.TH runlevel 7 2009-07-09 "Upstart"
.\"
.SH NAME
runlevel \- event signalling change of system runlevel
.\"
.SH SYNOPSIS
.B runlevel
.BI RUNLEVEL\fR= RUNLEVEL
.BI PREVLEVEL\fR= PREVLEVEL
.RI [ ENV ]...
.\"
.SH DESCRIPTION
This page describes the
.B runlevel
Upstart event, and the general implementation of runlevels in the Upstart
system.  For the runlevel tool, see
.BR runlevel (8)
.\"
.SS The runlevel event
The
.B runlevel
event signals a change of system runlevel.  The new system runlevel is
given in the
.B RUNLEVEL
argument, and the previous system runlevel in the
.B PREVLEVEL
argument (which may be empty).

Additional environment may follow these depending on the runlevel, and the
tool that emitted the event.  The
.BR shutdown (8)
tool will supply an
.B INIT_HALT
variable set to either
.I HALT
or
.I POWEROFF
when called with
.B -H
or
.B -P
respectively.
.\"
.SS Runlevels
.\"
Runlevels are a concept from UNIX\*R System V used by the
.BR init (8)
daemon or other system initialisation system to define modes of system
operation.

Eight runlevels are permitted, the first seven are numbered
.BR 0 - 6
and the eighth is named
.B S
or
.B 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
.B /etc/init.d/rc
script executed on a change of runlevel
(by jobs run on the
.B runlevel
event in the Upstart system).  This script examines symlinks in the
.I /etc/rc?.d
directories, symlinks beginning
.B K
are services to be stopped and symlinks beginning
.B S
are services to be started.

The authoritative documentation for this process can be found in the
.I System run levels and init.d scripts
section of the
.I Debian Policy Manual.  This may be currently found at
.RB < http://www.debian.org/doc/debian-policy/ch-opersys.html#s-sysvinit >

Runlevels
.BR 0 ,
.BR 1 " and"
.B 6
are reserved.  Runlevel
.B 0
is used to halt the system and
.B 6
to reboot the system.  Runlevel
.B 1
is used to bring the system back down into single-user mode, after which
the runlevel will be
.BR S .
.\"
.SS System V initialisation in Upstart
The compatible implementation of runlevels permits Upstart jobs to be run
on the
.B runlevel
event that perform the same functionality as the original System V
.BR init (8)
daemon.

The
.I /etc/init/rc.conf
job is run on the
.B runlevel
event, thus receiving the
.B RUNLEVEL
and
.B PREVLEVEL
environment variables.  Its sole job is to execute the
.B /etc/init.d/rc
script, passing the new runlevel as an argument.

Initial system startup is provided by the
.I /etc/init/rc-sysinit.conf
job.  This is run on the
.BR startup (7)
event, and is primarily responsible for running the
.B /etc/init.d/rc
script with the special
.B S
argument and calling
.BR telinit (8)
to switch into the default runlevel when done.  This also handles the
.BR -b ,
.BR emergency ,
.B -s
and
.B single
kernel command-line options as well as specifying an alternate runlevel
on the kernel command-line.

Finally the
.I /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
.I rc-sysinit
job to re-enter the default runlevel.
.\"
.SS Implementation of runlevels in Upstart
The Upstart
.BR init (8)
daemon has no native concepts of runlevel, and unlike the System V daemon,
makes no attempt to keep track of the current runlevel.

Instead a compatible implementation is provided by the
.BR runlevel (8),
.BR telinit (8)
and
.BR shutdown (8)
tools supplied with Upstart.

The
.BR telinit (8)
and
.BR shutdown (8)
tools are used by system administrators to change the runlevel, thus they
both generate this
.B runlevel
event obtaining the value for the
.B PREVLEVEL
environment variable from their own environment (the
.B RUNLEVEL
variable) or the
.I /var/run/utmp
file.

Additionally they update the
.I /var/run/utmp
file with the new runlevel, and append a log entry to the
.I /var/log/wtmp
file.

The
.BR runlevel (8)
tool may be used by system administrators to obtain the current runlevel,
this reads the
.B RUNLEVEL
and
.B PREVLEVEL
variables from its own environment or reads the current and previous
runlevel from
.IR /var/run/utmp .

The
.BR who (1)
.B -r
command may also be used to read the current runlevel from
.IR /var/run/utmp .

This provides full compatibility with System V.

During the boot scripts, where the
.I /var/run/utmp
file may not yet be writable, the
.B RUNLEVEL
and
.B PREVLEVEL
environment variables will be available so
.BR telinit (8)
will still provide the correct values.

Once the boot scripts have finished, while the environment variables may no
longer be available, the
.I /var/run/utmp
file will be and the most recent
.BR telinit (8)
invocation should have successfully written to it.
.\"
.SS Boot time records
The
.BR telinit (8)
tool also takes care of writing the boot time record to both
.I /var/run/utmp
and
.IR /var/log/wtmp .

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
.B S
to the default runlevel, at which point the
.I /var/run/utmp
and
.I /var/log/wtmp
files are both writable, and the
.BR telinit (8)
invocation to do the switch has
.BI RUNLEVEL\fR= S
in its environment.
.\"
.SH EXAMPLE
A service running in the typical multi-user runlevels might use:

.RS
.nf
start on runlevel [2345]
stop on runlevel [!2345]
.fi
.RE
.\"
.SH SEE ALSO
.BR runlevel (8)
.BR init (8)
.BR telinit (8)
.BR shutdown (8)
.BR who (1)