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crond and atd are two very simple and important
services that everyone should be familiar with. crond does
the job of running commands periodically (daily, weekly), while
atd's main feature is run a command once at some future time.
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These two services are so basic that we are not going to detail
their package contents and invocation.
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The /etc/crontab file dictates a list of periodic jobs to be run
-- like updating the locate and whatis databases,
rotating logs, and possibly performing backup tasks. If there is
anything that needs to be done periodically, you can schedule that
job in this file. /etc/crontab is read by crond on
startup. crond will already be running on all but the most
broken of UNIX systems.
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After modifying /etc/crontab, you should restart
crond with /etc/rc.d/init.d/crond restart.
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/etc/crontab consists of single line definitions for what time of
the day/week/month a particular command should be run.
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Each line has the form:
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<time> <user> <executable>
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<time> is a time pattern that the current time must match for
the command to be executed. <user> tells under what user
the command is to be executed. <executable> is the
command to be run.
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The time pattern gives the minute, hour, month-day,
month, and week-day that the current time is compared. The
comparison is done at the start of every single minute. If
crond gets a match, it will execute the command. A simple
time pattern is as follows.
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50 13 2 9 6 root /usr/bin/play /etc/theetone.wav
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Which will play the given WAV file on Sat Sep 2 13:50:00
every year, while
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50 13 2 * * root /usr/bin/play /etc/theetone.wav
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will play it at 13:50:00 on the 2nd of every month, and
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50 13 * * 6 root /usr/bin/play /etc/theetone.wav
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will do the same on every Saturday. Further,
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50 13,14 * * 5,6,7 root /usr/bin/play /etc/theetone.wav
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will play at 13:50:00 and
at 14:50:00 on both Friday, Saturday and Sunday, while
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*/10 * * * 6 root /usr/bin/play /etc/theetone.wav
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Will play every 10 minutes the whole of Saturday. The
/ is a special notation meaning ``in steps of''.
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In the above examples, the play command is executed as
root.
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The following is an actual /etc/crontab file:
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# Environment variables first
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# Time specs
30 20 * * * root /etc/cron-alarm.sh
35 19 * * * root /etc/cron-alarm.sh
58 18 * * * root /etc/cron-alarm.sh
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
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Note that the # character is used
for comments as usual. crond also allows you to specify
environment variables under which commands are to be run.
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Your time additions should come like mine have, to remind
me of the last three Metro trains of the day.
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The last four entries are vender supplied. The
run-parts command is a simple script to run all
the commands listed under
/etc/cron.hourly, /etc/cron.daily etc. Hence, if you
have a script which needs to be run every day, but not at a specific
time, you needn't edit your crontab file: rather just place the
script with the others in /etc/cron.<interval>.
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My own /etc/cron.daily/ directory contains:
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total 14
drwxr-xr-x 2 root root 1024 Sep 2 13:22 .
drwxr-xr-x 59 root root 6144 Aug 31 13:11 ..
-rwxr-xr-x 1 root root 140 Aug 13 16:16 backup
-rwxr-xr-x 1 root root 51 Jun 16 1999 logrotate
-rwxr-xr-x 1 root root 390 Sep 14 1999 makewhatis.cron
-rwxr-xr-x 1 root root 459 Mar 25 1999 radiusd.cron.daily
-rwxr-xr-x 1 root root 99 Jul 23 23:48 slocate.cron
-rwxr-xr-x 1 root root 103 Sep 25 1999 tetex.cron
-rwxr-xr-x 1 root root 104 Aug 30 1999 tmpwatch
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It is advisable to go through each of these now to see what
your system is doing to itself behind your back.
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at will execute a command at some future time. I
suppose it is essential to know, although I never used it myself until
writing this chapter. at is the front to the atd daemon
which, like crond will almost definitely be running.
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Try our wave file example, remembering to press
Ctrl-D to get the <EOT> (End Of Text):
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[root@cericon /etc]# at 14:19
at> /usr/bin/play /etc/theetone.wav
at> <EOT>
warning: commands will be executed using /bin/sh
job 3 at 2000-09-02 14:19
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You can the type atq to get a list of current
jobs:
a means is the queue name, 3 is the
job number, and 2000-09-02 14:19 is the scheduled time of
execution. While play is executing, atq will give:
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The at and atd man pages contain
additional information.
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