Runlevel

The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. This system replaces the traditional /etc/rc and /etc/rc.local scripts used in BSD UNIX. Conventionally, seven runlevels exist, numbered from zero to six, plus an alias runlevel named S and/or s .

=Standard runlevels=

= Almost all systems use runlevel 1 for this purpose. This mode is intended to provide a safe environment to perform system maintenance. Originally this runlevel provided a single terminal (console) interface running a root login operating system shell. The increasing trend towards physical access to the booting process has led to changes in this area.

=Linux=

The GNU/Linux operating system can make use of runlevels through the programs of the sysvinit project. After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.

Linux users may have the ability to ascertain the current runlevel of their operating environment by issuing either of the following commands:

$ runlevel

$ who -r

==Typical Linux runlevels==

Most Linux distributions, in addition to the standard runlevels, define the following additional runlevels:

= The additional behavior of this runlevel varies greatly. All distributions provide at least one virtual terminal. Some distributions start a login shell as the Superuser; some require correctly entering the superuser s password first; others provide a login prompt, allowing any user access.

= In some cases, runlevels 2 and 3 function identically; offerring a Multi-User Mode with Networking.

==Slackware Linux runlevels==

Slackware Linux provides simpler and somewhat different runlevels than those of other Linux distributions:

Additionally, runlevels 2 and 5 both equate to runlevel 3.