1.\" 2.\" Copyright (c) 2002-2003 Todd C. Miller <millert@openbsd.org> 3.\" 4.\" Permission to use, copy, modify, and distribute this software for any 5.\" purpose with or without fee is hereby granted, provided that the above 6.\" copyright notice and this permission notice appear in all copies. 7.\" 8.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 9.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 10.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 11.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 12.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 13.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 14.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 15.\" 16.\" Sponsored in part by the Defense Advanced Research Projects 17.\" Agency (DARPA) and Air Force Research Laboratory, Air Force 18.\" Materiel Command, USAF, under agreement number F39502-99-1-0512. 19.\" 20.\" $OpenBSD: cron.8,v 1.36 2019/01/25 00:19:27 millert Exp $ 21.\" 22.Dd $Mdocdate: January 25 2019 $ 23.Dt CRON 8 24.Os 25.Sh NAME 26.Nm cron 27.Nd clock daemon 28.Sh SYNOPSIS 29.Nm cron 30.Op Fl n 31.Op Fl l Ar load_avg 32.Sh DESCRIPTION 33The 34.Nm 35daemon schedules commands to be run at specified dates and times. 36Commands that are to be run periodically are specified within 37.Xr crontab 5 38files. 39Commands that are only to be run once are scheduled via the 40.Xr at 1 41and 42.Xr batch 1 43commands. 44Normally, the 45.Nm 46daemon is started from the 47.Pa /etc/rc 48command script. 49Because it can execute commands on a user's behalf, 50.Nm 51should be run late in the startup sequence, 52as close to the time when logins are accepted as possible. 53.Pp 54.Nm 55loads 56.Xr crontab 5 57and 58.Xr at 1 59files when it starts up and also when changes are made via the 60.Xr crontab 1 61and 62.Xr at 1 63commands. 64Additionally, 65.Nm 66checks the modification time on the system crontab file 67.Pq Pa /etc/crontab , 68the crontab spool 69.Pq Pa /var/cron/tabs , 70and the at spool 71.Pq Pa /var/cron/atjobs 72once a minute. 73If the modification time has changed, the affected files are reloaded. 74.Pp 75Any output produced by a command is sent to the user specified in the 76.Ev MAILTO 77environment variable as set in the 78.Xr crontab 5 79file or, if no 80.Ev MAILTO 81variable is set (or if this is an 82.Xr at 1 83or 84.Xr batch 1 85job), to the job's owner. 86If a command produces no output or if the 87.Ev MAILTO 88environment variable is set to the empty string, no mail will be sent. 89The exception to this is 90.Xr at 1 91or 92.Xr batch 1 93jobs submitted with the 94.Fl m 95flag. 96In this case, mail will be sent even if the job produces no output. 97.Ss Daylight Saving Time and other time changes 98Local time changes of less than three hours, such as those caused 99by the start or end of Daylight Saving Time, are handled specially. 100This only applies to jobs that run at a specific time and jobs that 101are run with a granularity greater than one hour. 102Jobs that run more frequently are scheduled normally. 103.Pp 104If time has moved forward, those jobs that would have run in the 105interval that has been skipped will be run immediately. 106Conversely, if time has moved backward, care is taken to avoid running 107jobs twice. 108.Pp 109Time changes of more than 3 hours are considered to be corrections to 110the clock or time zone, and the new time is used immediately. 111.Pp 112The options are as follows: 113.Bl -tag -width Ds 114.It Fl l Ar load_avg 115If the current load average is greater than 116.Ar load_avg , 117.Xr batch 1 118jobs will not be run. 119The default value is 1.5. 120To allow 121.Xr batch 1 122jobs to run regardless of the load, a value of 0.0 may be used. 123.It Fl n 124By default, 125.Nm 126will detach from the current tty and become a daemon. 127The 128.Fl n 129option disables this behavior and causes it to run in the foreground. 130.El 131.Sh FILES 132.Bl -tag -width "/var/run/cron.sock" -compact 133.It Pa /etc/crontab 134system crontab file 135.It Pa /var/cron/atjobs 136directory containing 137.Xr at 1 138jobs 139.It Pa /var/cron/log 140cron's log file 141.It Pa /var/cron/tabs 142directory containing individual crontab files 143.It Pa /var/run/cron.sock 144used by 145.Xr crontab 1 146to tell 147.Nm 148to check for crontab changes immediately 149.El 150.Sh SEE ALSO 151.Xr at 1 , 152.Xr crontab 1 , 153.Xr syslog 3 , 154.Xr crontab 5 155.Sh AUTHORS 156.An Paul Vixie Aq Mt vixie@isc.org 157.Sh CAVEATS 158.Xr crontab 5 159files will be ignored if they do not have the proper file mode. 160For user crontab files created by 161.Xr crontab 1 , 162the mode must be 0600. 163If the system crontab file is used, 164.Pa /etc/crontab 165must not be writable by any user other than root and must not have 166the execute, set-user-ID, set-group-ID or sticky bits set. 167