Add unregistered CommandViewer

src/PIL/ImageShow.py

@@ -258,6 +258,26 @@ except ImportError:
 else:
     register(IPythonViewer)
 
+class CommandViewer(Viewer):
+    """
+    The viewer that use commands.
+    Should be suitable for any OS
+    """
+
+    def get_command(self, file, **options):
+        """
+        Substitute ``{}`` specifiers in ``command``
+        :param options: kwargs that should have 'command' in it
+        """
+
+        if not "command" in options:
+            raise TypeError("CommandViewer missing required keyword-only argument 'command'")
+        command = options["command"]
+        if not isinstance(command, str):
+            raise TypeError(f"'command' must be 'str' not '{type(command)}'")
+        values = {'file': file, **options}
+        return command.format(values)
+
 
 if __name__ == "__main__":