As you learned earlier, you must log in to be able to use a Mac OS X?powered Mac. To log in, you simply select or enter the name of the user account, enter the password, and press Return. You then move to the desktop for that user account.
Under older versions of the Mac OS, when you were done working with the computer and wanted to "turn it off," you shut it down using the Shut Down command. Under Mac OS X, you can still do this, but most of the time you will log out instead. When you log out, all the processes currently running are stopped and the user account is "closed." To log out of the current user account, select the Apple menu, and then select Log Out (or press Shift--Q). In the resulting confirmation dialog box, press Return or click Log Out (if you don't do this within 2 minutes, the system logs you out automatically). The processes that are currently running are stopped (you are first prompted to save any open documents that have unsaved changes), and you return to the Login dialog box.
If you have enabled fast user switching, you can also select the Login Window command on the User Switching menu to protect access to your account while keeping it logged in.
Logging out and then logging back in is a lot faster than stopping and starting your machine. In fact, there aren't many reasons to shut down your machine, and if you use it to serve Web pages, you shouldn't shut it down. To secure your Mac when you aren't using it, just log out.
Although Mac OS X is much more stable than previous versions, there might be occasions when you need to restart your Mac to correct some problem that is occurring. Also, occasionally, you will need to restart your Mac after making system changes or installing new software.
Restarting Mac OS X is similar to restarting previous versions of the OS, except that after the restart, you return to the Login dialog box rather than the desktop (unless you have set the Automatic Login mode to be active, in which case you do return to the desktop). You can restart Mac OS X in a couple of ways:
Click Restart in the Login window.
Select the Apple menu and select Restart.
Some Mac-compatible keyboards (not produced by Apple) and some older Apple keyboards have a Power key, which you can use to start the machine when it is off or to bring up a dialog box that enables you to shut down, restart, or put your Mac to sleep. Newer Apple keyboards don't have this key, and some Macs running Mac OS X don't support it even if the keyboard has the Power key. If your keyboard has the Power key, press it and see whether anything happens. If the dialog box appears, you can use its controls to shut down, restart, or put your Mac to sleep. If nothing happens, you know that you never need to press the Power key again.
Because today's machines use very little energy, there isn't really much reason to shut them off. Most of the time, when you are done working with your computer, you should simply log out. This stops all the processes that are running and puts your Mac in a safe condition. Logging in is much faster because you don't have to wait for your computer to start up.
Still, if you are leaving your Mac for a long time, you might want to shut it down. You can do so in the following ways:
Select the Apple menu and then select Shut Down.
Press Shift--Q to log out, confirm the logout, and then click Shut Down in the Login window.
To turn on most modern Macs, you need to press the Power key located on the CPU.
If you use an Apple Studio or Cinema Display, you can also turn on the Mac by pressing the Power button located on the display. If you press the Power button on the display while your Mac is on, it goes to sleep. If you press and hold down the Power button for a few seconds, your Mac is powered off without going through the shutdown process (use this only when the machine is totally locked up).
Also, with certain Apple displays, you will see the Options tab on the Displays pane of the System Preferences utility. You can use this tab to configure what happens when you press the Power button on these displays. Your options are to have it control the display's power only or to control the computer's power too. This tab isn't available for newer Apple displays.
You can perform a hard shutdown by pressing the Power key and holding it down until the Mac turns off. You won't have a chance to save any open files so you should use this method only when all other options have failed.