1.\"/* Copyright 1988,1990,1993 by Paul Vixie 2.\" * All rights reserved 3.\" * 4.\" * Distribute freely, except: don't remove my name from the source or 5.\" * documentation (don't take credit for my work), mark your changes (don't 6.\" * get me blamed for your possible bugs), don't alter or remove this 7.\" * notice. May be sold if buildable source is provided to buyer. No 8.\" * warrantee of any kind, express or implied, is included with this 9.\" * software; use at your own risk, responsibility for damages (if any) to 10.\" * anyone resulting from the use of this software rests entirely with the 11.\" * user. 12.\" * 13.\" * Send bug reports, bug fixes, enhancements, requests, flames, etc., and 14.\" * I'll try to keep a version up to date. I can be reached as follows: 15.\" * Paul Vixie <paul@vix.com> uunet!decwrl!vixie!paul 16.\" */ 17.\" 18.\" $FreeBSD$ 19.\" 20.Dd December 20, 1993 21.Dt CRON 8 22.Os 23.Sh NAME 24.Nm cron 25.Nd daemon to execute scheduled commands (Vixie Cron) 26.Sh SYNOPSIS 27.Nm 28.Op Fl j Ar jitter 29.Op Fl J Ar rootjitter 30.Op Fl s 31.Op Fl o 32.Op Fl x Ar debugflag Ns Op , Ns Ar ... 33.Sh DESCRIPTION 34The 35.Nm 36utility should be started from 37.Pa /etc/rc 38or 39.Pa /etc/rc.local . 40It will return immediately, 41so you do not need to start it with '&'. 42.Pp 43The 44.Nm 45utility searches 46.Pa /var/cron/tabs 47for crontab files which are named after accounts in 48.Pa /etc/passwd ; 49crontabs found are loaded into memory. 50The 51.Nm 52utility also searches for 53.Pa /etc/crontab 54which is in a different format (see 55.Xr crontab 5 ) . 56The 57.Nm 58utility 59then wakes up every minute, examining all stored crontabs, checking each 60command to see if it should be run in the current minute. 61When executing 62commands, any output is mailed to the owner of the crontab (or to the user 63named in the 64.Ev MAILTO 65environment variable in the crontab, if such exists). 66.Pp 67Additionally, 68.Nm 69checks each minute to see if its spool directory's modification time (or 70the modification time on 71.Pa /etc/crontab ) 72has changed, and if it has, 73.Nm 74will then examine the modification time on all crontabs and reload those 75which have changed. 76Thus 77.Nm 78need not be restarted whenever a crontab file is modified. 79Note that the 80.Xr crontab 1 81command updates the modification time of the spool directory whenever it 82changes a crontab. 83.Pp 84Available options: 85.Bl -tag -width indent 86.It Fl j Ar jitter 87Enable time jitter. 88Prior to executing commands, 89.Nm 90will sleep a random number of seconds in the range from 0 to 91.Ar jitter . 92This will not affect superuser jobs (see 93.Fl J ) . 94A value for 95.Ar jitter 96must be between 0 and 60 inclusive. 97Default is 0, which effectively disables time jitter. 98.Pp 99This option can help to smooth down system load spikes during 100moments when a lot of jobs are likely to start at once, e.g., 101at the beginning of the first minute of each hour. 102.It Fl J Ar rootjitter 103Enable time jitter for superuser jobs. 104The same as 105.Fl j 106except that it will affect jobs run by the superuser only. 107.It Fl s 108Enable special handling of situations when the GMT offset of the local 109timezone changes, such as the switches between the standard time and 110daylight saving time. 111.Pp 112The jobs run during the GMT offset changes time as 113intuitively expected. 114If a job falls into a time interval that disappears 115(for example, during the switch from 116standard time) to daylight saving time or is 117duplicated (for example, during the reverse switch), then it is handled 118in one of two ways: 119.Pp 120The first case is for the jobs that run every at hour of a time interval 121overlapping with the disappearing or duplicated interval. 122In other words, if the job had run within one hour before the GMT offset change 123(and cron was not restarted nor the 124.Xr crontab 5 125changed after that) 126or would run after the change at the next hour. 127They work as always, skip the skipped time or run in the added 128time as usual. 129.Pp 130The second case is for the jobs that run less frequently. 131They are executed exactly once, they are not skipped nor 132executed twice (unless cron is restarted or the user's 133.Xr crontab 5 134is changed during such a time interval). 135If an interval disappears 136due to the GMT offset change, such jobs are 137executed at the same absolute point of time as they would be in the 138old time zone. 139For example, if exactly one hour disappears, this 140point would be during the next hour at the first minute that is 141specified for them in 142.Xr crontab 5 . 143.It Fl o 144Disable the special handling of situations when the GMT offset of the local 145timezone changes, to be compatible with the old (default) behavior. 146If both options 147.Fl o 148and 149.Fl s 150are specified, the option specified last wins. 151.It Fl x Ar debugflag Ns Op , Ns Ar ... 152Enable writing of debugging information to standard output. 153One or more of the following comma separated 154.Ar debugflag 155identifiers must be specified: 156.Pp 157.Bl -tag -width ".Cm proc" -compact 158.It Cm bit 159currently not used 160.It Cm ext 161make the other debug flags more verbose 162.It Cm load 163be verbose when loading crontab files 164.It Cm misc 165be verbose about miscellaneous one-off events 166.It Cm pars 167be verbose about parsing individual crontab lines 168.It Cm proc 169be verbose about the state of the process, including all of its offspring 170.It Cm sch 171be verbose when iterating through the scheduling algorithms 172.It Cm test 173trace through the execution, but do not perform any actions 174.El 175.El 176.Sh SEE ALSO 177.Xr crontab 1 , 178.Xr crontab 5 179.Sh AUTHORS 180.An Paul Vixie Aq paul@vix.com 181