Single user mode

Single user mode is a mode in which a multiuser computer operating system boots into a single superuser. 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 running fsck on a network 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 lost superuser 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-style runlevels, BSD-style boot-loader options, or other boot-time options.

The runlevel is usually changed using the init 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. 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 bootup to start in single user mode. In Solaris the command reboot -- -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 the firmware. In OS X El Capitan and later releases of macOS, the mode can be reversed to single user mode 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. Single User Mode is different from a Safe 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 run fsck or other command line utilities as needed (or installed).

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