title: What is the essential difference between compound command and normal command in bash?
body: <p>I am learning bash. Now I learning [[ ... ]] command and (( ... )) command. They are called compound command, which is discriminated from normal command, like [. I read the article of "Compound Commands" in bash manual. It seems they runs with their own rules. But unfortunately, I could not think of why they are called as "compound".
  Someone who knows it, please let me know. Thank you very much.</p>