title: Logback is not generating log files on Ubuntu
body: <p>We have configured logback for our logging needs in our application. When we run our application on Windows machine, it works as expected and generates the log files with proper logs. However when we deployed the same runnable jar file on Ubuntu machine, it is not generating log files. Following is the code in logback.xml</p>
  
  <pre><code>&lt;configuration&gt;
    &lt;appender name="SIFT" class="ch.qos.logback.classic.sift.SiftingAppender"&gt;
      &lt;!-- in the absence of the class attribute, it is assumed that the
           desired discriminator type is
           ch.qos.logback.classic.sift.MDCBasedDiscriminator --&gt;
  &lt;discriminator&gt;
    &lt;key&gt;uniqueNumber&lt;/key&gt;
    &lt;defaultValue&gt;unknown&lt;/defaultValue&gt;
  &lt;/discriminator&gt;
  &lt;sift&gt;
    &lt;appender name="FILE-${uniqueNumber}" class="ch.qos.logback.core.rolling.RollingFileAppender"&gt;
      &lt;file&gt;${uniqueNumber}.log&lt;/file&gt;
      &lt;append&gt;true&lt;/append&gt;
       &lt;rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy"&gt;
                      &lt;FileNamePattern&gt;${uniqueNumber}_%i.log&lt;/FileNamePattern&gt;
                      &lt;MinIndex&gt;1&lt;/MinIndex&gt;
                      &lt;MaxIndex&gt;10000&lt;/MaxIndex&gt;
              &lt;/rollingPolicy&gt;
              &lt;triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy"&gt;
                      &lt;MaxFileSize&gt;5MB&lt;/MaxFileSize&gt;
              &lt;/triggeringPolicy&gt;
      &lt;layout class="ch.qos.logback.classic.PatternLayout"&gt;
        &lt;pattern&gt;%d [%thread] %level %mdc %logger{35} - %msg%n&lt;/pattern&gt;
      &lt;/layout&gt;
    &lt;/appender&gt;
  &lt;/sift&gt;
    &lt;/appender&gt;
    &lt;root level="DEBUG"&gt;
      &lt;appender-ref ref="SIFT" /&gt;
    &lt;/root&gt;
  &lt;/configuration&gt;
  </code></pre>