182 lines
4.1 KiB
Groff
182 lines
4.1 KiB
Groff
.\" $NetBSD: cron.8,v 1.5 2011/10/12 22:50:31 wiz Exp $
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.\"
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.\" Id: cron.8,v 1.8 2004/01/23 19:03:32 vixie Exp
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.\"
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.Dd October 12, 2011
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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 daemon to execute scheduled commands (ISC Cron V4.1)
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.Sh SYNOPSIS
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.Nm
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.Op Fl n
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.Op Fl x Ar debugflags
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.Sh DESCRIPTION
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.Nm
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is normally started during system boot by
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.Xr rc.d 8
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framework, if cron is switched on in
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.Xr rc.conf 5 .
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.Pp
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It will return immediately so you don't have to start it with
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.Sq \*[Am] .
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.Pp
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.Nm
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searches
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.Pa /var/cron/tabs
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for crontab files which are named after accounts in
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.Pa /etc/passwd .
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Crontabs found are loaded into memory.
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.Nm
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also searches for
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.Pa /etc/crontab
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which is in a different format (see
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.Xr crontab 5 ) .
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Finally
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.Nm
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looks for crontabs in
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.Pa /etc/cron.d
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if it exists, and executes each file as a crontab.
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.Pp
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When
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.Nm
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looks in a directory for crontabs (either in
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.Pa /var/cron/tabs
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or
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.Pa /etc/cron.d )
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it will not process files that:
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.Bl -dash -compact -offset indent
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.It
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Start with a
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.Sq \&.
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or a
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.Sq # .
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.It
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End with a
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.Sq ~
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or with
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.Dq .rpmsave ,
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.Dq .rpmorig ,
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or
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.Dq .rpmnew .
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.It
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Are of zero length.
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.It
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Their length is greater than
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.Dv MAXNAMLEN .
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.El
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.Pp
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.Nm
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then wakes up every minute, examining all stored crontabs, checking each
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command to see if it should be run in the current minute.
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When executing commands, any output is mailed to the owner of the
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crontab (or to the user named in the
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.Ev MAILTO
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environment variable in the crontab, if such exists).
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.Pp
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Events such as
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.Dv START
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and
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.Dv FINISH
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are recorded in the
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.Pa /var/log/cron
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log file with date and time details.
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This information is useful for a number of reasons, such as
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determining the amount of time required to run a particular job.
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By default, root has an hourly job that rotates these log files
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with compression to preserve disk space.
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.Pp
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Additionally,
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.Nm
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checks each minute to see if its spool directory's modtime (or the modtime
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on
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.Pa /etc/crontab
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or
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.Pa /etc/cron.d )
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has changed, and if it has,
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.Nm
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will then examine the modtime on all crontabs and reload those which have
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changed.
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Thus
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.Nm
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need not be restarted whenever a crontab file is modified.
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Note that the
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.Xr crontab 1
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command updates the modtime of the spool directory whenever it changes a
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crontab.
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.Pp
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The following options are available:
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.Bl -tag -width indent
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.It Fl x
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This flag turns on some debugging flags.
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.Ar debugflags
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is comma-separated list of debugging flags to turn on.
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If a flag is turned on,
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.Nm
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writes some additional debugging information to system log during
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its work.
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Available debugging flags are:
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.Bl -tag -width proc -compact
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.It Ar sch
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scheduling
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.It Ar proc
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process control
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.It Ar pars
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parsing
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.It Ar load
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database loading
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.It Ar misc
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miscellaneous
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.It Ar test
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test mode - do not actually execute any commands
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.It Ar bit
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show how various bits are set (long)
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.It Ar ext
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print extended debugging information
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.El
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.It Fl n
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Stay in the foreground and don't daemonize
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.Nm .
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.El
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.Sh 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 timezone, and the new time is used immediately.
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.Sh SIGNALS
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On receipt of a
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.Dv SIGHUP ,
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the cron daemon will close and reopen its
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log file.
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This is useful in scripts which rotate and age log files.
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Naturally this is not relevant if cron was built to use
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.Xr syslog 3 .
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.Sh FILES
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.Bl -tag -width /var/cron/tabs -compact
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.It Pa /var/cron/tabs
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.Nm
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spool directory
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.It Pa /etc/crontab
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system crontab file
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.It Pa /etc/cron.d/
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system crontab directory
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.It Pa /var/log/cron
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log file for cron events
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.El
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.Sh SEE ALSO
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.Xr crontab 1 ,
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.Xr crontab 5
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.Sh AUTHORS
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.An Paul Vixie
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.Aq vixie@isc.org
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