title: Can you explain this Spring environment variable resolution?
body: <p>The <a href="http://docs.spring.io/spring-boot/docs/current/reference/html/howto-logging.html" rel="noreferrer">Spring Boot docs</a> have the following sample logging file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;configuration&gt;
      &lt;include resource="org/springframework/boot/logging/logback/defaults.xml" /&gt;
      &lt;property name="LOG_FILE" value="${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}"/&gt;
      &lt;include resource="org/springframework/boot/logging/logback/file-appender.xml" /&gt;
      &lt;root level="INFO"&gt;
          &lt;appender-ref ref="FILE" /&gt;
      &lt;/root&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>Can you help me understand the line <code>${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}</code>? What are the <code>-</code> for?</p>