Most of the time, you operate the system in multiuser mode so that users can log in. But there is a special state called single-user mode, where Unix is running but there is no login prompt. When you're in single-user mode, you're basically super user (root). You may have to enter this mode during installation if something goes wrong. Single-user mode is important for certain routine system administration tasks, like checking corrupted filesystems. (This is not fun; try not to corrupt your filesystem. For instance, always shut down the system through a shutdown command before you turn off the power. This is described in the section "Section 5.5, "Shutting Down the System"" later in this chapter.)
Under single-user mode, the system is nearly useless; very little configuration is done, filesystems are unmounted, and so on. This is necessary for recovering from certain kinds of system problems; see the section "Section 8.6, "What to Do in an Emergency"" in Chapter 8, "Other Administrative Tasks" for details.
Note that Unix is still a multiprocessing system, even in single-user mode. You can run multiple programs at once. Servers can run in the background, so that special functions, such as the network, can operate. But if your system supports more than one terminal, only the console can be used. And the X Window System cannot run.
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