cron(8) [bsd man page]

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

NAME

       cron - clock daemon

SYNOPSIS

       /usr/sbin/cron

DESCRIPTION

       Cron  executes commands at specified dates and times according to the instructions in the files /etc/crontab and /etc/crontab.local.  None,
       either one, or both of these files may be present.  Since cron never exits, it should only be executed once.  This is best done by  running
       cron from the initialization process through the file /etc/rc; see init(8).

       The  crontab files consist of lines of seven fields each.  The fields are separated by spaces or tabs.  The first five are integer patterns
       to specify:

       o minute (0-59)

       o hour (0-23)

       o day of the month (1-31)

       o month of the year (1-12)

       o day of the week (1-7 with 1 = Monday)

       Each of these patterns may contain:

       o a number in the range above

       o two numbers separated by a minus meaning a range inclusive

       o a list of numbers separated by commas meaning any of the numbers

       o an asterisk meaning all legal values

       The sixth field is a user name: the command will be run with that user's uid and permissions.  The seventh field consists of all  the  text
       on  a  line  following  the sixth field, including spaces and tabs; this text is treated as a command which is executed by the Shell at the
       specified times.  A percent character (``%'') in this field is translated to a new-line character.

       Both crontab files are checked by cron every minute, on the minute.

FILES

       /etc/crontab
       /etc/crontab.local

7th Edition							 October 23, 1996							   CRON(8)

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

NAME

       cron - daemon to execute scheduled commands (ISC Cron V4.1)

SYNOPSIS

       cron [-l load_avg] [-n]

DESCRIPTION

       Cron  should  be started from /etc/rc or /etc/rc.local.	It will return immediately, so you don't need to start it with '&'.  The -n option
       changes this default behavior causing it to run in the foreground.  This can be useful when starting it out of init.

       Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory.  Cron
       also  searches  for  /etc/crontab  and the files in the /etc/cron.d directory, which are in a different format (see crontab(5)).  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 execut-
       ing  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).

       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.

   Daylight Saving Time and other time changes
       Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially.  This
       only  applies  to jobs that run at a specific time and jobs that are run with a granularity greater than one hour.  Jobs that run more fre-
       quently are scheduled normally.

       If time has moved forward, those jobs that would have run in the interval that has been skipped will be run  immediately.   Conversely,	if
       time has moved backward, care is taken to avoid running jobs twice.

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

   PAM Access Control
       On  SUSE  LINUX	systems,  crond  now  supports	access	control with PAM - see pam(8).	A PAM configuration file for crond is installed in
       /etc/pam.d/crond .  crond loads the PAM environment from the pam_env module, but these can be overriden by settings in the crontab file.

SIGNALS

       On receipt of a SIGHUP, the cron daemon will close and reopen its log file.  This is useful in scripts which  rotate  and  age  log  files.
       Naturally this is not relevant if cron was built to use syslog(3).

CAVEATS

       In this version of cron, /etc/crontab must not be writable by any user other than root.	No crontab files may be links, or linked to by any
       other file.  No crontab files may be executable, or be writable by any user other than their owner.

SEE ALSO

       crontab(1), crontab(5), pam(8)

AUTHOR

       Paul Vixie <vixie@isc.org>

4th Berkeley Distribution					 10 January 1996"							   CRON(8)