6.4 System V Runlevels

Now that you’ve learned about systemd and how it works, let’s shift gears and look at some aspects of the traditional System V init. At any given time on a Linux system, a certain base set of processes (such as crond and udevd) is running. In System V init, this state of the machine is called its runlevel, which is denoted by a number from 0 through 6. A system spends most of its time in a single runlevel, but when you shut down the machine, init switches to a different runlevel in order to terminate the system services in an orderly fashion and tell the kernel to stop.

You can check your system’s runlevel with the who -r command like this:

$ who -r
run-level 5  2019-01-27 16:43

p. 157

This output tells us that the current runlevel is 5, as well as the date and time that the runlevel was established.

Runlevels serve various purposes, but the most common one is to distinguish between system startup, shutdown, single-user mode, and console mode states. For example, most systems traditionally used runlevels 2 through 4 for the text console; a runlevel of 5 means that the system starts a GUI login.

But runlevels are becoming a thing of the past. Even though systemd supports them, it considers runlevels obsolete as end states for the system, preferring target units instead. To systemd, runlevels exist primarily to start services that support only the System V init scripts.