Using the STATE expression to write flexible plugins with check_multi

For a individual interpretation of data in check_multi the builtin state evaluation is a good means. It internally works with perl eval and therefore is extremely flexible.

Look at the following SNMP example as a simple introduction:

# sensor.cmd
#
# (c) Matthias Flacke
# 30.12.2007
#
# Flexible interpretation of smmp results
command [ sensor ] = /usr/bin/snmpget -v 1 -c $COMMUNITY$ -Oqv $HOSTNAME$ XYZ-MIB::Sensor.0
 
state [ UNKNOWN  ] =  $sensor$  !~ /[4627859]/
state [ OK       ] = ( $sensor$ == 4 || $sensor$ == 6 )
state [ WARNING  ] = ( $sensor$ == 2 || $sensor$ == 7 || $sensor$ == 8 )
state [ CRITICAl ] = ( $sensor$ == 5 || $sensor$ == 9 )

What is happening here?
The first part, the sensor command is just a plain snmpget, as you probably often use in self written plugins.

But instead of a big if-else-clause check_multi uses state expressions to assign SNMP values to the different result values OK, WARNING and CRITICAL.

UNKNOWN has a special role here. It is used, when the SNMP value is not member of a specific group of numbers.

So this example really does not more than standard plugin also can do. But it shows how fast and reliable you can do develop such a SNMP plugin with check_multi. And if you want to change a value afterwards, you does not need to bother a developer. Any administrator can do this as well.

check_multi can do all the standard tasks like a normal plugin:

• gathering data
• validating the retrieved data (→ state [ UNKNOWN ] line)
• evaluating the different results against rules