CRON(8) 						      System Manager's Manual							   CRON(8)

NAME

       cron - daemon to execute scheduled commands (Vixie Cron)

SYNOPSIS

       cron [-f] [-l] [-L loglevel]

DESCRIPTION

       cron is started automatically from /etc/init.d on entering multi-user runlevels.

OPTIONS

       -f      Stay in foreground mode, don't daemonize.

       -l      Enable  LSB  compliant  names  for  /etc/cron.d	files.	This  setting,	however,  does	not  affect  the  parsing  of  files under
	       /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly or /etc/cron.monthly.

       -L loglevel
	       Tell cron what to log about jobs (errors are logged regardless of this value) as the sum of the following values:

		   1	  will log the start of all cron jobs

		   2	  will log the end of all cron jobs

		   4	  will log all failed jobs (exit status != 0)

		   8	  will log the process number of all cron jobs

	       The default is to log the start of all jobs (1). Logging will be disabled if levels is set to zero (0). A  value  of  fifteen  (15)
	       will select all options.

NOTES

       cron  searches  its spool area (/var/spool/cron/crontabs) for crontab files (which are named after accounts in /etc/passwd); crontabs found
       are loaded into memory.	Note that crontabs in this directory should not be accessed directly - the  crontab  command  should  be  used	to
       access and update them.

       cron  also  reads /etc/crontab, which is in a slightly different format (see crontab(5)).  In Debian, the content of /etc/crontab is prede-
       fined to run programs under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly. This configuration  is  specific	to
       Debian, see the note under DEBIAN SPECIFIC below.  in order

       Additionally, in Debian, cron reads the files in the /etc/cron.d directory.  cron treats the files in /etc/cron.d as in the same way as the
       /etc/crontab file (they follow the special format of that file, i.e. they include  the  user  field).  However,	they  are  independent	of
       /etc/crontab: they do not, for example, inherit environment variable settings from it. This change is specific to Debian see the note under
       DEBIAN SPECIFIC below.

       Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes. In general, the system administrator  should  not  use
       /etc/cron.d/, but use the standard system crontab /etc/crontab.

       /etc/crontab  and  the files in /etc/cron.d must be owned by root, and must not be group- or other-writable. In contrast to the spool area,
       the files under /etc/cron.d or the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly may also  be  sym-
       links,  provided  that  both  the  symlink and the file it points to are owned by root.	The files under /etc/cron.d do not need to be exe-
       cutable, while the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly do, as they are run  by  run-parts
       (see run-parts(8) for more information).

       cron  then  wakes  up  every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute.
       When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable  in  the
       crontab, if such exists).  The children copies of cron running these processes have their name coerced to uppercase, as will be seen in the
       syslog and ps output.

       Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on the /etc/crontab file) has changed, and if
       it  has,  cron  will  then  examine the modtime on all crontabs files and reload those which have changed.  Thus cron need not be restarted
       whenever a crontab file is modified.  Note that the crontab(1) command updates the modtime of the spool directory  whenever  it	changes  a
       crontab.

       Special considerations exist when the clock is changed by less than 3 hours, for example at the beginning and end of daylight savings time.
       If the time has moved forwards, those jobs which would have run in the time that was skipped will be  run  soon	after  the  change.   Con-
       versely, if the time has moved backwards by less than 3 hours, those jobs that fall into the repeated time will not be re-run.

       Only  jobs  that run at a particular time (not specified as @hourly, nor with '*' in the hour or minute specifier) are affected. Jobs which
       are specified with wildcards are run based on the new time immediately.

       Clock changes of more than 3 hours are considered to be corrections to the clock, and the new time is used immediately.

       cron logs its action to the syslog facility 'cron', and logging may be controlled using the standard syslogd(8) facility.

ENVIRONMENT

       If configured in /etc/default/cron in Debian systems, the cron daemon localisation settings environment can be managed through the  use	of
       /etc/environment  or  through the use of /etc/default/locale with values from the latter overriding values from the former. These files are
       read and they will be used to setup the LANG, LC_ALL, and LC_CTYPE environment variables. These variables are then used to set the  charset
       of mails, which defaults to 'C'.

