title: Why is COMPOSER_HOME empty?
body: <p>I have a question about the programming tool <a href="https://getcomposer.org/download/" rel="noreferrer">composer</a> which is installed as an executable in <code>/usr/local/bin/composer</code> on 3 different machines that I use between work and home. According to:</p>
  
  <pre class="lang-sh prettyprint-override"><code>composer help global
  </code></pre>
  
  <blockquote>
  <pre><code>COMPOSER_HOME is c:\Users&lt;user&gt;\AppData\Roaming\Composer on Windows
  and /home/&lt;user&gt;/.composer on unix systems.
  
  Note: This path may vary depending on customizations to bin-dir in
  composer.json or the environmental variable COMPOSER_BIN_DIR.
  </code></pre>
  </blockquote>
  
  <p>But both of these are empty:</p>
  
  <pre class="lang-sh prettyprint-override"><code>echo $COMPOSER_HOME
  echo $COMPOSER_BIN_DIR
  </code></pre>
  
  <p>When I run:</p>
  
  <pre class="lang-sh prettyprint-override"><code>composer global require &lt;package&gt;
  </code></pre>
  
  <p>It installs to <code>/home/&lt;user&gt;/.config/composer/vendor/bin</code> <em>only</em> on my Ubuntu 16.04 desktop, but everywhere else it installs appropriately to <code>/home/&lt;user&gt;/.composer/vendor/bin</code></p>
  
  <p>Why is it installing to <code>~/.config</code> instead of <code>~/.composer</code> and not setting the <code>COMPOSER_HOME</code> variable?</p>
  
  <p>I notice this because I source my common dotfiles which includes the global composer bin path. I know I can workaround this by manually setting <code>COMPOSER_HOME</code> in my bashrc, but I would rather understand why this is happening in the first place.</p>