title: How to evaluate if a environment variable is set in vimrc
body: <p>How to evaluate if an bash environment variable is set</p>
  
  <p>for example </p>
  
  <pre><code>function! Myfoo(arg)
    if $SomeVar is set/exist ----&gt; how to eval the SomeVar has been set
       ....
    endif
  endfunction
  </code></pre>