Additional environment may follow these depending on the runlevel, and the tool that emitted the event. The shutdown(8) tool will supply an INIT_HALT variable set to either HALT or POWEROFF when called with -H or -P respectively.
Eight runlevels are permitted, the first seven are numbered 0-6 and the eighth is named S or s (both are permitted).
Services and other system components are said to exist in one or more runlevels. When switching from one runlevel to another, the services that should not exist in the new runlevel are stopped and the services that only exist in the new runlevel are started.
This is performed by the /etc/init.d/rc script executed on a change of runlevel (by jobs run on the runlevel event in the Upstart system). This script examines symlinks in the /etc/rc?.d directories, symlinks beginning K are services to be stopped and symlinks beginning S are services to be started.
The authoritative documentation for this process can be found in the System run levels and init.d scripts section of the Debian Policy Manual. This may be currently found at <http://www.debian.org/doc/debian-policy/ch-opersys.html#s-sysvinit>
Runlevels 0, 1 and 6 are reserved. Runlevel 0 is used to halt the system and 6 to reboot the system. Runlevel 1 is used to bring the system back down into single-user mode, after which the runlevel will be S.
The /etc/init/rc.conf job is run on the runlevel event, thus receiving the RUNLEVEL and PREVLEVEL environment variables. Its sole job is to execute the /etc/init.d/rc script, passing the new runlevel as an argument.
Initial system startup is provided by the /etc/init/rc-sysinit.conf job. This is run on the startup(7) event, and is primarily responsible for running the /etc/init.d/rc script with the special S argument and calling telinit(8) to switch into the default runlevel when done. This also handles the -b, emergency, -s and single kernel command-line options as well as specifying an alternate runlevel on the kernel command-line.
Finally the /etc/init/rcS.conf job handles the special case of entering the single-user runlevel and providing a login shell. Once that shell terminates, this restarts the rc-sysinit job to re-enter the default runlevel.
Instead a compatible implementation is provided by the runlevel(8), telinit(8) and shutdown(8) tools supplied with Upstart.
The telinit(8) and shutdown(8) tools are used by system administrators to change the runlevel, thus they both generate this runlevel event obtaining the value for the PREVLEVEL environment variable from their own environment (the RUNLEVEL variable) or the /var/run/utmp file.
Additionally they update the /var/run/utmp file with the new runlevel, and append a log entry to the /var/log/wtmp file.
The runlevel(8) tool may be used by system administrators to obtain the current runlevel, this reads the RUNLEVEL and PREVLEVEL variables from its own environment or reads the current and previous runlevel from /var/run/utmp.
The who(1) -r command may also be used to read the current runlevel from /var/run/utmp.
This provides full compatibility with System V.
During the boot scripts, where the /var/run/utmp file may not yet be writable, the RUNLEVEL and PREVLEVEL environment variables will be available so telinit(8) will still provide the correct values.
Once the boot scripts have finished, while the environment variables may no longer be available, the /var/run/utmp file will be and the most recent telinit(8) invocation should have successfully written to it.
This is written if the previous runlevel in these files does not match the previous runlevel obtained from its environment. In general this occurs when switching from runlevel S to the default runlevel, at which point the /var/run/utmp and /var/log/wtmp files are both writable, and the telinit(8) invocation to do the switch has RUNLEVEL=S in its environment.
start on runlevel  stop on runlevel [!2345]