runlevel(7)                                                                                                          runlevel(7)



NAME
       runlevel - event signalling change of system runlevel

SYNOPSIS
       runlevel RUNLEVEL=RUNLEVEL PREVLEVEL=PREVLEVEL [ENV]...

DESCRIPTION
       This  page  describes the runlevel Upstart event, and the general implementation of runlevels in the Upstart system.  For
       the runlevel tool, see runlevel(8)

   The runlevel event
       The runlevel event signals a change of system runlevel.  The new system runlevel is given in the RUNLEVEL  argument,  and
       the previous system runlevel in the PREVLEVEL argument (which may be empty).

       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.

   Runlevels
       Runlevels are a concept from UNIX(R) System V used by the init(8) daemon or other system initialisation system to  define
       modes of system operation.

       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  authorative  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.

   System V initialisation in Upstart
       The  compatible  implementation  of  runlevels permits Upstart jobs to be run on the runlevel event that perform the same
       functionality as the original System V init(8) daemon.

       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.

   Implementation of runlevels in Upstart
       The  Upstart  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  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.

   Boot time records
       The telinit(8) tool also takes care of writing the boot time record to both /var/run/utmp and /var/log/wtmp.

       This is written if the previous runlevel in these files does not match the previous runlevel obtained from  its  environ-
       ment.   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  environ-
       ment.

EXAMPLE
       A service running in the typical multi-user runlevels might use:

              start on runlevel [2345]
              stop on runlevel [!2345]

SEE ALSO
       runlevel(8) init(8) telinit(8) shutdown(8) who(1)



Upstart                                                    2009-07-09                                                runlevel(7)