If you've used any modern Linux distribution, chances are good you
have become familiar with systemd. You might have also happened upon
a systemd issue when an app or service refuses to start. When that's
the case, you might have found yourself in a situation where you're
not quite sure how to begin troubleshooting.
Fortunately for every Linux admin, there's a built-in tool to help
you with that. Said tool is journalctl. Journalctl is the utility
used for querying the systemd logging utility, journald. With the
help of journald and journalctl, you can begin the process of
troubleshooting why a service is refusing to start. Journald also
tracks logs to a specific boot. With this mechanism, you can compare
system boots to see when a service was working properly versus when
it wasn't.