title: What does $ROOT mean in a Mac Terminal?
body: <p>I received some command line instructions:</p>
  
  <pre><code>ROOT=`cmd1 | cmd2 | cmd3`
  echo ROOT=$ROOT
  ...
  </code></pre>
  
  <p>It seems that this $ROOT stores the output of the three pipelined cmds. But what is $ROOT? Is it an environment variable? Is it a register? How should I call it? Can I have other variables like it? For example $APPLE? I hope to learn more about $ROOT :)</p>