Perrunlevel

Perrunlevel is a term used to describe the per-runlevel configuration policy in Unix-like init systems. It refers to how, for each defined runlevel, the system determines which services should be started, stopped, or kept running. The concept is most closely associated with SysV init, where different runlevels represent distinct system states such as halt, single-user mode, multi-user mode, and graphical mode.

In a SysV-style setup, perrunlevel configuration is implemented through runlevel-specific directories that contain symbolic links to service scripts. Typically, the system uses directories named rc0.d, rc1.d, rc2.d, and so on, up to rc6.d. Inside each directory, links prefixed with S start a service, while those prefixed with K stop a service, when the system enters the corresponding runlevel. The numeric part of the link determines the order in which services are started or stopped. Tools such as update-rc.d or chkconfig are used to create or modify these per-runlevel links.

The perrunlevel mechanism ensures the system reaches an appropriate state for each mode. For example, runlevel 3 is commonly used for multi-user operations without a graphical interface, while runlevel 5 typically enables a graphical environment. Transitioning between runlevels involves enabling or disabling services according to the per-runlevel rules.

With the rise of systemd, runlevels are largely superseded by targets, and per-runlevel management is achieved through target-based dependencies rather than separate rc?.d directories. However, many distributions maintain SysV-compatible scripts and links to preserve behavior in environments that still rely on classic init semantics.

Note: perrunlevel as a term is descriptive rather than a formal standard; it describes how services are associated with specific runlevels in traditional init systems. See also runlevel, SysV init, and systemd targets.