cron/cron/cron.8

.\"/* Copyright 1988,1990,1993,1996 by Paul Vixie
.\" * All rights reserved
.\" */
.\"
.\"Copyright (c) 1997 by Internet Software Consortium
.\"
.\"Permission to use, copy, modify, and distribute this software for any
.\"purpose with or without fee is hereby granted, provided that the above
.\"copyright notice and this permission notice appear in all copies.
.\"
.\"THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM DISCLAIMS
.\"ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
.\"OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL INTERNET SOFTWARE
.\"CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
.\"DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
.\"PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
.\"ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
.\"SOFTWARE.
.\"
.\" $Id: cron.8,v 1.2 1998/08/14 00:32:36 vixie Exp $
.\"
.Dd February 9, 2022
.Dt CRON 8
.Os
.Sh NAME
.Nm cron
.Nd daemon to execute scheduled commands (Vixie Cron)
.Sh SYNOPSIS
.Nm
.Op Fl j Ar jitter
.Op Fl J Ar rootjitter
.Op Fl m Ar mailto
.Op Fl n
.Op Fl s
.Op Fl o
.Op Fl x Ar debugflag Ns Op , Ns Ar ...
.Sh DESCRIPTION
The
.Nm
utility should be started from
.Pa /etc/rc
or
.Pa /etc/rc.local .
It will return immediately,
so you do not need to start it with '&'.
.Pp
The
.Nm
utility searches
.Pa /var/cron/tabs
for crontab files which are named after accounts in
.Pa /etc/passwd ;
crontabs found are loaded into memory.
The
.Nm
utility also searches for
.Pa /etc/crontab
and files in
.Pa /etc/cron.d
and
.Pa /usr/local/etc/cron.d
which are in a different format (see
.Xr crontab 5 ) .
.Pp
The
.Nm
utility
then wakes up every minute, examining all stored crontabs, checking each
command to see if it should be run in the current minute.
Before running a command from a per-account crontab file,
.Nm
checks the status of the account with
.Xr pam 3
and skips the command if the account is unavailable,
e.g., locked out or expired.
Commands from
.Pa /etc/crontab
bypass this check.
When executing
commands, any output is mailed to the owner of the crontab (or to the user
named in the
.Ev MAILTO
environment variable in the crontab, if such exists).
The from address of this mail may be set with the
.Ev MAILFROM
environment variable.
.Pp
Additionally,
.Nm
checks each minute to see if its spool directory's modification time (or
the modification time on
.Pa /etc/crontab )
has changed, and if it has,
.Nm
will then examine the modification time on all crontabs and reload those
which have changed.
Thus
.Nm
need not be restarted whenever a crontab file is modified.
Note that the
.Xr crontab 1
command updates the modification time of the spool directory whenever it
changes a crontab.
.Pp
Available options:
.Bl -tag -width indent
.It Fl j Ar jitter
Enable time jitter.
Prior to executing commands,
.Nm
will sleep a random number of seconds in the range from 0 to
.Ar jitter .
This will not affect superuser jobs (see
.Fl J ) .
A value for
.Ar jitter
must be between 0 and 60 inclusive.
Default is 0, which effectively disables time jitter.
.Pp
This option can help to smooth down system load spikes during
moments when a lot of jobs are likely to start at once, e.g.,
at the beginning of the first minute of each hour.
.It Fl J Ar rootjitter
Enable time jitter for superuser jobs.
The same as
.Fl j
except that it will affect jobs run by the superuser only.
.It Fl m Ar mailto
Overrides the default recipient for
.Nm
mail.
Each
.Xr crontab 5
without
.Ev MAILTO
explicitly set will send mail to the
.Ar mailto
mailbox.
Sending mail will be disabled by default if
.Ar mailto
set to a null string, usually specified in a shell as
.Li ''
or
.Li \*q\*q .
.It Fl n
Do not daemonize; run in foreground instead.
.It Fl s
Enable special handling of situations when the GMT offset of the local
timezone changes, such as the switches between the standard time and
daylight saving time.
.Pp
The jobs run during the GMT offset changes time as
intuitively expected.
If a job falls into a time interval that disappears
(for example, during the switch from
standard time) to daylight saving time or is
duplicated (for example, during the reverse switch), then it is handled
in one of two ways:
.Pp
The first case is for the jobs that run every at hour of a time interval
overlapping with the disappearing or duplicated interval.
In other words, if the job had run within one hour before the GMT offset change
(and cron was not restarted nor the
.Xr crontab 5
changed after that)
or would run after the change at the next hour.
They work as always, skip the skipped time or run in the added
time as usual.
.Pp
The second case is for the jobs that run less frequently.
They are executed exactly once, they are not skipped nor
executed twice (unless cron is restarted or the user's
.Xr crontab 5
is changed during such a time interval).
If an interval disappears
due to the GMT offset change, such jobs are
executed at the same absolute point of time as they would be in the
old time zone.
For example, if exactly one hour disappears, this
point would be during the next hour at the first minute that is
specified for them in
.Xr crontab 5 .
.It Fl o
Disable the special handling of situations when the GMT offset of the local
timezone changes, to be compatible with the old (default) behavior.
If both options
.Fl o
and
.Fl s
are specified, the option specified last wins.
.It Fl x Ar debugflag Ns Op , Ns Ar ...
Enable writing of debugging information to standard output.
One or more of the following comma separated
.Ar debugflag
identifiers must be specified:
.Pp
.Bl -tag -width ".Cm proc" -compact
.It Cm bit
currently not used
.It Cm ext
make the other debug flags more verbose
.It Cm load
be verbose when loading crontab files
.It Cm misc
be verbose about miscellaneous one-off events
.It Cm pars
be verbose about parsing individual crontab lines
.It Cm proc
be verbose about the state of the process, including all of its offspring
.It Cm sch
be verbose when iterating through the scheduling algorithms
.It Cm test
trace through the execution, but do not perform any actions
.El
.El
.Sh FILES
.Bl -tag -width /usr/local/etc/cron.d -compact
.It Pa /etc/crontab
System crontab file
.It Pa /etc/cron.d
Directory for optional/modularized system crontab files.
.It Pa /etc/pam.d/cron
.Xr pam.conf 5
configuration file for
.Nm
.It Pa /usr/local/etc/cron.d
Directory for third-party package provided crontab files.
.It Pa /var/cron/tabs
Directory for personal crontab files
.El
.Sh SEE ALSO
.Xr crontab 1 ,
.Xr pam 3 ,
.Xr crontab 5 ,
.Xr pam.conf 5 ,
.Xr periodic 8
.Sh AUTHORS
.An Paul Vixie Aq Mt paul@vix.com