SingleUser

Single-user mode, often referred to as single-user, is a privileged startup state in Unix-like operating systems in which only the root user has access and only essential system services are started. The primary purpose is maintenance, recovery, and repair tasks when normal multi-user operation is unavailable or undesirable. In this mode, remote logins are typically disabled and networking is halted, reducing the system to a minimal, secure environment.

In traditional SysVinit systems, single-user mode corresponds to runlevel 1 and can be entered with commands like init 1 or by selecting the single-user option at boot. Modern Linux distributions using systemd provide a similar state called rescue mode, reachable via systemctl isolate rescue.target or by booting with the kernel parameter systemd.unit=rescue.target. Emergency mode is even more minimal, offering a root shell without mounting all filesystems or starting basic services.

The environment is limited: only essential services are started, many daemons are not running, and login is typically restricted to the root account. The root filesystem may be mounted read-only initially; maintenance tasks such as filesystem checks, password resets, bootloader repairs, or configuration edits are performed here. Some systems require root authentication to proceed.

Exiting single-user mode returns the system to its normal multi-user boot sequence, allowing normal logins and networking to resume. Single-user mode is not intended for regular use, but serves as a safe, controlled environment for troubleshooting and recovery operations.