1 .\"/* Copyright 1988,1990,1993 by Paul Vixie
2 .\" * All rights reserved
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
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
18 .\" $FreeBSD: src/usr.sbin/cron/cron/cron.8,v 1.7.2.9 2003/03/11 21:13:48 trhodes Exp $
19 .\" $DragonFly: src/usr.sbin/cron/cron/cron.8,v 1.3 2006/03/18 20:29:50 dillon Exp $
26 .Nd daemon to execute scheduled commands (Vixie Cron)
30 .Op Fl J Ar rootjitter
33 .Op Fl x Ar debugflag Ns Op , Ns Ar ...
37 utility should be started from
41 It will return immediately,
42 so you don't need to start it with '&'.
48 for crontab files which are named after accounts in
50 crontabs found are loaded into memory.
53 utility also searches for
55 which is in a different format (see
60 then wakes up every minute, examining all stored crontabs, checking each
61 command to see if it should be run in the current minute. When executing
62 commands, any output is mailed to the owner of the crontab (or to the user
65 environment variable in the crontab, if such exists).
69 checks each minute to see if its spool directory's modification time (or
70 the modification time on
72 has changed, and if it has,
74 will then examine the modification time on all crontabs and reload those
75 which have changed. Thus
77 need not be restarted whenever a crontab file is modified. Note that the
79 command updates the modification time of the spool directory whenever it
83 .Bl -tag -width indent
86 Prior to executing commands,
88 will sleep a random number of seconds in the range from 0 to
90 This won't affect superuser jobs (see
94 must be between 0 and 60 inclusive.
95 Default is 0, which effectively disables time jitter.
97 This option can help to smooth down system load spikes during
98 moments when a lot of jobs are likely to start at once, e.g.,
99 at the beginning of the first minute of each hour.
100 .It Fl J Ar rootjitter
101 Enable time jitter for superuser jobs.
104 except that it will affect jobs run by the superuser only.
106 Enable special handling of situations when the GMT offset of the local
107 timezone changes, such as the switches between the standard time and
108 daylight saving time.
110 The jobs run during the GMT offset changes time as
111 intuitively expected.
112 If a job falls into a time interval that disappears
113 (for example, during the switch from
114 standard time) to daylight saving time or is
115 duplicated (for example, during the reverse switch), then it's handled
118 The first case is for the jobs that run every at hour of a time interval
119 overlapping with the disappearing or duplicated interval.
120 In other words, if the job had run within one hour before the GMT offset change
121 (and cron was not restarted nor the
124 or would run after the change at the next hour.
125 They work as always, skip the skipped time or run in the added
128 The second case is for the jobs that run less frequently.
129 They are executed exactly once, they are not skipped nor
130 executed twice (unless cron is restarted or the user's
132 is changed during such a time interval).
133 If an interval disappears
134 due to the GMT offset change, such jobs are
135 executed at the same absolute point of time as they would be in the
137 For example, if exactly one hour disappears, this
138 point would be during the next hour at the first minute that is
139 specified for them in
142 Disable the special handling of situations when the GMT offset of the local
143 timezone changes, to be compatible with the old (default) behavior.
148 are specified, the option specified last wins.
149 .It Fl x Ar debugflag Ns Op , Ns Ar ...
150 Enable writing of debugging information to standard output.
151 One or more of the following comma separated
153 identifiers must be specified:
155 .Bl -tag -width ".Cm proc" -compact
159 make the other debug flags more verbose
161 be verbose when loading crontab files
163 be verbose about miscellaneous one-off events
165 be verbose about parsing individual crontab lines
167 be verbose about the state of the process, including all of its offspring
169 be verbose when iterating through the scheduling algorithms
171 trace through the execution, but do not perform any actions
178 .An Paul Vixie Aq paul@vix.com