def on_startup(self, *, command: Literal['build', 'gh-deploy', 'serve'], dirty: bool) -> None:
        """
        The `startup` event runs once at the very beginning of an `mkdocs` invocation.

        New in MkDocs 1.4.

        The presence of an `on_startup` method (even if empty) migrates the plugin to the new
        system where the plugin object is kept across builds within one `mkdocs serve`.

        Note that for initializing variables, the `__init__` method is still preferred.
        For initializing per-build variables (and whenever in doubt), use the `on_config` event.

        Args:
            command: the command that MkDocs was invoked with, e.g. "serve" for `mkdocs serve`.
            dirty: whether `--dirty` flag was passed.
        """

        The `startup` event runs once at the very beginning of an `mkdocs` invocation.

        New in MkDocs 1.4.

        The presence of an `on_startup` method (even if empty) migrates the plugin to the new
        system where the plugin object is kept across builds within one `mkdocs serve`.

        Note that for initializing variables, the `__init__` method is still preferred.
        For initializing per-build variables (and whenever in doubt), use the `on_config` event.

        Args:
            command: the command that MkDocs was invoked with, e.g. "serve" for `mkdocs serve`.
            dirty: whether `--dirty` flag was passed.
        
mkdocs/plugins.py