sudo journalctl -u
sudo journalctl -u <service> [-n N] [-f] [-g word]
Risk: Safe
What it does
Reads the journal, systemd's diary of what services said.
When to use it
A service failed and you need to know why.
Example
$ sudo journalctl -u nginx -n 20The last 20 lines from nginx.
$ sudo journalctl -u nginx -fWatch new lines live. Ctrl+C stops.
$ sudo journalctl -u nginx -g emergOnly lines with emerg.
Learn it in: Logs with journalctl · Official docs