Catching Errors in PowerShell

I’ve been recently writing a lot of PowerShell for the SA-ModularInput-PowerShell addon. It’s amazingly flexible at capturing data that is embedded in the .NET framework and many Microsoft products provide convenient access to their monitoring counters via PowerShell. This modular input can replace perfmon, regmon, WMI and all the other things we used to use for monitoring Windows boxes. However, sometimes bad things happen. Scripts don’t work as expected. In the Splunk world, permissions, connectivity and other problems make the diagnosis of scripted inputs a problem. I can run the script myself and get the right stuff, but when I put it in an inputs.conf file, it breaks.

One way to get some diagnostics in there is to ensure the script throws exceptions when necessary and then use a wrapper script to capture those exceptions and produce log events from them. We use this a lot within new apps, and if you have signed up for the Splunk App for SQL Server Beta Program, you will know that all our PowerShell scripts are wrapped in this manner. You can download and view the script on Github, so I am not going to reproduce it here.

This script traps errors. Along the way it writes out two events for you. The first (with sourcetype=PowerShell:ScriptExecutionSummary) contains an Identity field (more on that later), InvocationLine and TerminatingError fields. The more important one from a diagnostics point of view is the second event (with sourcetype=PowerShell:ScriptExecutionErrorRecord) has a ParentIdentity (which matches the Identity field from the first event so you can correlate the two events), and all the Error information as fields. Just in case that wans’t enough, it adds timing information to the ScriptExecutionSummary so you can see how long your script is running.

Using this script is easy. In your addon, create a bin directory for your PowerShell scripts and place the above script in the bin directory as “Invoke-MonitoredScript.ps1” as well. Let’s take a look at the normal running of a script and the wrapped version. Here is our normal inputs.conf stanza for a typical script, taken from the addon for Microsoft SQL Server:

[powershell://DBInstances]
script = & "$SplunkHome\etc\apps\TA-SQLServer\bin\dbinstances.ps1"
schedule = 0 */5 * ? * *
index = mssql
sourcetype = MSSQL:Instance:Information
source = Powershell

Now let’s take a look at the modified version for producing the error information:

[powershell://DBInstances]
script = & "$SplunkHome\etc\apps\TA-SQLServer\bin\Invoke-MonitoredScript.ps1" -Command ".\dbinstances.ps1"
schedule = 0 */5 * ? * *
index = mssql
sourcetype = MSSQL:Instance:Information
source = Powershell

The script you want to run is not affected – only the execution of the script is adjusted. Now you will be able to see any errors that are produced within the monitored script. I have added an Errors dashboard that shows the errors I get combined with the parent invocation information to show timing as well.

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