Fix negative temperature value alert in check_pcmeasure2 plugin

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A few months ago I wrote about temperatures way above the mean temperature in Zurich, Switzerland. These temperatures were all measured with temperature sensors and the Nagios plugin check_pcmeasure2.pl by www.messpc.de. 

Now its winter in Switzerland and temperatures dropped significantly:

Temperature Sensor Graph

On November 28th, the temperature dropped below 0° Celcius and the plugin turned CRITICAL, although the thresholds were set to 37 and 40 degrees:

./check_pcmeasure2.pl  -H outsidesensor -p 4000 -S com1.2 -w 37 -c 40 -l temperature -F "%.1fC"
T CRITICAL - -0.1C | temperature=-0.1;37;40

Of course it doesn't make sense that the plugin now is in a CRITICAL status only because the temperature value became negative.

To fix this, I have modified check_pcmeasure2.pl to not pass the current temperature ($value) to the Nagios::Plugin (perl plugin):

# -----------------------------------------------
# finish: return to caller
# -----------------------------------------------
#my $result = $np->check_threshold($value);

# 2013-11-28 Claudio Kuenzler: fix threshold check when negative values occur
my $result = "";
if ( $value > $crit_threshold ) {
  $result = "CRITICAL";
}
elsif ( $value > $warn_threshold ) {
  $result = "WARNING";
}
else {
  $result = "OK";
}

# -- for future use alarm(0);
my $text_output = sprintf "$format_string", $value;
$np->nagios_exit($result, $text_output);

Another possible fix, which I haven't tested though, is to define the thresholds according to the Nagios Plugin Development Guidelines:

# -- thresholds
$np->set_thresholds(
   warning  => ~:$warn_threshold,
   critical => ~:$crit_threshold,
);

Note the "~:" in front of the threshold variables? This declares a negative infinity. But as written above, I haven't tested this fix.

After my fix, the negative values are now handled correctly:

./check_pcmeasure2.pl  -H 212.71.111.200 -p 4000 -S com1.2 -w 37 -c 40 -l temperature -F "%.1fC"
T OK - -0.1C | temperature=-0.1;37;40



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