an IO you should use to print messages to the standard out.</li><li><code>#err</code> - on IO you should use to print messages to the standard error.</li><li><code>#system!</code> - the method you should use to spawn child processes. This is preferable to <code>Kernel.system</code> because the command executed will be logged, and your app will raise if the command fails. This makes it more straightfoward to safely script other command line invocations.</li></ul><h3 id="advanced-options" tabindex="-1">Advanc