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<TITLE>The Answer Guy 33:
Conditional Execution Based on Host Availability
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Conditional Execution Based on Host Availability
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<H4>"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<H4 align="center">By James T. Dennis,
<a href="mailto:answerguy@ssc.com">answerguy@ssc.com</a>
<BR>Starshine Technical Services, <A HREF="http://www.starshine.org/">http://www.starshine.org/</A>
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<H3><img src="../../gx/dennis/qbub.gif" alt="(?)"width="50" height="28"
align="left" border="0">Conditional Execution Based on Host Availability</H3>
<p><strong>From Vladimir Kukuruzovic on
the Linux Users Support Team mailing list
on 20 Sep 1998 </strong></p>
<!-- begin body -->
<p><strong>
Hi, regarding to your answer guy message
</strong></p>
<font color="navy"><em>
<p><strong>
Conditional Execution Based on Host Availability
</strong></p>
<p><strong>
From the L.U.S.T Mailing List on 07 Aug 1998
</strong></p>
<pre><strong>#!/path/to/perl
$ping = Ping -c 1 10.10.10.10;
exec ("program") if $ping =~ /100\% packet loss/;
</strong></pre>
<p><strong>
What's wrong with a simple:
</strong></p>
<p><strong><code>
ping -c 1 $target && $do_something $target || $complain
</code></strong></p>
<p><strong>
... where you fill $do_something and $complain with commands that you
actually want to run on success or failure of the 'ping'.
</strong></p>
<p><strong>
That's what shell "conditional execution operators" (<tt>&&</tt>
and <tt>||</tt>) are for after all.
</strong></p>
</em></font>
<p><strong>
your program does not work well with current release of net-tools and ipv6
support.
<br>you should rewrite it this way:
</strong></p>
<p><strong><code>
ping -c 1 -q $target 2> /dev/null | fgrep "1 packets received" \
<br> /dev/null && $do_something $target || $complain
</strong></p>
<blockquote><img src="../../gx/dennis/bbub.gif" height="28" width="50"
alt="(!)" border="0"
>This doesn't look right to me. My example simply
sends a ping packet and tests the return value.
It's possible that this host might not be reachable
by <EM>some</EM> ping's (ICMP's) --- that there might be
some lossage. However, I was just giving the simple
case of a "well-connected" system on the local LAN.
</blockquote>
<blockquote>
I should <EM>not</EM> have to use '<tt>grep</tt>' and parse the output
from the ping command. It should return an error level
that reflects the results.
</blockquote>
<blockquote>
If it doesn't do that in some new release --- I'll
hack it back in myself. (Ideally it might offer an
option to specify a threshold lossage percentage
--- on which it returns an error. But adding a
command line option to '<tt>ping</tt>' for this might be
"gilding the lily" --- and adding anything to it
(since it is, by nature, an SUID program) is a
unpleasant prospect.
</blockquote>
<p><strong><img src="../../gx/dennis/qbub.gif" height="28" width="50"
alt="(?)" border="0"
>kind regards,
<br>Vladimir
</strong></p>
<p><strong>
p.s. the original program would say that everything is ok when
$target is in DNS, but is not reachable (no route to host)
</strong></p>
<blockquote>
By that I presume you're referring to the fragment
of perl code. Mine did not seem to do this (since
I tested it with several degenerate cases).
</blockquote>
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<H5 align="center"><a href="http://www.linuxgazette.com/ssc.copying.html"
>Copyright ©</a> 1998, James T. Dennis <BR>
Published in <I>Linux Gazette</I> Issue 33 October 1998</H5>
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