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Linux 2.6 - man page for cron (linux section 8)

CRON(8) 										  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

       -L loglevel
	       Sets  the  loglevel for cron. The standard logging level (1) will log the start of
	       all the cron jobs. A higher loglevel (2) will cause cron to log also  the  end  of
	       all  cronjobs,  which  can  be useful to audit the behaviour of tasks run by cron.
	       Logging will be disabled if the loglevel is set to zero (0).

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)).
       Additionally, cron reads the files in /etc/cron.d: it 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. The intended purpose of
       this feature is to allow packages that require finer control of their scheduling than  the
       /etc/cron.{daily,weekly,monthly}  directories  to  add a crontab file to /etc/cron.d. Such
       files should be named after the package that supplies them. Files 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. If the -l option is specified,  then  they
       must  conform to the LSB namespace specification, exactly as in the --lsbsysinit option in
       run-parts.

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

       In contrast to the spool area, files in /etc/cron.d may be symlinks,  provided  that  both
       the symlink and the file it points to are owned by root.

       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 vari-
       able 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 mod-
       time on /etc/crontab) has changed, and if it has, cron will then examine  the  modtime  on
       all  crontabs  and reload those which have changed.  Thus cron need not be restarted when-
       ever 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.
       Conversely, 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 set-
       tings 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.

SEE ALSO
       crontab(1), crontab(5)

AUTHOR
       Paul Vixie <paul@vix.com>

4th Berkeley Distribution		  19 April 2010 				  CRON(8)


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