Which Runlevels Start Appropriate Services To Support Networking?

Linux runlevels distinguish betwixt system operating states. Switching runlevel volition modify the services which are running. Yous can customise runlevels and then each level starts with a specific set of services.

Standard Runlevels

The Linux kernel defines seven standard runlevels. Individual operating organisation distributions may customise these runlevels or add additional ones. Users are besides gratuitous to create and alter runlevels.

Runlevel 0 – No activity, equivalent to shutdown. The arrangement is ready for a physical loss of power.
Runlevel i – Running in single-user (rescue) style.
Runlevel 2 – Running in multi-user mode without networking and graphics.
Runlevel 3 – Multi-user mode with the add-on of networking support.
Runlevel four – Left for users/distributions to define.
Runlevel five – Multiple user mode with a graphical user interface.
Runlevel 6 – System reboot mode.

Most Linux systems will boot into Runlevel 5. This is the runlevel y'all're probably most familiar with. The broadest range of services are available, including a brandish server and full networking stack. If you're on a headless server without a display stack, y'all might be in Runlevel 3.

Some single-user systems will kickoff in Runlevel ane.. You'll also encounter this runlevel if you utilise a recovery mode to rescue your system. This lets you obtain beat access without logging in as a regular user.

Runlevels ii, 3 and four can vary beyond distribution. Some distributions will create a clear distinction between each runlevel; in others, all three may have the aforementioned outcome. Generally, y'all can wait any of these three to requite you a single-user text vanquish with networking available.

Your system can only exist in ane runlevel at a fourth dimension. A unmarried runlevel volition be selected after kick. Y'all won't transition been runlevels unless you explicitly initiate a alter. Normally, the but runlevel transitions that occur are from levels ane-5 into level 0 or 6, when your organization shuts down or reboots.

College runlevels betoken increasing levels of available functionality. This convention is followed by all distributions. A low runlevel usually means that only a subset of the system's services are running.

Runlevels and Startup Services

Startup services are coupled to runlevels. Linux traditionally adds startup services to runlevels. When your system enters a runlevel, all the services associated with that level will be started.

The fashion in which services are really handled depends on the service director you're using. Individual distributions transport with dissimilar service managers. The original service manager, init, defines its services inside /etc/init.d. Scripts created here get symlinked into /etc/rc where they're sorted by runlevel:

/etc/rc0.d – Scripts placed in here are executed by runlevel 0…
/etc/rc1.d – Scripts for runlevel i…
/etc/rc2.d – …etc.

To add new startup services, create or edit a script in /etc/init.d:

ms:345:respawn:/usr/bin/service_executable

Next, apply chkconfig to enable the service:

sudo chkconfig service_executable on

The 345:respawn in the script line instructs init to run service_executable when the system enters runlevels 3, 4 or 5.

Many newer distributions accept replaced init with more modern alternatives. These can provide a higher-level abstraction over runlevels and the init organisation.

The majority of Linux distributions, including Debian, Ubuntu, CentOS, Curvation and Red Hat, now apply systemd. This doesn't reference runlevels direct; instead, they're converted to "targets" which are identified by name. While Linux tin only be in one runlevel at a time, systemd supports multiple concurrently active targets. The mapping between runlevels and targets is simply approximate; it's provided for compatibility purposes.

The special default target defines what the system will boot into. The default target is usually linked to the multi-user target – equivalent to a runlevel of two or higher.
Each target's services are stored in a "wants" directory. This will contain .service files. These are besides symbolic links that point back to service definitions in /usr/lib/systemd/system.

ls -l /etc/systemd/arrangement/multi-user.target.wants/*.service

This allows service definitions to be created once and shared with multiple "targets". When Linux boots, systemd will select the target indicated by the runlevel. The services wanted past that target will and so exist loaded upwards. systemd supports service dependencies, so private services tin exist fabricated to wait on other services before they beginning.

Inspecting Your Organisation'southward Runlevel

Y'all can find out your system'southward runlevel using the runlevel control. This volition print two characters to the last. They show your organisation'southward previous and current runlevels.

North 5 is a typical output for a desktop Linux system. The five indicates you're in a multi-user graphical session with networking available. Due north means the previous runlevel couldn't be determined, unremarkably because the organization was shutdown.

On some systems, y'all may be able to get the current and previous runlevels using the $RUNLEVEL and $PREVLEVEL environment variables. If these variables are set, the runlevel command merely emits their values.

You tin can also inspect your current runlevel using the who control. Run who -r to see the runlevel number and the fourth dimension at which the runlevel was entered.

Switching Between Runlevels

Y'all can switch betwixt runlevels using the telinit command. This lets you manually trigger different modes, such as single-user mode or rescue fashion.

Running telinit 0 volition shutdown your car; telinit 6 should initiate a reboot. Using telinit with runlevels 1 – v will activate the respective operating mode. If you lot're currently in text-only way, telinit five will try to enable the display server and enter graphical style.

When using systemd, the systemctl isolate command lets you actuate a different "target". All running services will be stopped; services associated with the new target then go started up.

Hither'southward how to activate the multi-user target:

sudo systemctl isolate multi-user.target

If you want to make a permanent change to the runlevel, employ the systemctl set-default command:

sudo systemctl set-default multi-user.target

Now the multi-user target will be selected whenever your arrangement reboots.

Summary

Linux runlevels describe unlike states within the operating organization. Each runlevel layers additional functionality in the form of extra running services.

Low runlevels are commonly used for recovery modes and text-only operation. A desktop Linux system with a graphical environment will normally exist in runlevel 5. You should check your distribution's documentation for a detailed description of available runlevels, as implementations tin can vary.