runlevels

• In OpenRC and SysVInit, different runlevels represented different states of Linux environment.
• Scripts were configured to run as a runlevel was started or stopped
• Format was /etc/rc[runlevel].d/[SK][Priority]-[Process]
  • E.g. /etc/rc3.d/S20-NetworkManager , a script starting with S in the name, would start NetworkManager when entering runlevel 3. The priority would ensure the file order, so S20-NetworkManager would run ahead of S25-my-process
  • /etc/rc5.d/K10-gnome, a script starting with K in the name, would run a script when exiting runlevel 5

Main runlevels:

• Runlevel 0 = Shut down the system
• Runlevel 1 = Rescue system (single user)
• Runlevel 3 = Multiuser system (standard command line system)
• Runlevel 5 = GUI system (e.g. start X11 and a Desktop Environment)
• Runlevel 6 = Restart the system

Not applicable in systemd systems, where control is replaced with unit files