Tricky PowerShell Pipeline Tricks – Playing With WMI

Here’s a quick task: Get the WMI object win32_bios for a computer. Using PowerShell, that’s really easy. You just run Get-WMIObject win32_bios. Now what if you wanted all the extended properties of the object (not just the five that it normally returns) and ONLY to return the properties that actually have a value assigned?

Well this question just got trickier. win32_bios isn’t very big so it’s an easy one to play with in this example. Let’s step through a couple commands so we know what we’re dealing with.

Well okay, there are the five properties that we knew the command returns by default. We know the extended properties are in there and we can prove it.

There’s everything! That’s a lot of blanks, though. Depending on the type of reporting you’re doing, that might not be so nice to look at. I know that I’d like to get them out. Luckily with PowerShell 4.0, it’s really easy if you use the -PipelineVariable parameter.

I’ve left out the output since it’s every property that has a value and none of the ones that have a blank. Let’s take a look at what’s actually happening here.

On Line 1, we’re running the same command we ran the last two times except we assigned a Pipeline Variable which will be named $bios (you omit the $ when assigning the name of the variable). We enter a foreach loop on Line 2.

On Line 3, we’re setting the value of $props and on Line 4, we’re writing out the part of the WMI object that contains it. The tricky thing here is how we get the value of $props. Look at how we use the Pipeline Variable and the .PSObject.Properties.Name property to identify the items with a value.

Well that’s great but what if I don’t have PowerShell 4.0 or found that example really confusing?

Don’t worry, this is pretty easy to do in earlier versions of PowerShell, too.

I started using some more shortcuts (gmi and % instead of Get-WMIObject and foreach).

In Line 1, I’m doing the same ol’ Get-WMIObject command that I did before and in down the pipe, I’m assigning the value to $wmi. I could have also done “$wmi = gmi win32_bios” but this strategy has benefits if you’re planning on scaling this script out.

In Line 2, I’m doing some tricky Select-Object work. The input for the command is $wmi, which isn’t so tricky, but the property that we’re selecting is pretty tricky. -Property takes an array, so we can put a command in there as long as it returns an array.

$wmi has a property of .Properties.Name which is an array of all the names of the properties attached to the WMI object that got returned in Line 1 (that we are using as our input). We don’t just want all the properties, though, so we need to select only the ones with a value (not null). We do that by piping the list of all the properties into a Where-Object command.

The Where-Object command has a tricky property called -FilterScript which basically acts as an implicit IF statement. If you wrote Where-Object -FilterScript {$true} then you would return every object in the pipe. If you wrote Where-Object -FilterScript {($num = $((Get-Random -Minimum 1 -Maximum 100) % 2) -eq 1)} you would get random properties because sometimes the statement will be true and sometimes the statement will be false. These are all silly items to filter on, though.

In my script, I’m filtering on if the current property the script is looking at has a value in $wmi. If the property is 0 or null and therefore doesn’t exist, $wmi.item($_) will return false and that line won’t be returned. It’s basically a test to see if there’s a string or not. Consider this example:

Because $var1 doesn’t have an actual value assigned to it, an if ($var1) will return false. That’s the same logic we are using all throughout the above code.

Tricky, right?

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