There are a lot of tools for this kind of thing. My previous employer has something that is massive overkill but might be interesting to you. It's called Telegraf and is an open-source agent for monitoring all kinds of things (like a ludicrous number of plugins for all manner of software). However, one of the basic things it does well is monitor the system-level stuff.

It can monitor disk space (and only that, if it's all you're interested in), and send that info to InfluxDB (which the above screenshot is from) either self-hosted or in the *cloud* (your choice). From there you can trigger email (or other) Threshold or Deadman alerts to let you know when things go awry. 

The docs for Telegraf are at https://docs.influxdata.com/telegraf/v1.27/

It's not hard to set up, and the developers are cool people who are happy to help when/if it breaks.

Obviously, this could be considered massively overkill, compared to just emailing yourself a copy of 'df -h' :D

Hope that helps.

Al.