167 lines
4.5 KiB
Groff
167 lines
4.5 KiB
Groff
.\"
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.\" Copyright (c) 2002-2003 Todd C. Miller <millert@openbsd.org>
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.\" Sponsored in part by the Defense Advanced Research Projects
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.\" Agency (DARPA) and Air Force Research Laboratory, Air Force
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.\" Materiel Command, USAF, under agreement number F39502-99-1-0512.
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.\"
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.\" $OpenBSD: cron.8,v 1.36 2019/01/25 00:19:27 millert Exp $
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.\"
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.Dd $Mdocdate: January 25 2019 $
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.Dt CRON 8
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.Os
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.Sh NAME
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.Nm cron
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.Nd clock daemon
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.Sh SYNOPSIS
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.Nm cron
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.Op Fl n
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.Op Fl l Ar load_avg
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.Sh DESCRIPTION
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The
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.Nm
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daemon schedules commands to be run at specified dates and times.
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Commands that are to be run periodically are specified within
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.Xr crontab 5
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files.
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Commands that are only to be run once are scheduled via the
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.Xr at 1
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and
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.Xr batch 1
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commands.
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Normally, the
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.Nm
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daemon is started from the
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.Pa /etc/rc
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command script.
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Because it can execute commands on a user's behalf,
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.Nm
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should be run late in the startup sequence,
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as close to the time when logins are accepted as possible.
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.Pp
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.Nm
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loads
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.Xr crontab 5
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and
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.Xr at 1
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files when it starts up and also when changes are made via the
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.Xr crontab 1
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and
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.Xr at 1
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commands.
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Additionally,
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.Nm
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checks the modification time on the system crontab file
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.Pq Pa /etc/crontab ,
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the crontab spool
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.Pq Pa /var/cron/tabs ,
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and the at spool
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.Pq Pa /var/cron/atjobs
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once a minute.
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If the modification time has changed, the affected files are reloaded.
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.Pp
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Any output produced by a command is sent to the user specified in the
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.Ev MAILTO
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environment variable as set in the
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.Xr crontab 5
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file or, if no
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.Ev MAILTO
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variable is set (or if this is an
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.Xr at 1
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or
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.Xr batch 1
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job), to the job's owner.
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If a command produces no output or if the
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.Ev MAILTO
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environment variable is set to the empty string, no mail will be sent.
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The exception to this is
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.Xr at 1
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or
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.Xr batch 1
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jobs submitted with the
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.Fl m
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flag.
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In this case, mail will be sent even if the job produces no output.
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.Ss Daylight Saving Time and other time changes
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Local time changes of less than three hours, such as those caused
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by the start or end of Daylight Saving Time, are handled specially.
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This only applies to jobs that run at a specific time and jobs that
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are run with a granularity greater than one hour.
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Jobs that run more frequently are scheduled normally.
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.Pp
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If time has moved forward, those jobs that would have run in the
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interval that has been skipped will be run immediately.
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Conversely, if time has moved backward, care is taken to avoid running
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jobs twice.
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.Pp
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Time changes of more than 3 hours are considered to be corrections to
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the clock or time zone, and the new time is used immediately.
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl l Ar load_avg
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If the current load average is greater than
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.Ar load_avg ,
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.Xr batch 1
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jobs will not be run.
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The default value is 1.5.
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To allow
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.Xr batch 1
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jobs to run regardless of the load, a value of 0.0 may be used.
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.It Fl n
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By default,
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.Nm
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will detach from the current tty and become a daemon.
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The
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.Fl n
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option disables this behavior and causes it to run in the foreground.
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.El
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.Sh FILES
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.Bl -tag -width "/var/run/cron.sock" -compact
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.It Pa /etc/crontab
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system crontab file
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.It Pa /var/cron/atjobs
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directory containing
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.Xr at 1
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jobs
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.It Pa /var/cron/log
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cron's log file
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.It Pa /var/cron/tabs
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directory containing individual crontab files
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.It Pa /var/run/cron.sock
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used by
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.Xr crontab 1
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to tell
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.Nm
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to check for crontab changes immediately
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.El
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.Sh SEE ALSO
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.Xr at 1 ,
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.Xr crontab 1 ,
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.Xr syslog 3 ,
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.Xr crontab 5
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.Sh AUTHORS
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.An Paul Vixie Aq Mt vixie@isc.org
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.Sh CAVEATS
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.Xr crontab 5
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files will be ignored if they do not have the proper file mode.
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For user crontab files created by
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.Xr crontab 1 ,
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the mode must be 0600.
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If the system crontab file is used,
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.Pa /etc/crontab
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must not be writable by any user other than root and must not have
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the execute, set-user-ID, set-group-ID or sticky bits set.
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