cron.8

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th CRON VIII 10/25/74
.sh NAME
cron \*- clock daemon
.sh SYNOPSIS
.bd /etc/cron
.sh DESCRIPTION
.it Cron
executes commands at specified dates and times
according to the instructions in the file
/usr/lib/crontab.
Since
.it cron
never exits,
it should only be executed once.
This is best done by running
.it cron
from the initialization
process through the file
/etc/rc;
see
.it init
(VIII).
.s2
Crontab
consists of lines of six fields each.
The fields are separated by spaces or tabs.
The first 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 sixth 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.
.s2
Crontab is examined by
.it cron
every hour.
Thus it could take up to an hour
for entries to become effective.
If it receives
a hangup signal, however, the table is examined immediately;
so `kill \-1 ...' can be used.
.sh FILES
/usr/lib/crontab
.sh "SEE ALSO"
init(VIII), sh(I), kill (I)
.sh DIAGNOSTICS
None \- illegal lines in
crontab are ignored.
.sh BUGS
A more efficient algorithm could be used.
The overhead in running
.it cron
is about one percent of the
machine,
exclusive of any commands executed.