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+ <title>Crontab - doing things periodically</title>
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+ <meta name="Description" content="Software and systems for realtime digital communications between amateur radio stations for the provision of information on propagation conditions and stations operating">
+ <meta name="Author" content="Dirk Koopman G1TLH">
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+ <hr>
+ <h2>Crontab - doing things periodically</h2>
+ <hr>
+ </font>
+
+
+ <address><a href="mailto:djk@tobit.co.uk">Dirk Koopman G1TLH</a></address>
+ <p>
+ <!-- Created: Sun Dec 13 20:25:14 GMT 1998 -->
+ <!-- hhmts start -->
+Last modified: Mon Dec 28 01:06:43 GMT 1998
+<!-- hhmts end -->
+ <h4>Introduction</h4>
+
+ There are a number of jobs that need to be done periodically. The
+ principle one being starting <a
+ href="connect.html">connections</a> to other clusters if you are
+ not connected. The <tt>crontab</tt> allows you to do this.
+
+ <h4>Location</h4>
+
+ There two locations for the <tt>crontab</tt> files. The first and standard one (which
+ in common with other 'issue' files should not be changed) lives at <b>/spider/cmd/crontab</b>.
+ The sysop changable one lives at <b>/spider/local_cmd/crontab</b>.
+
+ <p>The files will automatically be re-read whenever you change them.
+
+ <h4>The <tt>Crontab</tt> File </h4>
+
+ The <tt>crontab</tt> file defines what is to be done and
+ when. It consists of lines of text created by your favorite editor. Completely blank
+ lines or lines starting with '#' are ignored.
+
+ <p>Each line of a <tt>crontab</tt> file contains six fields
+ each separated with white space. The first five fields are times when the
+ command is to be executed and the last field is the command
+ itself. The time fields consist of:-
+
+ <p><table border=2>
+ <tr><td>field</td><td>Allowed Values</td></tr>
+ <tr><td>minute</td><td>0 - 59</td></tr>
+ <tr><td>hour</td><td>0 - 23</td></tr>
+ <tr><td>day of month</td><td>1 - 31</td></tr>
+ <tr><td>month</td><td>1 - 12</td></tr>
+ <tr><td>day of week</td><td>0 - 6 (0 is Sunday)</td></tr>
+ </table>
+
+ <p>A field may be '*', which means 'any when' for that field.
+
+ <p>Ranges of numbers are allowed. Ranges are two numbers
+ separated with a hyphen. The specified range is inclusive. For
+ example, 8-11 for an <tt>hours</tt> entry specifies execution at hours
+ 8, 9, 10 and 11.
+
+ <p>Lists are allowed. A list is a set of numbers (or ranges)
+ separated by commas. Examples: <tt>1,2,5,9</tt> or <tt>0-3,5,8-12</tt>.
+
+ <p>Commands are actually small snippets of perl. They can be anything legal within
+ perl and the context of the DXSpider <tt>cluster.pl</tt> daemon. The normal use will be connecting
+ to another cluster and a set of routines are specially provided in the context
+ of the <tt>DXCron</tt> package to make this easy. For example
+ <pre>
+ start_connect('gb7tlh') if !connected('gb7tlh')
+ </pre>
+ will attempt to start a <a href="connect.html">connection</a> process to GB7TLH if it isn't
+ already locally connected.
+
+ <p>There is absolutely no reason why you could not do something more complicated using information
+ contained inside the DXSpider daemon, but this will obviously require a more complex line of code.
+ You can also write your own functions, include them within the DXSpider system and call them from
+ the <tt>crontab</tt>
+
+ <p>A full <tt>crontab</tt> file could like like:-
+ <pre>
+ #
+ # This is a sample crontab file
+ #
+ #
+ # check every 10 minutes to see if gb7tlh is connected and if not
+ # start a connect job going
+
+ 0,10,20,30,40,50 * * * * start_connect('gb7tlh') if !connected('gb7tlh')
+
+ # at 03:15 on Sundays start a job called 'analyse.pl' which does something
+ # or other. This starts a new process and runs to completion, be careful
+ # what you do with stdin and stdout as they are the same as those of
+ # cluster.pl
+
+ 15 3 * * 0 spawn('/spider/local/analyse.pl')
+
+ # this is a pointless command which will echo the string shown
+ # on the same terminal as the cluster.pl program after substituting
+ # the values for mycall and version
+
+ 15,30 * * * spawn("echo $main::mycall is a DXSpider Version $main::version DX Cluster system")
+ </pre>
+
+ It is important remember that these <tt>crontab</tt> routines execute in line with the main
+ cluster code, so if you create a long, slow <tt>crontab</tt> commands, it will impact on the speed
+ and usability of the cluster as a whole.
+
+ <h4>Standard Routines</h4>
+
+ As mentioned earlier, there are a small number of routines that are declared in <tt>DXCron</tt>
+ context. They are there basically to make the starting of connections and external programs easy.
+ They are:-
+
+ <ul>
+ <p><li><b>connected(<callsign>)</b> - returns true if the <callsign> is directly connected
+ to this cluster node.
+ <p><li><b>start_connect(<script-name>)</b> - starts a <a href="connect.html">connection</a>
+ script just as if you had typed in <tt>connect script-name</tt> on the sysop console client.
+ <p><li><b>spawn(<command>)</b> - start a <command> as a new process. This is used to do
+ various batch jobs that you may wish to happen at certain times of the day or week that operate
+ on your machine but don't require access to the real-time internals of the cluster daemon. You can
+ execute just about any command you like, but <em>be warned</em> <b>stdin</b> and <b>stdout</b> are
+ still connected to the same terminal (if any) as the cluster daemon. Any unix command and arguments
+ can used, see <tt>exec</tt> in the <a href="http://www.perl.com">perl</a> documentation.
+ </ul>
+
+ <h4>Caveats</h4>
+
+ There seems to be an intermittent problem when running
+ (especially?) with the debugger on. Essentially you will
+ experience random crashes with nonsensical error messages. I
+ believe that this is caused by stack tracing trying to work inside
+ <tt>forked</tt> processes.
+
+<!-- Standard Footer!! -->
+ <p> </p>
+ <p>
+ <FONT COLOR="#606060"><hr></font>
+ <font color="#FF0000" size=-2>
+ Copyright © 1998 by Dirk Koopman G1TLH. All Rights Reserved<br>
+ </font>
+ <font color="#000000" size=-2>$Id$</font>
+ </body>
+</html>
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