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<title>Crontab - doing things periodically</title>
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<h2>Crontab - doing things periodically</h2>
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<address><a href="mailto:djk@tobit.co.uk">Dirk Koopman G1TLH</a></address>
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<!-- Created: Sun Dec 13 20:25:14 GMT 1998 -->
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Last modified: Mon Apr 23 01:00:44 BST 2001
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<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') unless connected('gb7tlh')
</pre>
this could have also been written:
<pre>
start_connect('gb7tlh') if !connected('gb7tlh')
</pre>
but the first method is more 'perlish',
<p>Either of these commands 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') unless 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")
# then there is always the highly contentious one like this little jem which
# checks every hour to see if a certain callsign is connected to another cluster
# and silently disconnects him. This is an example only (of course...)
23 * * * * rcmd('rcmd/gb7dxm disc/noinform G9TLH') if present_on('G9TLH', 'GB7DXM')
# some people like to do an hourly announce to say who they are. There is a
# slight complication about this because of the announce duplicate checking
# so you need to make each announce unique. I do this by adding a date and time
# stamp on the end
0 * * * * run_cmd('ann CLUSTER: GB7DJK JO02LQ at ' . cldate . ' ' . ztime)
</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> command, it will impact on the speed
and usability of the cluster as a whole.
<p>If you want to see what commands are being run and/or the syntax errors in the
crontab, then run: <em>set/debug cron</em> on the console and monitor the
debuging output (I use <em>watchdbg</em> in another window).
<P> To set the debugging back to normal do: <em>unset/debug cron</em>.
<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>
<li><b>run_cmd(&lt;cluster command string>)</b> - run any cluster command as
the node callsign. Any output is sent to the 'cron' debug channel
(<em>set/debug cron</em> to see this).
<br><br><li><b>connected(&lt;callsign>)</b> - returns true if the &lt;callsign> is directly connected
to this cluster node.
<br><br><li><b>start_connect(&lt;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.
<br><br><li><b>spawn(&lt;command>)</b> - start a &lt;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.
<br><br><li><b>disconnect(&lt;callsign>)</b> - disconnects a locally connected station from your node.
<br><br><li><b>rcmd(&lt;node-call>, &lt;command>)</b> - send a command to another node in exactly the
same way as, for example, <tt>RCMD/GB7TLH disc GB7DJK</tt> typed on a sysop console.
<br><br><li><b>present(&lt;exact-callsign>)</b> and <b>presentish(&lt;callsign-no-ssid>)</b> - returns
true if the
callsign is connected anywhere on the cluster either with the exact callsign or with the callsign
minus its ssid respectively.
<br><br><li><b>present_on(&lt;exact-callsign>, &lt;node>)</b> and <b>presentish_on(&lt;callsign-no-ssid>, &lt;node>)</b> - returns
true if the
callsign is connected on the node specified either with the exact callsign or with the callsign
minus its ssid respectively.
<br><br><li><b>last_connect(&lt;callsign>)</b> - Returns the last connect time of the callsign or the
current time if it is currently connected locally.
</ul>
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Copyright &copy; 1998 by Dirk Koopman G1TLH. All Rights Reserved<br>
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