Add target config setting and front matter variables so one could create multiple outputs from a single site codebase with various target site included files

lib/jekyll/collection.rb

@@ -185,7 +185,8 @@ module Jekyll
     #
     # Returns true if the 'write' metadata is true, false otherwise.
     def write?
-      !!metadata.fetch("output", false)
+      !!metadata.fetch("output", false) &&
+        (site.target == metadata.fetch("target", site.target))
     end
 
     # The URL template to render collection's documents at.

lib/jekyll/configuration.rb

@@ -30,6 +30,7 @@ module Jekyll
       "limit_posts"         => 0,
       "future"              => false,
       "unpublished"         => false,
+      "target"              => "default",
 
       # Plugins
       "whitelist"           => [],

lib/jekyll/document.rb

@@ -348,17 +348,20 @@ module Jekyll
     end
 
     # Determine whether this document should be written.
-    # Based on the Collection to which it belongs.
+    # Based on the Collection to which it belongs
+    # and site and document target.
     #
     # True if the document has a collection and if that collection's #write?
-    # method returns true, and if the site's Publisher will publish the document.
+    # method returns true, and if the site's Publisher will publish the document,
+    # and if the document's target matches the site target or is undefined.
     # False otherwise.
     #
     # rubocop:disable Naming/MemoizedInstanceVariableName
     def write?
       return @write_p if defined?(@write_p)
 
-      @write_p = collection&.write? && site.publisher.publish?(self)
+      @write_p = collection&.write? && site.publisher.publish?(self) &&
+        (site.target == data.fetch("target", site.target))
     end
     # rubocop:enable Naming/MemoizedInstanceVariableName
 

lib/jekyll/drops/document_drop.rb

@@ -15,7 +15,7 @@ module Jekyll
       private delegate_method_as :data, :fallback_data
 
       delegate_methods :id, :output, :content, :to_s, :relative_path, :url, :date
-      data_delegators "title", "categories", "tags"
+      data_delegators "title", "categories", "tags", :target
 
       def collection
         @obj.collection.label

lib/jekyll/page.rb

@@ -180,7 +180,7 @@ module Jekyll
     end
 
     def write?
-      true
+      site.config["target"] == data.fetch("target", site.config["target"])
     end
 
     def excerpt_separator

lib/jekyll/site.rb

@@ -6,7 +6,7 @@ module Jekyll
                   :file_read_opts, :future, :gems, :generators, :highlighter,
                   :include, :inclusions, :keep_files, :layouts, :limit_posts,
                   :lsi, :pages, :permalink_style, :plugin_manager, :plugins,
-                  :reader, :safe, :show_drafts, :static_files, :theme, :time,
+                  :reader, :safe, :target, :show_drafts, :static_files, :theme, :time,
                   :unpublished
 
     attr_reader :cache_dir, :config, :dest, :filter_cache, :includes_load_paths,
@@ -47,7 +47,7 @@ module Jekyll
     def config=(config)
       @config = config.clone
 
-      %w(safe lsi highlighter baseurl exclude include future unpublished
+      %w(safe lsi highlighter baseurl exclude include future unpublished target
          show_drafts limit_posts keep_files).each do |opt|
         send(:"#{opt}=", config[opt])
       end
@@ -360,7 +360,7 @@ module Jekyll
     end
 
     def each_site_file
-      pages.each { |page| yield page }
+      pages.each { |page| yield (page) if page.write? }
       static_files.each { |file| yield(file) if file.write? }
       collections.each_value { |coll| coll.docs.each { |doc| yield(doc) if doc.write? } }
     end