lk/system/systemd/journal.md

title	tags
journal	Documentation systemd

See a running log of all system messages:

journalctl -f

Or just one unit (sshd):

journalctl -f -u sshd

Find errors since November

journalctl --since=2018-11-01 --grep="EXT4-fs error"

Limit size to 2G.

journalctl --vacuum-size=2G

Log the fact that you've installed your own dnsmasq on your system to journalctl, so that you can notice why your system's broken:

logger "Installed new dnsmasq"
sudo journalctl -f