a /etc/systemd/journald.conf
systemctl force-reload systemd-journald

You can specify directory in which journal messages saved. It would be convinient, for example, when you read other system's journal messages which is mounted on any mount point.

journalctl -D /mnt/var/log/journal -x