How can I access a higher level $_ pipeline variable from a nested pipeline?

Question

I am writing a script that will look at a directory of Parent VHD files and then evaluate which VMs are using those parent VHDs.

I have the mechanics of it working but I am running into an issue where I really need to reference a automatic pipeline variable ($_) from the context of a nested pipeline

The sudo code would be something like:

For each File in Files
Iterate over all VMs that have differencing disks
and return all the VMs that have a disk whose parent disk is File

Here is the actual powershell code I have implemented so far to do this:

$NAVParentFiles = get-childitem '\\hypervc2n2\c$\ClusterStorage\Volume1\ParentVHDs' | where {$_.Name -notLike "*diff*"} | select name
$NAVParentFiles | % { Get-VM | where {$_.VirtualHardDisks | where {$_.VHDType -eq "Differencing" -and ($_.ParentDisk.Location | split-path -leaf) -like <$_ from the outer for each loop goes here> } }

Thanks for any help you can provide me on how to elegantly access an outer pipeline variable from a nested pipeline.

Sometimes nestead foreaches is more readable than nested pipelines. — stej
– stej, Commented Nov 23, 2011 at 14:49
Similar question with similar solution: stackoverflow.com/q/26715634/11942268 — stackprotector
– stackprotector, Commented Mar 9, 2021 at 7:57

CJBS · Accepted Answer · 2022-09-19 02:23:58Z

52

You can assign the $_ to a variable and use that:

 1..10 | %{ $a = $_; 1..10 | %{ write-host $a} }

Anyway, consider refactoring your script. It is too nested. Concentrate on readability. It is not always necessary to pipe, you can use a foreach loop if that helps improve readability.

edited Sep 19, 2022 at 2:23

CJBS

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answered Nov 23, 2011 at 4:33

manojlds

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1 Comment

Chris Magnuson Over a year ago

That did the trick =). I am looking at refactoring the code as you suggested and possibly going to nested foreach loops where it makes sense.

js2010 · Accepted Answer · 2019-08-18 14:20:07Z

5

I don't have that command, but maybe the -pipelinevariable common parameter can be of use here.

Get-VM -PipelineVariable vm | Get-VHD |
  Select-Object @{n='Name'; e={$vm.name}}, path, parentpath

edited Aug 18, 2019 at 14:20

answered Aug 18, 2019 at 13:55

js2010

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