How to write a PowerShell function to use Confirm, Verbose and WhatIf

In my last blog post I showed how to run a script with the WhatIf parameter. This assumes that the commands within the script have been written to use the common parameters Confirm, Verbose and WhatIf.

Someone asked me how to make sure that any functions that they write will be able to do this.

it is very easy

When we define our function we are going to add [cmdletbinding(SupportsShouldProcess)] at the top

and every time we perform an action that will change something we put that code inside a code block like this

and alter The Item and The Change as appropriate.

I have created a snippet for VS Code to make this quicker for me. To add it to your VS Code. Click the settings button bottom right, Click User Snippets, choose the powershell json and add the code below between the last two }’s (Don’t forget the comma)

and save the powershell.json file

Then when you are writing your code you can simply type “ifs” and tab and the code will be generated for you

As an example I shall create a function wrapped around Set-Content just so that you can see what happens.

I have done this before because if the file does not exist then Set-Content will create a new file for you, but with this function I can check if the file exists first with the ValidateScript before running the rest of the function.

As you can see I add variables from my PowerShell code into the “The Item” and “The Change”. If I need to add a property of an object I use $($Item.Property).

So now, if I want to see what my new function would do if I ran it without actually making any changes I have -WhatIf added to my function automagically.

If I want to confirm any action I take before it happens I have -Confirm

As you can see it also give the confirm prompts for the Set-Content command

You can also see the verbose messages with

So to summarise, it is really very simple to add Confirm, WhatIf and Verbose to your functions by placing  [cmdletbinding(SupportsShouldProcess)] at the top of the function and placing any code that makes a change inside

with some values that explain what the code is doing to the The Item and The Change.

Bonus Number 1 – This has added support for other common parameters as well – Debug, ErrorAction, ErrorVariable, WarningAction, WarningVariable, OutBuffer, PipelineVariable, and OutVariable.

Bonus Number 2 – This has automatically been added to your Help

Bonus Number 3 – This has reduced the amount of comments you need to write and improved other peoples understanding of what your code is supposed to do 🙂 People can read your code and read what you have entered for the IfShouldProcess and that will tell them what the code is supposed to do 🙂

Now you have seen how easy it is to write more professional PowerShell functions

How to run a PowerShell script file with Verbose, Confirm or WhatIf

Before you run a PowerShell command that makes a change to something you should check that it is going to do what you expect. You can do this by using the WhatIf parameter for commands that support it. For example, if you wanted to create a New SQL Agent Job Category you would use the awesome dbatools module and write some code like this

before you run it, you can check what it is going to do using

which gives a result like this

This makes it easy to do at the command line but when we get confident with PowerShell we will want to write scripts to perform tasks using more than one command. So how can we ensure that we can check that those will do what we are expecting without actually running the script and see what happens? Of course, there are Unit and integration testing that should be performed using Pester when developing the script but there will still be occasions when we want to see what this script will do this time in this environment.

Lets take an example. We want to place our SQL Agent jobs into specific custom categories depending on their name. We might write a script like this

You can run this script against any SQL instance by calling  it and passing an instance parameter from the command line like this

If you wanted to see what would happen, you could edit the script and add the WhatIf parameter to every changing command but that’s not really a viable solution. What you can do is

this will set all commands that accept WhatIf to use the WhatIf parameter. This means that if you are using functions that you have written internally you must ensure that you write your functions to use the common parameters

Once you have set the default value for WhatIf as above, you can simply call your script and see the WhatIf output

which will show the WhatIf output for the script

Once you have checked that everything is as you expected then you can remove the default value for the WhatIf parameter and run the script

and get the expected output

If you wish to see the verbose output or ask for confirmation before any change you can set those default parameters like this

and set them back by setting to false

 

Pester 4.2.0 has a Because…… because :-)

I was going through my demo for the South Coast User Group meeting tonight and decided to add some details about the Because parameter available in the Pester pre-release version 4.2.0.

To install a pre-release version you need to get the latest  PowerShellGet module. This is pre-installed with PowerShell v6 but for earlier versions open PowerShell as administrator and run

You can try out the Pester pre-release version (once you have the latest PowerShellGet) by installing it from the PowerShell Gallery with

There are a number of improvements as you can see in the change log I particularly like the

  • Add -BeTrue to test for truthy values
  • Add -BeFalse to test for falsy values

This release adds the Because parameter to the all assertions. This means that you can add a reason why the test has failed. As JAKUB JAREŠ writes here

  • Reasons force you think more
  • Reasons document your intent
  • Reasons make your TestCases clearer
  • So you can do something like this

Which gives an error message like this 🙂

As you can see the Expected gives the expected value and then your Because statement and then the actual result. Which means that you could write validation tests like

and get a result like this

Or if you were looking to validate your SQL Server you could write something like this

or maybe your security policies allow Windows Groups as logins on your SQL instances. You could easily link to the documentation and explain why this is important. This way you could build up a set of tests to validate your SQL Server is just so for your environment

Just for fun, these would look like this

and the code looks like

This will be a useful improvement to Pester when it is released and enable you to write validation checks with explanations.

