Uses FFI and works all relevant operating systems.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

If you'd try to do it yourself, you'd have to use FFI which is a bit awkward (see the gem's code), or call commands like df -B1 and search the strings for your relevant data.
That gem takes the pain away and works nicely.