Improve the docs about scrapy parse --pipelines (#7876)

docs/topics/commands.rst

@@ -507,7 +507,7 @@ Supported options:
 * ``--cbkwargs``: additional keyword arguments that will be passed to the callback.
   This must be a valid json string. Example: --cbkwargs='{"foo" : "bar"}'
 
-* ``--pipelines``: process items through pipelines
+* ``--pipelines``: :ref:`process items through pipelines <test-item-pipeline>`
 
 * ``--rules`` or ``-r``: use :class:`~scrapy.spiders.CrawlSpider`
   rules to discover the callback (i.e. spider method) to use for parsing the

docs/topics/item-pipeline.rst

@@ -330,6 +330,36 @@ passes through ``PricePipeline`` before it reaches the :ref:`feed exports
 .. _books.toscrape.com: https://books.toscrape.com/
 
 
+.. _test-item-pipeline:
+
+Testing an item pipeline
+========================
+
+To send the items from a single URL through your item pipelines, use the
+:command:`parse` command with the ``--pipelines`` option::
+
+    scrapy parse --pipelines "https://books.toscrape.com/"
+
+To test specific item data instead, add a callback that builds an item out of
+its keyword arguments:
+
+.. skip: next
+.. code-block:: python
+
+    class BooksSpider(scrapy.Spider):
+        # ...
+
+        def parse_item(self, response, **fields):
+            yield BookItem(**fields)
+
+and pass those keyword arguments in the command line::
+
+    scrapy parse --pipelines -c parse_item --cbkwargs '{"title": "Test", "price": 10}' "https://books.toscrape.com/"
+
+Pass any URL that your spider handles; it is downloaded even though the
+callback ignores it.
+
+
 Common pitfalls
 ===============