V8/usr/man/man8/cron.8

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.TH CRON 8 
.SH NAME
cron \- clock daemon
.SH SYNOPSIS
.B /etc/cron
.SH DESCRIPTION
.I Cron
executes commands at specified dates and times
according to the instructions in the file
.IR /etc/crontab .
Since
.I cron
never exits,
it should be executed only once,
by running
.I cron
from the initialization
process through the file
.IR /etc/rc ;
see
.IR init (8).
.PP
.I Crontab
consists of lines of seven fields each.
The fields are separated by spaces or tabs.
The first field specifies the user under
whose
.I uid
and
.I gid
the command should be executed.
The next five are integer patterns to
specify the
minute (0-59),
hour (0-23),
day of the month (1-31),
month of the year (1-12),
and day of the week (1-7 with 1=Monday).
Each of these patterns may
contain a number in the range above;
two numbers separated by
a minus
meaning a range inclusive;
a list of numbers separated by
commas meaning any of the numbers;
or an asterisk meaning all legal values.
The seventh field is a string
that is executed by the shell at the
specified times.
A percent character
in this field is translated to a new-line
character.
Only the first line (up to a % or end of line)
of the command field is executed by the Shell.
The other lines are made available to the
command as standard input.
.PP
.I Crontab
is examined by
.I cron
every minute.
.SH FILES
/etc/crontab