Have you ever told your machine to shut down, and then remembered that you had one last thing you wanted to do—send an email, check the weather forecast, or pay a bill online, for instance? Once the shut down process has started, it seems you’re out of luck—there’s no obvious way to cancel a shut down command.
However, there are two things you can do that will prevent your machine from shutting down. Either one should work, assuming you have enough open applications that the shut down process is taking a bit of time. First, you can try launching a program from your Dock, preferably a larger app that takes a bit of time to open. The launching application will cancel the shut down process.
If that trick doesn’t work for you, it’s also possible to stop the shut down process by having a visible Save dialog box in any application. The quickest way to get one of these onscreen is to press Shift-Command-S, which executes the File: Save As command. After realizing you want to cancel your shut down, use Command-Tab to quickly get to a still-running application that allows Save As, and then press Shift-Command-S. As soon as the shut down process reaches the app with its visible Save As dialog, the process will terminate.
Once the shut down has been cancelled, you can do whatever it is you needed to do, then shut down again when done. And don’t worry, no harm can come to your machine by canceling the shut down process.