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runlevel(7)                                                        runlevel(7)

       runlevel - event signalling change of system runlevel


       This page describes the runlevel Upstart event, and the general implementation of runlevels in the Upstart sys-
       tem.  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 argu-
       ment, 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

       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  per-

       Services  and other system components are said to exist in one or more runlevels.  When switching from one run-
       level 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  <

       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 environ-
       ment (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  RUN-
       LEVEL  and  PREVLEVEL  variables  from  its  own  environment  or  reads the current and previous runlevel from

       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 envi-
       ronment 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
       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.

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

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

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

Upstart                           2009-07-09                       runlevel(7)