The Ubuntu runlevels are defined for the Ubuntu system in /etc/inittab.
Each runlevel tells the init command what services to start or stop. Although runlevels might all have custom definitions, Ubuntu has adopted some standards for runlevels:
• Runlevel 0 Known as "halt," this runlevel is used to shut down the system.
• Runlevel 1 This is a special runlevel, defined as "single," which boots Ubuntu to a root access shell prompt where only the root user may log in. It has networking, X, and multi-user access turned off. This is the maintenance or rescue mode. It allows the system administrator to perform work on the system, make backups, or repair configuration or other files.
• Runlevel 2 This runlevel dictates that Ubuntu be booted to a console, or textbased mode, with multi-user access.
• Runlevel 3 This runlevel is identical to runlevel 2, except that it also starts any networking services.
• Runlevel 4 This runlevel is undefined, and it can readily be configured to boot Ubuntu to a custom system state.
• Runlevel 5 This runlevel boots Ubuntu to a networking, multi-user state with an active X session. This is the most common runlevel for home users who want a graphical interface.
• Runlevel 6 This runlevel is used to reboot the system.
Runlevel 1 (also known as single-user mode or maintenance mode) is most commonly used to repair file systems and change the root password on a system when the password has been forgotten. Trespassers with physical access to the machine can also use runlevel 1 to access your system.
Never forget that uncontrolled physical access is virtually a guarantee of access to your data by an intruder.
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