What do runlevels define in Linux?

Runlevels in Linux define the state of the system, particularly in relation to the services that are started or stopped during the boot process and while the system is running. Each runlevel corresponds to a specific mode of operation and determines which processes and services are active. For example, runlevel 0 is typically used for shutting down the system, while runlevel 5 might be used for booting into a graphical interface.

The runlevel configuration allows system administrators to manage the startup and shutdown sequence and to set up different environments or operational modes tailored to specific needs. This flexibility is crucial for maintaining system performance, security, and usability across various scenarios.

Understanding runlevels is essential for administration tasks, including system recovery, troubleshooting, and optimizing system resources based on how the system is utilized at any given time. Each runlevel also plays a vital role in defining what the system should do under various operating conditions.