Run Levels
Table of Contents
1. Levels
A runlevel is a preset operating state on a Unix-like operating system.
A system can be booted into (i.e., started up into) any of several runlevels, each of which is represented by a single digit integer. Each runlevel designates a different system configuration and allows access to a different combination of processes (i.e., instances of executing programs).
The are differences in the runlevels according to the operating system. Seven runlevels are supported in the standard Linux kernel (i.e., core of the operating system). They are:
0 - System halt; no activity, the system can be safely powered down. 1 - Single user; rarely used. 2 - Multiple users, no NFS (network filesystem); also used rarely. 3 - Multiple users, command line (i.e., all-text mode) interface; the standard runlevel for most Linux-based server hardware. 4 - User-definable 5 - Multiple users, GUI (graphical user interface); the standard runlevel for most Linux-based desktop systems. 6 - Reboot; used when restarting the system.
By default Linux boots either to runlevel 3 or to runlevel 5. The former permits the system to run all services except for a GUI. The latter allows all services including a GUI.
2. Checking Level
We can check with who:
who -r
Or with systemd:
systemctl get-default
Run level 0 is matched by poweroff.target Run level 1 is matched by rescue.target Run level 3 is emulated by multi-user.target Run level 5 is emulated by graphical.target Run level 6 is emulated by reboot.target
3. Changing levels
We can change levels with the call:
systemctl set-default multi-user.target