runlevelid

runlevelid is a concept often encountered in Unix-like operating systems, particularly those based on System V init. It refers to a specific operating mode or state that the system can be in. Each runlevel is assigned a unique numerical identifier, typically from 0 to 6, though some systems may support additional levels. These levels dictate which services and processes are active and available to the user.

Common runlevels include:

0: System halt. The system shuts down completely.

1: Single-user mode. This is a maintenance mode, usually with only the root user logged in and a minimal set of services running. It's useful for system administration tasks.

2: Multi-user mode without networking. Similar to runlevel 3 but without network services.

3: Full multi-user mode. This is the standard operating mode for most server systems, with networking and multiple users allowed.

4: User-defined. This level is often unused or can be configured by the system administrator for specific purposes.

5: Graphical multi-user mode. This is the typical runlevel for desktop environments, enabling the graphical user interface.

6: System reboot. The system shuts down all processes and then restarts.

The system's default runlevel is usually configured in a system file, and users can often switch between runlevels using commands like `init` or `telinit` followed by the desired runlevel number. Understanding runlevels is crucial for system administration, allowing for controlled startup, shutdown, and maintenance of operating system services.