1.\" $NetBSD: cron.8,v 1.3 2010/05/08 11:55:01 wiz Exp $ 2.\" 3.\" Id: cron.8,v 1.8 2004/01/23 19:03:32 vixie Exp 4.\" 5.Dd May 5, 2010 6.Dt CRON 8 7.Os 8.Sh NAME 9.Nm cron 10.Nd daemon to execute scheduled commands (ISC Cron V4.1) 11.Sh SYNOPSIS 12.Nm 13.Op Fl n 14.Op Fl x Ar debugflags 15.Sh DESCRIPTION 16.Nm 17is normally started during system boot by 18.Xr rc.d 8 19framework, if cron is switched on in 20.Xr rc.conf 5 . 21.Pp 22It will return immediately so you don't have to start it with 23.Sq \*[Am] . 24.Pp 25.Nm 26searches 27.Pa /var/cron/tabs 28for crontab files which are named after accounts in 29.Pa /etc/passwd . 30Crontabs found are loaded into memory. 31.Nm 32also searches for 33.Pa /etc/crontab 34which is in a different format (see 35.Xr crontab 5 ) . 36.Nm 37then wakes up every minute, examining all stored crontabs, checking each 38command to see if it should be run in the current minute. 39When executing commands, any output is mailed to the owner of the 40crontab (or to the user named in the 41.Ev MAILTO 42environment variable in the crontab, if such exists). 43.Pp 44Events such as 45.Dv START 46and 47.Dv FINISH 48are recorded in the 49.Pa /var/log/cron 50log file with date and time details. 51This information is useful for a number of reasons, such as 52determining the amount of time required to run a particular job. 53By default, root has an hourly job that rotates these log files 54with compression to preserve disk space. 55.Pp 56Additionally, 57.Nm 58checks each minute to see if its spool directory's modtime (or the modtime 59on 60.Pa /etc/crontab ) 61has changed, and if it has, 62.Nm 63will then examine the modtime on all crontabs and reload those which have 64changed. 65Thus 66.Nm 67need not be restarted whenever a crontab file is modified. 68Note that the 69.Xr crontab 1 70command updates the modtime of the spool directory whenever it changes a 71crontab. 72.Pp 73The following options are available: 74.Bl -tag -width indent 75.It Fl x 76This flag turns on some debugging flags. 77.Ar debugflags 78is comma-separated list of debugging flags to turn on. 79If a flag is turned on, 80.Nm 81writes some additional debugging information to system log during 82its work. 83Available debugging flags are: 84.Bl -tag -width proc -compact 85.It Ar sch 86scheduling 87.It Ar proc 88process control 89.It Ar pars 90parsing 91.It Ar load 92database loading 93.It Ar misc 94miscellaneous 95.It Ar test 96test mode - do not actually execute any commands 97.It Ar bit 98show how various bits are set (long) 99.It Ar ext 100print extended debugging information 101.El 102.It Fl n 103Stay in the foreground and don't daemonize 104.Nm . 105.El 106.Sh Daylight Saving Time and other time changes 107Local time changes of less than three hours, such as those caused 108by the start or end of Daylight Saving Time, are handled specially. 109This only applies to jobs that run at a specific time and jobs that 110are run with a granularity greater than one hour. 111Jobs that run more frequently are scheduled normally. 112.Pp 113If time has moved forward, those jobs that would have run in the 114interval that has been skipped will be run immediately. 115Conversely, if time has moved backward, care is taken to avoid running 116jobs twice. 117.Pp 118Time changes of more than 3 hours are considered to be corrections to 119the clock or timezone, and the new time is used immediately. 120.Sh SIGNALS 121On receipt of a 122.Dv SIGHUP , 123the cron daemon will close and reopen its 124log file. 125This is useful in scripts which rotate and age log files. 126Naturally this is not relevant if cron was built to use 127.Xr syslog 3 . 128.Sh FILES 129.Bl -tag -width /var/cron/tabs -compact 130.It Pa /var/cron/tabs 131.Nm 132spool directory 133.It Pa /etc/crontab 134system crontab 135.It Pa /var/log/cron 136log file for cron events 137.El 138.Sh SEE ALSO 139.Xr crontab 1 , 140.Xr crontab 5 141.Sh AUTHORS 142.An Paul Vixie 143.Aq vixie@isc.org 144