Fix #5970

scrapy/commands/crawl.py

@@ -1,6 +1,10 @@
+import logging
+
 from scrapy.commands import BaseRunSpiderCommand
 from scrapy.exceptions import UsageError
 
+logger = logging.getLogger(__name__)
+
 
 class Command(BaseRunSpiderCommand):
     requires_project = True
@@ -12,6 +16,27 @@ def short_desc(self):
         return "Run a spider"
 
     def run(self, args, opts):
+        if opts.output:
+            extensions_base = self.settings.getdict("EXTENSIONS_BASE")
+            extensions = self.settings.getdict("EXTENSIONS")
+
+            feed_exporter_base = extensions_base.get(
+                "scrapy.extensions.feedexport.FeedExporter"
+            )
+            feed_exporter = extensions.get("scrapy.extensions.feedexport.FeedExporter")
+
+            if feed_exporter_base is None:
+                logger.warning(
+                    "A file output was specified but the FeedExporter extension is not enabled in EXTENSIONS_BASE."
+                )
+            elif (
+                "scrapy.extensions.feedexport.FeedExporter" in extensions.keys()
+                and feed_exporter is None
+            ):
+                logger.warning(
+                    "A file output was specified but the FeedExporter extension is not enabled in EXTENSIONS."
+                )
+
         if len(args) < 1:
             raise UsageError()
         elif len(args) > 1: