[Sussex] ADSL

[Geoffrey Teale]

Angelo,

At the risk of being rude... 

man cron

... or for the emacs literate:

Meta-X woman RETURN cron RETURN

.. either of which will yield:


CRON(8) -- 20 December 1993

NAME
	cron - daemon to execute scheduled commands (Vixie Cron)

SYNOPSIS
	cron [-f]

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

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  extensions to  the  /etc/crontab file  (they follow  the
	special  format of that  file, i.e.  they include  the  user
	field).  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  allow 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.

	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  /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  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.  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  syslog(8)
	facility.

SEE ALSO
	crontab(1), crontab(5)

AUTHOR
		Paul Vixie <paul at vix.com>


-- 
Geoffrey Teale <gteale at cmedltd.com>
Cmed Technology