and 5 identically configured for a console (with all services active); and runlevel 4 adds the X Window System. Gentoo Linux Debian GNU/Linux Unix System V Releases 3 and 4 Solaris Starting from Solaris 10, SMF (Service Management Facility) is used instead of SVR4 run levels. The latter are emulated to preserve compatibility with legacy startup scripts. HP-UX AIX AIX does not follow the System V R4 (SVR4) runlevel specification, with runlevels from 0 to 9 available, as well as from a to c (or h). 0 and 1 are reserved, 2 is the default normal multi-user mode and runlevels from 3 to 9 are free to be defined by the administrator. Runlevels from a to c (or h) allow the execution of processes in that runlevel without killing processes started in another. The S, s, M and m runlevels are not true runlevels, but are used to tell the init command to enter maintenance mode. When the system enters maintenance mode from another runlevel, only the system console is used as the terminal. See also Init Operating system service management systemd Upstart Notes References External links Runlevel Definition - by The Linux Information Project (LINFO) What are run levels? - LinuxQuestions.org FreeBSD system startup chkconfig, a utility for querying and updating runlevel-controlled services Unix Unix process- and task-management-related software