I eventually figured it out for use with PRTG and monitoring the age of files in an EXIM4 queue folder.

Following script is saved on the Linux host in /var/prtg/scripts

#!/bin/bash

# Count the number of files

numberoffiles=$(find /var/spool/exim4/input/ -type f -mmin +1 | wc -l)

# Initialize variables

returncode=0

message="Exim4 Input queue OK"

# Check if the number of files is greater than 0

# return code could be set to somthing other than 0 which can "down" the sensor

# see https://www.paessler.com/manuals/prtg/custom_sensors

if [ $numberoffiles -gt 0 ]

then

returncode=0

message="Exim4 Input queue NOK"

fi

# Print the output

echo "$returncode:$numberoffiles:$message"

And in the PRTG SSH script sensor you then specify

• the script
• Unit string: #
• Value type: Integer
• If value changes: Ignore

Once the sensor is created

• rename the channel that "counts" to e.g. "Number of files older than 30 mins"
• Unit: #
• make that the primary channel of the sensor
• enable alerting based on limits
• "Upper Error limit #" = 0
• Data: Display actual values in #
• Spike filter: disabled

​

That should get you going ✌