default site.url in dev environment to `http://localhost:4000`

take `host`, `port` and `ssl` options into account

lib/jekyll/commands/serve.rb

@@ -33,6 +33,7 @@ module Jekyll
               opts["serving"] = true
               opts["watch"  ] = true unless opts.key?("watch")
               config = opts["config"]
+              opts["url"] = default_url(opts) if Jekyll.env == "development"
               Build.process(opts)
               opts["config"] = config
               Serve.process(opts)
@@ -47,11 +48,7 @@ module Jekyll
           destination = opts["destination"]
           setup(destination)
 
-          server = WEBrick::HTTPServer.new(webrick_opts(opts)).tap { |o| o.unmount("") }
-          server.mount(opts["baseurl"], Servlet, destination, file_handler_opts)
-          Jekyll.logger.info "Server address:", server_address(server, opts)
-          launch_browser server, opts if opts["open_url"]
-          boot_or_detach server, opts
+          start_up_webrick(opts, destination)
         end
 
         # Do a base pre-setup of WEBRick so that everything is in place
@@ -101,6 +98,17 @@ module Jekyll
           opts
         end
 
+        #
+
+        private
+        def start_up_webrick(opts, destination)
+          server = WEBrick::HTTPServer.new(webrick_opts(opts)).tap { |o| o.unmount("") }
+          server.mount(opts["baseurl"], Servlet, destination, file_handler_opts)
+          Jekyll.logger.info "Server address:", server_address(server, opts)
+          launch_browser server, opts if opts["open_url"]
+          boot_or_detach server, opts
+        end
+
         # Recreate NondisclosureName under utf-8 circumstance
 
         private
@@ -127,6 +135,16 @@ module Jekyll
 
         #
 
+        def default_url(opts)
+          config = configuration_from_options(opts)
+          host = config["host"] == "127.0.0.1" ? "localhost" : config["host"]
+          port = config["port"]
+          protocol = config["ssl_cert"] && config["ssl_key"] ? "https" : "http"
+          "#{protocol}://#{host}:#{port}"
+        end
+
+        #
+
         private
         def launch_browser(server, opts)
           address = server_address(server, opts)

test/test_commands_serve.rb

@@ -19,6 +19,12 @@ class TestCommandsServe < JekyllUnitTest
           p
         )
       end
+      Jekyll.sites.clear
+      allow(SafeYAML).to receive(:load_file).and_return({})
+      allow(Jekyll::Commands::Build).to receive(:build).and_return("")
+    end
+    teardown do
+      Jekyll.sites.clear
     end
 
     should "label itself" do
@@ -79,16 +85,53 @@ class TestCommandsServe < JekyllUnitTest
         custom_options = {
           "config"  => %w(_config.yml _development.yml),
           "serving" => true,
-          "watch"   => false # for not having guard output when running the tests
+          "watch"   => false, # for not having guard output when running the tests
+          "url"     => "http://localhost:4000"
         }
-        allow(SafeYAML).to receive(:load_file).and_return({})
-        allow(Jekyll::Commands::Build).to receive(:build).and_return("")
 
         expect(Jekyll::Commands::Serve).to receive(:process).with(custom_options)
         @merc.execute(:serve, { "config" => %w(_config.yml _development.yml),
                                 "watch"  => false })
       end
 
+      context "in development environment" do
+        setup do
+          expect(Jekyll).to receive(:env).and_return("development")
+          expect(Jekyll::Commands::Serve).to receive(:start_up_webrick)
+        end
+        should "set the site url by default to `http://localhost:4000`" do
+          @merc.execute(:serve, { "watch" => false, "url" => "https://jekyllrb.com/" })
+
+          assert_equal 1, Jekyll.sites.count
+          assert_equal "http://localhost:4000", Jekyll.sites.first.config["url"]
+        end
+
+        should "take `host`, `port` and `ssl` into consideration if set" do
+          @merc.execute(:serve, {
+            "watch"    => false,
+            "host"     => "example.com",
+            "port"     => "9999",
+            "url"      => "https://jekyllrb.com/",
+            "ssl_cert" => "foo",
+            "ssl_key"  => "bar"
+          })
+
+          assert_equal 1, Jekyll.sites.count
+          assert_equal "https://example.com:9999", Jekyll.sites.first.config["url"]
+        end
+      end
+
+      context "not in development environment" do
+        should "not update the site url" do
+          expect(Jekyll).to receive(:env).and_return("production")
+          expect(Jekyll::Commands::Serve).to receive(:start_up_webrick)
+          @merc.execute(:serve, { "watch" => false, "url" => "https://jekyllrb.com/" })
+
+          assert_equal 1, Jekyll.sites.count
+          assert_equal "https://jekyllrb.com/", Jekyll.sites.first.config["url"]
+        end
+      end
+
       context "verbose" do
         should "debug when verbose" do
           assert_equal custom_opts({ "verbose" => true })[:Logger].level, 5