# display logs from current boot
journalctl -b

sudo journalctl --since yesterday
journalctl --since "2015-01-10" --until "2015-01-11 03:00"
journalctl --since 09:00 --until "1 hour ago"

journalctl -u ssh --since "2019-11-04"

journalctl -u nginx.service

# journals disc usage
sudo journalctl --disk-usage

# show errors
journalctl -p err -b
journalctl -p 3 -xb

# show dmesg
journalctl has a --dmesg

journalctl --disk-usage

# Clear systemd journals older than X days
journalctl --vacuum-time=31d

# Clear systemd journals if they exceed X storage
journalctl --vacuum-size=1G

# show NetworkManager logs
journalctl -u NetworkManager

# show cron logs
journalctl -f -u cron

Links
https://www.digitalocean.com/community/tutorials/how-to-use-journalctl-to-view-and-manipulate-systemd-logs
http://0pointer.de/blog/projects/journalctl.html
http://blog.delouw.ch/2013/07/24/why-journalctl-is-cool-and-syslog-will-survive-for-another-decade/