138 lines
4.6 KiB
Groff
138 lines
4.6 KiB
Groff
.TH CRONTAB 5
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.SH NAME
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crontab \- table of jobs to be performed by cron
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.SH SYNOPSIS
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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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.SH DESCRIPTION
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The
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.BR cron (8)
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daemon runs jobs at regular intervals. These jobs are listed in
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.B crontab
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files. The format of entries in a crontab file are five fields of numbers
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specifying the minute (0\-59), hour (0\-23), day of the month (1\-31), month
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(1\-12), and day of the week (0\-6 with 0 = Sunday) that a task must be
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executed. The task to be executed follows as a shell command.
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.PP
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The time numbers can be given as a comma separated list of simple numbers,
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ranges ("2\-5" is the same as "2,3,4,5"), and repeats ("2:5" means
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"2,7,12,17,22" in the hour field). A repeat is cyclic affair, i.e. 2:5
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and 12:5 are the same thing. A single "*" can be used in a field to
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indicate all valid numbers in that field, so it translates to "always". In
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the minute field you can use "?" for the current minute that the crontab
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file is loaded. It can be used in a repeat, i.e. "?:10" for every 10
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minutes. This keeps machines with identical crontabs from executing tasks
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at exactly the same time, causing a burst of traffic if anything is done
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over a network.
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.PP
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If a given time is valid in all five fields then a command is executed.
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Here are a few examples that illustrate the possibilities:
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.PP
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.if t .RS
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.if t .ft C
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.nf
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# min hour mday mon wday command
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? 3 * * * /usr/etc/daily # Daily system cleanup
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0 * * * * date # Print date on the hour
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30 4 * * 2\-6 /var/etc/backup # After workdays on 4:30
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0 9 25 12 * \-u ast sing # Andy sings on Xmas morning
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0 0 13 * 5 echo Beware! # For the superstitious
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.fi
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.if t .ft P
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.if t .RE
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.PP
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The command may optionally be prefixed by
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.BI \-u " user"
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to specify under which user the command should be run. Commands from
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crontabs in the spool directory are always run under the id of the crontab's
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owner, the
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.B \-u
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flag is ignored.
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.PP
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A command can be placed on the same line as the time fields, or on the next
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line indented by one TAB character. (A TAB, not eight spaces.) More TAB
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indented lines can be added for a multiline command. The tabs are removed
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from the command when passed to the shell. If a command is put on the same
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line as the time fields then percent characters are changed into newlines,
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this is not done for a TAB indented command. The following three entries
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give the same output:
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.PP
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.RS
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.if t .ft C
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.nf
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.ta +8n
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0 12 * * * echo 'Hello'; echo ' World!'
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#1
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0 12 * * * echo 'Hello% World!' #2
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0 12 * * * #3
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cat <<EOF #4
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Hello
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\& World!
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EOF
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.fi
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.if t .ft P
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.RE
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.PP
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Comments start with a "#" character and continue until end of line. They,
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excess whitespace, and empty lines are ignored. Of the comments in the
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example above #1 and #3 are ignored by
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.BR cron ,
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but #2 and #4 are not recognized as comments, but are seen as part of a
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command and are passed to the shell who then happens to ignore them. There
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is no interpretation of command characters other than the percent in a
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oneliner. The time fields must all be on the same line.
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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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.SH "SEE ALSO"
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.BR crontab (1),
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.BR cron (8).
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.SH NOTES
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The "?" in the minute field, the repeat field (e.g. "2:5"), TAB indented
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multiline commands and the
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.B \-u
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option are unique to this cron implementation. This doesn't mean you
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shouldn't use these features, but just that you should be aware of the
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differences with other systems. You are even advised to use these features
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and avoid the percent hack for multiline commands.
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.PP
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Other crons allow one to specify input to a job in some way, something this
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cron can't. Simply use the << shell feature to do that. Other crons often
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choke on empty lines.
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.PP
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It is a common bug to use 0 for Sunday instead of 7. This cron, like most
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other crons out there accepts this without comment.
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.PP
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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 AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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.\"
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.\" $PchId: crontab.5,v 1.3 2000/07/17 18:53:05 philip Exp $
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