runlevelkäsitettä

The runlevel concept, often referred to as a "runlevel," is a mode of operation for Unix-like operating systems. Each runlevel defines a specific set of processes and services that are running on the system. By changing the runlevel, an administrator can switch the system between different states, such as shutting down, booting up, a single-user maintenance mode, or a multi-user graphical desktop environment.

Historically, systems had a fixed set of runlevels, typically numbered from 0 to 6. For example, runlevel 0 was often used for halting the system, runlevel 1 for single-user mode, runlevel 3 for multi-user text mode, and runlevel 5 or 6 for graphical multi-user mode. The init process, the first process launched by the kernel during boot, was responsible for transitioning the system to the default runlevel and managing changes between them.

Modern Linux systems often use systemd, which has largely replaced the traditional init system and its runlevel concept. While systemd still has concepts that are analogous to runlevels, such as "targets," the terminology and implementation differ. Targets in systemd are more flexible and can represent a wider range of system states, including but not limited to the traditional runlevel definitions. Despite the shift to systemd, understanding the historical runlevel concept is still valuable for comprehending older systems and the evolution of operating system management.