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Linux Runlevels in the SysV Init System

Learn how traditional SysV Linux runlevels control services, how /etc/inittab sets the default state, and how to view or change runlevels safely.

A runlevel is a predefined operating state in the traditional System V (SysV) init model. The selected runlevel determines which services and processes should be running. For example, one runlevel may provide a networked text console, while another may start a graphical login.

Runlevels are a feature of SysV-style initialization. They are not a universal behavior implemented identically by every modern Linux system. Many current distributions use systemd, which represents system states with targets instead of traditional runlevels.

How Runlevels Work

During boot, the SysV init process reads its configuration, determines the default runlevel, and starts the services associated with that state. When the runlevel changes, init starts services that belong to the new state and stops services that no longer belong to it.

A service is a background process that provides a system function, such as networking, remote login, a database, or a graphical display manager. Service startup and shutdown actions are traditionally implemented with per-runlevel scripts and configuration. Their exact directories and naming conventions vary by distribution.

Traditional Runlevel Numbers

SysV systems conventionally use numeric runlevels from 0 through 6. Runlevels 0, 1, and 6 are special-purpose states and should not normally be selected as an everyday default. The meanings of runlevels 2, 3, and 4 are especially dependent on distribution and local configuration.

These descriptions are reference conventions, not identical guarantees. Before relying on a runlevel, inspect the local configuration and verify which services are enabled there. A distribution may assign different meanings to runlevels 2 through 5.

Single-User Mode and Maintenance

Single-user mode is a restricted maintenance state generally associated with runlevel 1, or with the aliases s and S. It is useful when repairing filesystems, correcting configuration, changing administrative settings, or diagnosing boot problems without starting the full collection of normal services.

Because this state may stop networking, SSH, and other services, remote administrators can lose their connection immediately after the transition. Use a physical console or an out-of-band management console before entering a maintenance-oriented runlevel. Notify users and consider the effect on production services before making the change.

/etc/inittab and the Default Runlevel

/etc/inittab is the traditional central configuration file for SysV init. It can define the default runlevel and actions to perform when runlevels change. The default runlevel is the state entered during a normal boot.

A generic configuration excerpt might look like this:

id:5:initdefault:

In this example, the initdefault entry requests runlevel 5 as the normal boot state. The exact syntax and supporting configuration are distribution-specific, so this line should be treated as an example rather than a universal modern configuration.

Other entries in /etc/inittab can describe actions associated with runlevel changes, console processes, or system initialization. Service scripts are commonly organized in distribution-specific directories such as runlevel-linked script directories, but the precise layout must be checked on the local system.

Viewing the Current and Previous Runlevel

The traditional runlevel command displays two values:

$ runlevel
N 5

The first character or value is the previous runlevel; the second is the current runlevel. In N 5, N means that no runlevel transition has occurred since boot. In the following example, the system changed from runlevel 3 to runlevel 5:

$ runlevel
3 5

Changing Runlevels with init

On a SysV system, the init command requests a transition to another runlevel. Administrative privileges are required because the change can stop services, disconnect users, and alter system availability.

Entering maintenance mode

1. Notify affected users and stop or protect important workloads.
2. Confirm that physical or out-of-band console access is available.
3. Request single-user mode:

# init 1

Perform the repair after the transition completes. Return to the normal configured operating state only after confirming that required services can be restored.

Shutdown and reboot states

Runlevel 0 requests a shutdown:

# init 0

Runlevel 6 requests a reboot:

# init 6

Both commands interrupt users and services. Use them only deliberately, with appropriate privileges and a maintenance plan.

Common Distribution Variations

Runlevel meanings are conventions. In particular, runlevels 2 through 5 can differ between distributions, releases, and administrator-maintained configurations. Runlevel 3 may be a networked multiuser text state on one system, while runlevel 5 may start a graphical display manager on another. A graphical login will not appear merely because the number 5 was selected if the display service is disabled or assigned elsewhere.

When investigating a system, distinguish the general reference meaning from its actual behavior. Inspect /etc/inittab if the system uses SysV init, review the per-runlevel service configuration, and confirm which processes are active after the transition.

SysV init Compared with Modern Init Systems

SysV init is the traditional System V-style initialization and service-management framework. Upstart uses event-driven job management, while systemd uses units and targets. A systemd target is a group of units representing a desired system state and broadly replaces the runlevel concept.

Some systemd installations provide compatibility mappings for commands such as runlevel and telinit, but the mapping does not make all SysV behavior identical. First identify the active init system, then use its native management tools and documentation.

Troubleshooting Runlevel Problems

The command or configuration file does not behave as expected

The likely cause is that the machine uses systemd or another init system rather than traditional SysV init. Identify the active init system and do not assume that /etc/inittab or classic runlevel commands control the machine.

Networking disappears after a transition

The selected runlevel may be configured without network services, or the distribution may assign that runlevel a different meaning. Use local console access, inspect the service configuration for that state, and return to the intended normal multiuser state.

A graphical login does not start in runlevel 5

The display manager may be disabled, not assigned to runlevel 5, or the distribution may use a different convention. Check the local runlevel service configuration. On systemd systems, investigate graphical.target instead.

The previous runlevel is shown as N

This is normal. It means that no runlevel switch has occurred since boot; it is not an error.

Remote administrators are disconnected

The transition probably stopped networking, SSH, or a related service. Plan such work from a console or out-of-band management channel, and warn users before changing the system state.

Exam-Relevant Summary

• A runlevel is an operating state in the traditional SysV init model.
• Runlevel 0 halts the system, 1 provides single-user maintenance, and 6 reboots.
• Runlevels 2 through 5 are conventions whose exact meanings vary by distribution and local configuration.
• /etc/inittab traditionally defines the default runlevel and other init actions.
• runlevel reports previous and current states; N means no transition has occurred since boot.
• init 1 can enter maintenance mode, but it may stop networking and disconnect remote users.
• Systemd uses targets rather than traditional runlevels, although compatibility mappings may exist.

For related study, see Linux runlevels and SysV initialization.