title: How to reference OS Environment Variables in nginx.conf
body: <p>In nginx.conf.</p>
  
  <p>After set a variable by <code>set $name value</code>,
  i can reference it like <code>$name</code>,</p>
  
  <p>But when I export an OS Environment Variable
  by <code>env name_from_env</code>, 
  like <a href="https://nginx.org/en/docs/ngx_core_module.html#env" rel="nofollow noreferrer">https://nginx.org/en/docs/ngx_core_module.html#env</a> said,
  and i am sure the name_from_env is valid which
  defined form nginx's parent process.</p>
  
  <p>But, my friends, how to reference it ?
  $name_from_env or ${name_from_env} or
  %name_from_env% didn't work what I've tried before.</p>