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

show system status

systemctl status

show running or failed units

systemctl list-units
systemctl
systemctl --failed

show installed unit files

systemctl list-unit-files
ls -l /usr/lib/systemd/system /etc/systemd/system

start, stop, restart or reload a unit

systemctl start unit
systemctl stop unit
systemctl restart unit
systemctl reload unit

show status of a unit

systemctl status unit

enable or disable a unit to be started on bootstrap

systemctl enable unit
systemctl disable unit

check whether a unit is enabled or not

systemctl is-enabled unit

mask or unmask a unit to make it impossible to start it

systemctl mask unit
systemctl unmask unit

reboot or poweroff the system

systemctl reboot
systemctl poweroff