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Single-user mode is a mode in which amultiuser computeroperating systemboots into a singlesuperuser. It is mainly used for maintenance of multi-user environments such as network servers. Some tasks may require exclusive access to shared resources, for example runningfsck on anetwork share. This mode can also be used for security purposes – network services are not run, eliminating the possibility of outside interference. On some systems a lostsuperuser password can be changed by switching to single-user mode, but not asking for the password in such circumstances is viewed as a security vulnerability.
Unix-like operating systems provide single-user mode functionality either through the System V-stylerunlevels, BSD-style boot-loader options, or other boot-time options.
The run-level is usually changed using theinit command, runlevel 1 or S will boot into single-user mode.
Boot-loader options can be changed during startup before the execution of the kernel.InFreeBSD andDragonFly BSD it can be changed before rebooting the system with the commandnextboot -o "-s" -k kernel, and its bootloader offers the option on bootup to start in single-user mode. InSolaris the commandreboot -- -s will cause a reboot into single-user mode.
macOS users can accomplish this by holding down⌘ S after powering the system. The user may be required to enter a password set in thefirmware. InOS X El Capitan and later releases of macOS, the mode can be reversed to single-user mode with the commandsudo launchctl reboot userspace -s in Terminal, and the system can be fully rebooted in single-user mode with the commandsudo launchctl reboot system -s. Single-user mode is different from asafe mode boot in that the system goes directly to the console instead of starting up the core elements of macOS (items in/System/Library/, ignoring/Library/,~/Library/, et al.). From there users are encouraged by a prompt to runfsck or othercommand line utilities as needed (or installed).
Microsoft Windows providesRecovery Console, Last Known Good Configuration,Safe Mode and recentlyWindows Recovery Environment as standard recovery means. Also, bootableBartPE-based third-party recovery discs are available.
Recovery Console and recovery discs are different from single-user modes in other operating systems because they are independent of the maintained operating system. This works more likechrooting into other environment with other kernel in Linux.
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