runlevel
also: init level, SysV runlevel
A runlevel is a predefined operating state of a Linux system that determines which services and daemons are started at boot or when switched to. Modern systemd systems use targets instead, but runlevels remain an important concept for understanding legacy systems.
Runlevels are numbered 0–6, each representing a different system state. For example, runlevel 3 is typically multi-user mode with networking but no GUI, while runlevel 5 is multi-user with a graphical display manager. Runlevel 0 halts the system and runlevel 6 reboots it.
The init system uses scripts in directories like /etc/rc.d/rc0.d/ through /etc/rc.d/rc6.d/ to start or stop services when entering a runlevel. You can check the current runlevel with runlevel or switch to another using telinit 3 (for runlevel 3).
On modern systemd-based distributions, runlevels are mapped to targets—runlevel 3 corresponds to multi-user.target and runlevel 5 to graphical.target. While the runlevel concept is largely superseded, understanding it helps with legacy systems and troubleshooting boot behavior.