Inheriting Preference Variables into Scriptblocks

So I recently created a module to using Powershell sessions to invoke commands on remote machines.

Preference Variables not inherting from Parent Scope

I hit an issue during testing and was trying to diagnose the issue so I added some Write-Verbose commands to assist with this except the verbose preference was not inheriting from the parent scope.

So I’ve been doing powershell long enough to know that calling a function should inherit the parent scope preference variables e.g.:

function Do-Stuff {
    [cmdletbinding()]
    param ()

    "Doing Stuff"
    "Doing Stuff Verbosely" | Write-Verbose
}

function Do-Something {
    [cmdletbinding()]
    param ()

    "Doing Something"
    Do-Stuff
}

Do-Something -Verbose

Gives you the output

Doing Something
Doing Stuff
VERBOSE: Doing Stuff Verbosely

So a bit strange that inheritance wasn’t working in a scriptblock being passed to a powershell session.

$scriptblock = {
    Param (
        [string]$sbparam
    )

    $sbparam
    "Some other Verbosity" | Write-Verbose
}

Invoke-Command -ScriptBlock $scriptblock -ArgumentList 'testing string' -Verbose

This did not trigger the verbose output in the scriptblock as intended.

My initial thought was maybe the scriptblock needs [cmdletbinding()] to access the verbose parameters to bind to them but that didn’t make sense.

Solution

Seems that this form of inheritance doesn’t work as the scriptblock is being run within the session of the remote machine so Chris Gardner gave me the solution that setting the $VerbosePreference within the scriptblock would trigger the verbose output from the remote machine.

$scriptblock = {
    Param (
        [string]$sbparam
    )

    $VerbosePreference = 'continue'

    $sbparam
    "Some other Verbosity" | Write-Verbose
}

Invoke-Command -ScriptBlock $scriptblock -ArgumentList 'testing string' -Verbose

Gives you the output

testing string
VERBOSE: Some other Verbosity

I think that everyone who picks up powershell has been tripped up with scope issues in the past. I know that I have in the past and more than likely will again in the future but this worked a treat.

It turns out that it wasn’t even my code that was the issue. I was attempting to run my commands in the old location of the agent I was interacting with so setting the new location worked and I learned something about scopes in the process.

Update

It appears that this was raised by Chris Bergmeister (MVP) and was discussed on Github around the default behavior of the preference variables in a scriptblock.

It’s brief but outlines what I went through above: https://github.com/PowerShell/PowerShell/issues/4040