Mocking Missing Cmdlet ErrorAction with Pester

Following on from the previous Mocking Missing Cmdlets with Pester post, I also encountered another interesting problem when attempting to mock cmdlets that were not present on the test system. This one is more of an edge-case, hence its own post. Just like last time, the tests worked when I had the Hyper-V cmdlets installed, but failed when running within an Appveyor VM.

It’s probably not uncommon that you will need to ensure that the code under test should throw an error here-and-there. Here is a pseudo-example that tests that Get-VM writes an error when passed with a non-existent VM name:

When this test is run it fails:

The problem here is that the stub function is not an advanced function and the –ErrorAction preference switch is ignored! This is easily resolved by adding the [CmdletBinding()] attribute to the stub function definition:

Running the test now results in the expected output:

This is not a Pester issue and it’s not a Powershell issue either. It’s just the way the normal Powershell functions work. But, just in case someone else runs into it I thought it would be worth quickly documenting!

Mocking Missing Cmdlets with Pester

When writing Pester unit tests for your Powershell code you will probably have a need to mock calls to external functions before too long. This process works as you would expect when Pester can locate a defined function/cmdlet with a matching name. However, if Pester cannot find a definition, it will fail.

This problem will normally surface in a Continuous Integration (CI) environment. For me, it was writing the first suite of tests for the xHyper-V DSC resource module. The Hyper-V cmdlets where present on my authoring machine but were not present on the Appveyor build VM.

Here is an overly simplified Pester test that I’ll use for demonstration purposes:

If we run the test and Pester cannot locate a defined function then it will report an error. Note: if you run this on a machine with the Hyper-V module installed (and you have a VM called ‘TestVM’) then it will pass – but you knew that already ;).

This is easily overcome by defining an empty function within the test file. Note: this will need to be defined within the ‘InModuleScope’ script block if you’re testing a module’s internals.

The test now passes as we would expect. Yay \o/

Now, what you really need to know is that for Pester to enumerate and mock parameter filters, those parameters need to be defined on the stub function. If we were to update the test to check for the passing of a particular –Name parameter like so:

When we run the test it will now fail again.

For Pester to enumerate the dynamic parameters on the function, it needs to have the parameters (only the one’s you’re interested in) defined. This can easily be fixed like so:

The tests will now once again pass successfully!

Hopefully this helps someone and saves some time. It took me a while to work out what was going on as I had the cmdlets available on my development machine but the tests were failing when running in an Appveyor VM. Perhaps I should submit a pull request to get this put into the Pester help documentation?!

Searching for String Properties with Powershell

I had a requirement recently to parse a configuration file (let’s just say for documentation purposes) and I needed to retrieve a property/value pair which may or may not be present in a text line. Now, depending on the product we wish to document, we might have a line in the configuration file constructed as follows:

Now, if wanted the “-intfType” value we could split the string using the Powershell .Split() method like so:

However, there are two issues here:

  1. What happens if there are additional or missing properties (the index/order will change)?
  2. We were hoping/expecting to see “Xen Virtual” output and not just “Xen.

Here’s a quick function that will solve issue #1 (complete with case insensitivity):

Solving issue #2 is a little more involved as we need find the quoted text, escape it and then restore it when needed. The following Get-StringProperty advanced function will first replace all quoted spaces with “^” and then replace the quotes themselves with “^^”. This permits the split function to work as expected. Finally, everything is put back together just before we need it!

That’s much better Smile. Just for good luck I have also created a complimentary Test-StringProperty advanced function that tests whether a property name is present or not. This removes a lot of if ((Get-StringPropertyValue $SearchString “PropertyName”) -ne $null) { Do-Something } calls.

Some final words of warning:

  • If there are escaped (double) quotes then this function won’t work without some modification.
  • If the $SearchString just so happens to natively contain either “^” and/or “^^” it won’t (currently) work either.
  • If you have any improvements or feedback then please let me know.
  • Please test in your environment before putting any 3rd party or external code into production!

RES Automation Manager Quick Tip – appending to existing registry values

I was recently asked (by one of our existing RES Automation Manager customers) how they go about adding to an existing registry value using RES Automation Manager. Well the answer is simple really – by using the @REGISTRY function. I’ll detail how you go about using this function in this blog post.

  1. Firstly start the RES Automation Manager console;
  2. Select “Modules” from the left hand pane, Right Click and select “Add”;
  3. Give the module a suitable name then select the “Tasks” Tab, Right Click and select “Add”.
  4. Select the task “Registry Setting (Apply,Query)” and select “Apply”.
  5. You will now be presented with a dialogue where you can select various methods to add the required registry value you wish to append too. In my example I’m going to APPEND a new string to the START of the existing USERINIT registry value. Select “HKEY_LOCAL_MACHINE” from the left hand pane, Right Click and select “Open HKEY_LOCAL_MACHINE…”.
  6. Browse to “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon” and select “Userinit”, this will add this value to the current dialog box. image
  7. Now we are going to add the @REGISTRY function to the Userinit value by Right Clicking on “Userinit” in the right hand pane and selecting “Modify”.
  8. In the “Value Data” field, Right Click and select “Insert Functions” >; “@[REGISTRY(;)]”.image
  9. RES Automation Manager now provides you with a nice GUI that allows you to browse to the registry value you wish to retrieve, when the job is executed on the agent. In my case this is going to be the registry value I selected in Step 6, as this is the value I’d like to append too.
  10. Now I simply add the new value that I wish to append, before the @REGISTRY function or after, depending where I’d like my value to appear – in my case this value is “MyNewValuetoAppend
    MyNewValuetoAppend,@[REGISTRY(HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon)]
  11. The resulted registry value now looks like this, once the job has been scheduled and completed 
    MyNewValuetoAppend,C:\Windows\system32\userinit.exe

That’s all there is to it! Smile

Nathan

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