[pp:exec] support more replacement fields for '--exec' (#4633)

- {_directory}
- {_filename}
- {_path} (alias for {})

gallery_dl/option.py

@@ -510,6 +510,8 @@ def build_parser():
         dest="postprocessors", metavar="CMD",
         action=AppendCommandAction, const={"name": "exec"},
         help=("Execute CMD for each downloaded file. "
+              "Supported replacement fields are "
+              "{} or {_path}, {_directory}, {_filename}. "
               "Example: --exec \"convert {} {}.png && rm {}\""),
     )
     postprocessor.add_argument(
@@ -518,7 +520,8 @@ def build_parser():
         action=AppendCommandAction, const={
             "name": "exec", "event": "finalize"},
         help=("Execute CMD after all files were downloaded successfully. "
-              "Example: --exec-after \"cd {} && convert * ../doc.pdf\""),
+              "Example: --exec-after \"cd {_directory} "
+              "&& convert * ../doc.pdf\""),
     )
     postprocessor.add_argument(
         "-P", "--postprocessor",