title: What is the use of eval `opam config env` or eval $(opam env) and their difference?
body: <p>After the installation of <code>opam</code>, it asks to do</p>
  <pre><code>eval `opam config env`
  </code></pre>
  <p>What is the exact usage of it?</p>
  <hr />
  <h1>Bounty: where to run <code>eval $(opam env)</code>?</h1>
  <p>When is <code>eval $(opam env)</code> supposed to be ran?</p>
  <p>after the opam init? before the activation of a switch? e.g. if I have:</p>
  <pre><code>conda install -c conda-forge opam
  opam init
  
  # - install coq: see 
  opam switch create debug_proj_4.09.1 4.09.1
  opam switch debug_proj_4.09.1
  opam repo add coq-released https://coq.inria.fr/opam/released
  # install the right version of coq and pins it to it so that future opam installs don't change the coq version
  opam pin add coq 8.11.0
  </code></pre>
  <p>where do I put <code>eval $(opam env)</code>?</p>