Single User Mode.

All versions of Unix have the concept of single and multi-user modes. On BSD based systems (SUN 4.x.x, Ultrix, NeXT) these are the only modes. On System V.x based systems there are additional modes (or run-levels ) available to the system administrator.

Single user mode brings the system up in the most minimal way possible. When brought up in single user mode only the root file system is mounted and the system will only allow a single login from the console. No deamon processes are running and no network is present. Only commands in directories that are part of the root file system are present (/etc/, /bin ).

Single user mode is useful when you are diagnosing hardware problems on disk drives, backing up file systems, restore file systems, or installing software (such as the OS).

A common activity in single user mode is to use the command fsck to correct file system problems. The fsck utility must be run on the raw file system and as such the file system must be unmounted.

To leave single user mode and enter multi-user mode root can enter a Control-d command or use the init command.

NOTE: If someone can get access to the system in single-user mode they have compromised the security of your machine! Most modern workstations now have a prom password that does not allow the machine to be booted in single-user mode. Consider using that feature on any machine you do not want users to gain root access. The same thing goes for access to your system console. If you have the console to your machine in an area where someone could access the machine and reboot it in single user mode you have NO security.