       This  does  NOT affect the environment of tasks running under cron. For more information on how to modify the environment of tasks, consult
       crontab(5)

       The daemon will use, if present, the definition from /etc/timezone for the timezone.

       The environment can be redefined in user's crontab definitions but cron will only handle tasks in a single timezone.

DEBIAN SPECIFIC

       Debian introduces some changes to cron that were not originally available upstream. The most significant changes introduced are:

       --     Support for /etc/cron.{hourly,daily,weekly,monthly} via /etc/crontab,

       --     Support for /etc/cron.d (drop-in dir for package crontabs),

       --     PAM support,

       --     SELinux support,

       --     auditlog support,

       --     DST and other time-related changes/fixes,

       --     SGID crontab(1) instead of SUID root,

       --     Debian-specific file locations and commands,

       --     Debian-specific configuration (/etc/default/cron),

       --     numerous other smaller features and fixes.

       Support for /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly is provided in Debian through the default setting	of
       the  /etc/crontab  file	(see  the system-wide example in crontab(5)).  The default sytem-wide crontab contains four tasks: run every hour,
       every day, every week and every month. Each of these tasks will execute run-parts providing each one of the  directories  as  an  argument.
       These tasks are disabled if anacron is installed (except for the hourly task) to prevent conflicts between both daemons.

       As  described  above, the files under these directories have to be pass some sanity checks including the following: be executable, be owned
       by root, not be writable by group or other and, if symlinks, point to files owned by root. Additionally, the file names must conform to the
       filename  requirements  of  run-parts:  they must be entirely made up of letters, digits and can only contain the special signs underscores
       ('_') and hyphens ('-'). Any file that does not conform to these requirements will not be executed by run-parts.   For  example,  any  file
       containing dots will be ignored.  This is done to prevent cron from running any of the files that are left by the Debian package management
       system when handling files in /etc/cron.d/ as configuration files (i.e. files ending in .dpkg-dist, .dpkg-orig, and .dpkg-new).

       This feature can be used by system administrators and packages to include tasks that will be run at defined  intervals.	Files  created	by
       packages in these directories should be named after the package that supplies them.

       Support	for  /etc/cron.d is included in the cron daemon itself, which handles this location as the system-wide crontab spool.  This direc-
       tory can contain any file defining tasks following the format used in /etc/crontab, i.e. unlike the user cron spool, these files must  pro-
       vide the username to run the task as in the task definition.

       Files  in  this directory have to be owned by root, do not need to be executable (they are configuration files, just like /etc/crontab) and
       must conform to the same naming convention as used by run-parts(8): they must consist solely of	upper-	and  lower-case  letters,  digits,
       underscores,  and  hyphens.  This means that they cannot contain any dots.  If the -l option is specified to cron (this option can be setup
       through /etc/default/cron, see below), then they must conform to the LSB namespace specification, exactly as in the --lsbsysinit option	in
       run-parts.

       The   intended	purpose   of   this   feature	is   to   allow  packages  that  require  finer  control  of  their  scheduling  than  the
       /etc/cron.{hourly,daily,weekly,monthly} directories to add a crontab file to /etc/cron.d. Such files should be named after the package that
       supplies them.

       Also,  the  default configuration of cron is controlled by /etc/default/cron which is read by the init.d script that launches the cron dae-
       mon. This file determines whether cron will read the system's environment variables and makes it possible to add additional options to  the
       cron program before it is executed, either to configure its logging or to define how it will treat the files under /etc/cron.d.

SEE ALSO

       crontab(1), crontab(5), run-parts(8)

AUTHOR

       Paul  Vixie  <paul@vix.com>  is the author of cron and original creator of this manual page. This page has also been modified for Debian by
       Steve Greenland, Javier Fernandez-Sanguino and Christian Kastner.

4th Berkeley Distribution					   19 April 2010							   CRON(8)