Runlevel | Article about runlevel by The Free Dictionary
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runlevelAn operating state of a Linux/Unix operating system. Runlevels are defined by the scripts that are executive; however, Linux typically defaults to runlevel 3 (all services without a GUI) or runlevel 5 (all services with GUI). Runlevel 1 is used to prevent other changes to files by users or programs (daemons) while administrative updates are made. It is also similar to the safe booting modes in Windows and Mac (see safe mode).