title: How do I set an environment variable on macOs Mojave?
body: <p>I'm installing 'THERMUS: A Thermal Model Package for ROOT' on macOs Mojave. One of the steps is (after unzipping) the following: "Set an environment variable `THERMUS' to point at the top-level directory containing the THERMUS code". I really don't know what to do. 
  I've seen the other post on stackoverflow: <a href="https://stackoverflow.com/questions/24792260/no-rule-to-make-target-with-no-reason">&#39;no rule to make target&#39; with no reason</a>, but, as I am a beginner, I really don't understand how I should set it. Please help me</p>