asdf to manage my Ruby version, but I was getting the error “ruby-build: definition not found: X.X.X” when I was trying to upgrade to a newer version. It took me a while to find out exactly why this was happening, but I eventually learned that I needed to update Ruby from
asdf like so:
asdf plugin-update ruby
After you run that, you can install your version of Ruby. Just replace
X.X.X with the version number you want:
asdf install ruby X.X.X
The more you know.