Add .Site.IsServer

Fixes #4478

docs/content/variables/site.md

@@ -52,6 +52,9 @@ The following is a list of site-level (aka "global") variables. Many of these va
 .Site.IsMultiLingual
 : whether there are more than one language in this site. See [Multilingual](/content-management/multilingual/) for more information.
 
+.Site.IsServer
+: a boolean to indicate if the site is being served with Hugo's built-in server. See [`hugo server`](/commands/hugo_server/) for more information.
+
 .Site.Language.Lang
 : the language code of the current locale (e.g., `en`).
 
@@ -122,4 +125,4 @@ You can use `.Site.Params` in a [partial template](/templates/partials/) to call
 <meta name="description" content="{{if .IsHome}}{{ $.Site.Params.description }}{{else}}{{.Description}}{{end}}" />
 {{< /code >}}
 
-[config]: /getting-started/configuration/
+[config]: /getting-started/configuration/

hugolib/site.go

@@ -467,6 +467,10 @@ func (s *SiteInfo) IsMultiLingual() bool {
 	return len(s.Languages) > 1
 }
 
+func (s *SiteInfo) IsServer() bool {
+	return s.owner.running
+}
+
 func (s *SiteInfo) refLink(ref string, page *Page, relative bool, outputFormat string) (string, error) {
 	var refURL *url.URL
 	var err error