166 lines
4.6 KiB
Groff
166 lines
4.6 KiB
Groff
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.TH CRON 8
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.SH NAME
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cron \- clock daemon
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.SH SYNOPSIS
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.B cron
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.RB [ \-d\c
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.RI [ level ]]
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.SH DESCRIPTION
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The
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.B cron
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daemon executes tasks that must be repeated every now and then (cron jobs),
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and tasks that must be run just once (at jobs). It is normally used to run
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daily or weekly system maintenance scripts. What it needs to run and when
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is specified in a number of "cron tables", or crontab files for short.
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These tables are:
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.PP
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.RS
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.nf
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.ft B
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/usr/lib/crontab
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/usr/local/lib/crontab
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/var/lib/crontab
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/var/opt/\fIname\fP/lib/crontab\ \ \fR(Minix-vmd only)\fB
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/usr/spool/crontabs/\fIuser\fP
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.ft R
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.fi
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.RE
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.PP
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These files follow the usual pattern: One for the standard things, one for
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local tasks per organization, one for tasks per system, and one crontab per
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installed package. (Cron reads
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.B /usr/lib/packages
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to find names of installed packages, it doesn't just grab everything in
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.BR /var/opt .)
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The last set of files fall outside the normal pattern, they are per user
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crontabs that one can create with the
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.BR crontab (1)
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command. The file names in
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.B /usr/spool/crontabs/
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are login names of the file owners.
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.PP
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The format of a crontab file is described in
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.BR crontab (5).
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.SS "AT jobs"
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.B Cron
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also takes care of the execution of jobs issued by
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.BR at (1)
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that are found in
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.BR /usr/spool/at/ .
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Cron simply runs the AT job as if there were an "sh at-job" as a cron job at
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the appropriate time under the user-id of the owner of the script. The
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script takes care of the rest. See
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.BR at (1)
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for the details.
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.SS "Job I/O"
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Standard input, output and error are the same as cron's if the job is
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started by the system crontabs or from package crontabs. This means that
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output from system jobs usually ends up on the console and in the log file.
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Output from personal cron jobs or at jobs are mailed to the owner of the
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job. No mail is sent if the job is silent.
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.SH OPTIONS
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.TP 5
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[\fB\-d\fR[\fIlevel\fR]]
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Set the debug level, by default 1. Makes
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.B cron
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print info on what it happens to be doing. Level 1 just tells about sleep
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times and what job is executed, level 2 also shows the internal crontab
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data on a table load. (With time fields translated to match those of
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.BR "struct tm" ,
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see
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.BR ctime (3).)
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.SS SIGNALS
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.B Cron
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takes the following actions when sent one of the following signals:
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.TP 12
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.B SIGHUP
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Reload the crontab tables if they changed since the last time they were
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loaded, and reexamine the AT job spool. Used by
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.BR at (1)
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and
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.BR crontab (1).
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.TP
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.B SIGUSR1
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Increase the debug level by 1.
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.TP
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.B SIGUSR2
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Turn debugging off.
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.SH ENVIRONMENT
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.B Cron
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sets the environment variables
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.BR USER ,
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.BR LOGNAME ,
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.BR HOME ,
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and
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.BR SHELL
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to the user's login name (2x), home directory, and shell if a job is
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executed for a given user. The working directory is set to the user's home
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directory. Everything else is inherited from
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.BR cron ,
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exactly as
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.B cron
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got it when it started. Note that commands are always passed to
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.BR /bin/sh ,
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not to the user's shell.
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.PP
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System cron jobs are in principle executed with
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.BR cron 's
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environment, use
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.B "\-u root"
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or the crontab file
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.B /usr/spool/crontabs/root
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if you want to give root the same treatment as ordinary users.
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.SH FILES
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.TP 25n
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.B /usr/lib/crontab
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Main MINIX 3 crontab file.
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.TP
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.B /usr/local/lib/crontab
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Local jobs for all systems in an organization.
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.TP
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.B /var/lib/crontab
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System specific jobs.
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.TP
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.B /var/opt/\fIname\fP/lib/crontab
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Per package jobs for Minix-vmd.
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.TP
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.B /usr/lib/packages
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List of installed packages.
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.TP
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.B /usr/spool/crontabs/\fIuser\fP
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Per user jobs.
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.TP
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.B /usr/spool/at/*
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Jobs issued by
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.BR at (1).
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.TP
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.B /usr/run/cron.pid
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Process id of cron when cron is running. Used by
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.BR at (1)
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and
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.BR crontab (1)
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to send cron a hangup signal.
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.SH "SEE ALSO"
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.BR at (1),
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.BR crontab (1).
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.SH NOTES
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A job is not reissued until a previous instance of it has exited. The next
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time to execute is computed from the previous time it ran. If job issuing
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lags behind on the system time then the next time to run it is computed from
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the current system time.
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.SH BUGS
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.B Cron
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doesn't like it if the system time is changed. If set forward then cron
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will react when it next wakes up by running all jobs within the skipped time
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once or twice before it catches up. Setting the clock backwards makes cron
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play dead until the system time passes the old time. (Changing the system
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time is bad idea anyway, and not just because of cron.)
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.PP
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Jobs that fall in the missing hour in a change to Daylight Saving Time are
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skipped. Nothing is done in the extra hour on the change out of DST.
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.SH AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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.\"
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.\" $PchId: cron.8,v 1.3 2000/07/17 18:49:18 philip Exp $
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