Mac OS X Panther in a Nutshell, 2nd Edition

Single-User Mode

As with other Unix systems, Mac OS X has an unadvertised feature known as single-user mode, which lets you boot the system under the most minimal terms. As the name implies, it allows only one user access—that user is whatever human seated at the keyboard directly plugged into the machine. No daemons run, the network interfaces lie dormant, and not even the root filesystem is mounted.

You will seldom, if ever, use your Mac in single-user mode. Some low-level diagnostic activities might require it; this is where you can manually and safely run /sbin/fsck -y to check and repair filesystem errors on a nonjournaled filesystem, for example, or force one on a journaled filesystem with the -f flag.^[16]

Because the one user who logs in during single-user mode happens to be root, you can also change all the system’s user passwords, including that of the root account itself. Thus, you can consider single-user mode an emergency back door into the system, should your passwords become lost, and you don’t have a system CD available.

Booting into Single-User Mode

You can activate single-user mode only during system startup. To enter single-user mode, hold down -S during startup, while the Apple logo is displayed. After a moment, instead of the blue Mac OS X startup screen, everything goes black, and startup console messages start spilling down the screen in plain white text, finally ...