Single-user mode

Single-user mode is a mode in which a multiuser computer operating system boots into a single administrator user, instead of the normal system. It is mainly used for maintenance of multi-user environments, including network servers, where operating on the normal system would be difficult or impossible – for example, some tasks may require exclusive access to resources that are shared, like repairing a broken filesystem used for a network share. This mode can also be used for security purposes, as network services are not run, eliminating the possibility of outside interference. On some systems, a forgotten superuser password can be changed in single-user mode, although not asking for the password in this case is viewed as a security vulnerability.

Unix family

Unix-like operating systems provide single-user mode functionality either through the System V-style runlevels, BSD-style boot-loader options, or other boot-time options.

In systems using System V-style runlevels, the runlevel is usually changed using the init command. Selecting a runlevel of 1, or S, will boot into single-user mode.

Boot-loader options can be changed during startup before the execution of the kernel. In FreeBSD and DragonFly BSD, it can be changed before rebooting the system with the command nextboot -o "-s" -k kernel, and its bootloader offers the option on boot to start in single-user mode. In Solaris, the command reboot -- -s will cause a reboot into single-user mode.

In OS X El Capitan and later releases of macOS, a userspace reboot to single-user mode can be performed with the command sudo launchctl reboot userspace -s in Terminal, and the system can be fully rebooted in single-user mode with the command sudo launchctl reboot system -s. On macOS High Sierra and earlier versions, a user can also enter single user mode by holding down ⌘ S at startup,^[1] which may require entering a password set in the firmware. Single-user mode is different from safe mode 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 run fsck or other command line utilities as needed (or installed).

Microsoft Windows

Microsoft Windows provides Recovery Console, Last Known Good Configuration, Safe Mode and recently the Windows Recovery Environment as standard recovery means. Bootable BartPE-based third-party recovery discs are available.

The Recovery Console and recovery discs are different from single-user modes in other operating systems because they are independent of the main operating system, whereas single-user mode is the same system operating in a different mode.