Chrissy has written about dbachecks the new up and coming community driven open source PowerShell module for SQL DBAs to validate their SQL Server estate. we have taken some of the ideas that we have presented about a way of using dbatools with Pester to validate that everything is how it should be and placed them into a meta data driven framework to make things easy for anyone to use. It is looking really good and I am really excited about it. It will be released very soon.

Chrissy and I will be doing a pre-con at SQLBits where we will talk in detail about how this works. You can find out more and sign up here

Using the AST in Pester for dbachecks

TagLine – My goal – Chrissy will appreciate Unit Tests one day 🙂

Chrissy has written about dbachecks the new up and coming community driven open source PowerShell module for SQL DBAs to validate their SQL Server estate. we have taken some of the ideas that we have presented about a way of using dbatools with Pester to validate that everything is how it should be and placed them into a meta data driven framework to make things easy for anyone to use. It is looking really good and I am really excited about it. It will be released very soon.

Chrissy and I will be doing a pre-con at SQLBits where we will talk in detail about how this works. You can find out more and sign up here

Cláudio Silva has improved my PowerBi For Pester file and made it beautiful and whilst we were discussing this we found that if the Pester Tests were not formatted correctly the Power Bi looked … well rubbish to be honest! Chrissy asked if we could enforce some rules for writing our Pester tests.

The rules were

The Describe title should be in double quotes
The Describe should use the plural Tags parameter
The Tags should be singular
The first Tag should be a unique tag in Get-DbcConfig
The context title should end with $psitem
The code should use Get-SqlInstance or Get-ComputerName
The Code should use the forEach method
The code should not use $_
The code should contain a Context block

She asked me if I could write the Pester Tests for it and this is how I did it. I needed to look at the Tags parameter for the Describe. It occurred to me that this was a job for the Abstract Syntax Tree (AST). I don’t know very much about the this but I sort of remembered reading a blog post by Francois-Xavier Cat about using it with Pester so I went and read that and found an answer on Stack Overflow as well. These looked just like what I needed so I made use of them. Thank you very much to Francois-Xavier and wOxxOm for sharing.

The first thing I did was to get the Pester Tests which we have located in a checks folder and loop through them and get the content of the file with the Raw parameter

Then I decided to look at the Describes using the method that wOxxOm (I know no more about this person!) showed.
As I understand it, this code is using the Parser on the $check (which contains the code from the file) and finding all of the Describe commands and creating an object of the title of the Describe with the StaticType equal to String and values from the Tag parameter.
When I ran this against the database tests file I got the following results
Then it was a simple case of writing some tests for the values

The Describes variable is inside @() so that if there is only one the ForEach Method will still work. The unique tags are returned from our command Get-DbcCheck which shows all of the checks. We will have a unique tag for each test so that they can be run individually.

Yes, I have tried to ensure that the tags are singular by ensuring that they do not end with an s (apart from statistics) and so had to not check  BackupPathAccess and statistics. Filename is a variable that we add to each Describe Tags so that we can run all of the tests in one file. I added a little if block to the Pester as well so that the error if the Tags parameter was not passed was more obvious

I did the same with the context blocks as well

This time we look for the Context command and ensure that the string value ends with psitem as the PowerBi parses the last value when creating columns
Finally I got all of the code and check if it matches some coding standards

I trim the title from the Describe block so that it is easy to see where the failures (or passes) are with some regex and then loop through each statement apart from the first line to ensure that the code is using our internal commands Get-SQLInstance or Get-ComputerName to get information, that we are looping through each of those arrays using the ForEach method rather than ForEach-Object and using $psitem rather than $_ to reference the “This Item” in the array and that each Describe block has a context block.

This should ensure that any new tests that are added to the module follow the guidance we have set up on the Wiki and ensure that the Power Bi results still look beautiful!

Anyone can run the tests using

before they create a Pull request and it looks like
if everything is Green then they can submit their Pull Request 🙂 If not they can see quickly that something needs to be fixed. (fail early 🙂 )
03 fails.png