Make middleman's preview server a daemon

middleman-cli/lib/middleman-cli/server.rb

@@ -49,6 +49,11 @@ module Middleman::Cli
                  aliases: '-l',
                  default: 0.5,
                  desc: 'Set file watcher latency, in seconds'
+    class_option :daemon,
+                 type: :boolean,
+                 aliases: '-d',
+                 default: false,
+                 desc: 'Daemonize preview server'
 
     # Start the server
     def server
@@ -73,7 +78,8 @@ module Middleman::Cli
         disable_watcher: options['disable_watcher'],
         reload_paths: options['reload_paths'],
         force_polling: options['force_polling'],
-        latency: options['latency']
+        latency: options['latency'],
+        daemon: options['daemon']
       }
 
       puts '== The Middleman is loading'

middleman-core/lib/middleman-core/preview_server.rb

@@ -58,6 +58,19 @@ module Middleman
 
         app.execute_callbacks(:before_server, [ServerInformationCallbackProxy.new(server_information)])
 
+        if @options[:daemon]
+          # To output the child PID, let's make preview server a daemon by hand
+          if child_pid = fork
+            app.logger.info "== Middleman preview server is running in background with PID #{child_pid}"
+            Process.detach child_pid 
+            exit 0
+          else
+            $stdout.reopen('/dev/null', 'w')
+            $stderr.reopen('/dev/null', 'w')
+            $stdin.reopen('/dev/null', 'r')
+          end
+        end
+
         loop do
           @webrick.start