SysV Runlevels
SysV Runlevels are a system initialization and service management concept from the System V Unix init system, used to define different operating states of a Unix-like system. Each runlevel (numbered 0-6) corresponds to a specific set of services and processes that are started or stopped, allowing the system to boot into modes like single-user maintenance, multi-user with networking, or shutdown. This mechanism controls which daemons and system services run at various stages, providing a structured way to manage system states.
Developers should learn SysV Runlevels when working with legacy Unix systems, older Linux distributions (e.g., RHEL 6 or earlier), or embedded systems that use traditional init systems, as it's essential for system administration tasks like troubleshooting, service management, and understanding boot processes. It's particularly useful for configuring startup scripts, managing daemons, and performing system maintenance in environments where modern init systems like systemd are not available, though its relevance has declined with the widespread adoption of systemd.