Getting an error when executing a nested ScriptBlock from Invoke-Command

Question

I'm looking for method to create a wrapper around Invoke-Command that restores the current directory that I'm using on the remote machine before invoking my command. Here's what I tried to do:

    function nice_invoke {
        param(
            [string]$Computer,
            [scriptblock]$ScriptBlock
        )
    
        Set-PSDebug -Trace 0

        $cwd = (Get-Location).Path
        write-host "cmd: $cwd"
    
        $wrapper = {
            $target = $using:cwd
            if (-not (Test-Path "$target")) {
                write-host "ERROR: Directory doesn't exist on remote"
                exit 1
            }
            else {
                Set-Location $target
            }
            $sb = $using:ScriptBlock
    
            $sb.Invoke() | out-host
       }
    
       # Execute Command on remote computer in Same Directory as Local Machine
       Invoke-Command -Computer pv3039 -ScriptBlock $wrapper    
    }

Command Line:

    PS> nice_invoke -Computer pv3039 -ScriptBlock {get-location |out-host; get-ChildItem | out-host }

Error Message:

Method invocation failed because [System.String] 
does not contain a method named 'Invoke'.
+ CategoryInfo          : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : MethodNotFound
+ PSComputerName        : pv3039

If I'm not mistaken the using shouldbe unecessary to prepare the second ScriptBlock. Did you investigate the datatype of $sb within the function? As you can see by the error $sb is only a string rather than a scripblock which is why it does not have a invoke option. In place it could be possible to just use $ScriptBlock. — Seth
– Seth, Commented Oct 27, 2021 at 17:09
@Seth unfortunately that is not the case, the ScriptBlock contents are internally a string-literal That is, variable expansion won't occur until the ScriptBlock is prepared for execution via .Invoke() or when using the call operator &. Either the $using: scope or providing an -ArgumentList is required here, but neither will preserve the original ScriptBlock type. — codewario
– codewario, Commented Oct 27, 2021 at 17:26

codewario · Accepted Answer · 2021-10-27 22:20:39Z

3

You can't pass a ScriptBlock like this with the $using: scope, it will get rendered to a string-literal first. Use the [ScriptBlock]::Create(string) method instead within your $wrapper block to create a ScriptBlock from a String:

$sb = [ScriptBlock]::Create($using:ScriptBlock)
$sb.Invoke() | Out-Host

Alternatively, you could also use Invoke-Command -ArgumentList $ScriptBlock, but you still have the same issue with the ScriptBlock getting rendered as a string. Nonetheless, here is an example for this case as well:

# Call `Invoke-Command -ArgumentList $ScriptBlock`
# $args[0] is the first argument passed into the `Invoke-Command` block
$sb = [ScriptBlock]::Create($args[0])
$sb.Invoke() | Out-Host

Note: While I kept the format here in the way you were attempting to run the ScriptBlock in your original code, the idiomatic way to run ScriptBlocks locally (from the perspective the nested ScriptBlock it is a local execution on the remote machine) is to use the Call Operator like & $sb rather than using $sb.Invoke().

With either approach, the nested ScriptBlock will execute for you from the nested block now. This limitation is similar to how some other types are incompatible with shipping across remote connections or will not survive serialization with Export/Import-CliXml; it is simply a limitation of the ScriptBlock type.

Worthy to note, this limitation persists whether using Invoke-Command or another cmdlet that initiates execution via a child PowerShell session such as Start-Job. So the solution will be the same either way.

edited Oct 27, 2021 at 22:20

answered Oct 27, 2021 at 17:21

codewario

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4 Comments

codewario Over a year ago

@SantiagoSquarzon That won't work because $ScriptBlock does not exist in the remote session. The $using: scope or -ArgumentList $ScriptBlock is required (the latter would require referencing $args[0] instead of $ScriptBlock in that case).

Santiago Squarzon Over a year ago

You're right, my bad sorry, I was having a really hard time understanding OP's code or even why is the function useful at all.

codewario Over a year ago

@SantiagoSquarzon Yeah a better approach would be to simply cd $using:pwd rather than create $cwd and do it that way. However, there is still value in understanding how to provide a nested ScriptBlock to a remote or parallel PowerShell session.

mklement0 Over a year ago

Nicely done, though I suggest at least mentioning that a more "natural" way to invoke script blocks is with & (or .). That script blocks deserialize as strings is certainly surprising and has been reported in GitHub issue #11698; unfortunately, the decision was made not to fix the problem, for security reasons I personally find unconvincing.

pico · Accepted Answer · 2021-10-27 17:12:18Z

-2

function nice_invoke {
    param(
        [string]$Computer,
        [scriptblock]$ScriptBlock
    )
    Set-PSDebug -Trace 0    
    $cwd = (Get-Location).Path
    write-host "cmd: $cwd"  
    $wrapper = {
        $target = $using:cwd
        if (-not (Test-Path "$target")) {
            write-host "ERROR: Directory doesn't exist on remote"
            exit 1
        }
        else {
            Set-Location $using:cwd
        }       
        $sb = [scriptblock]::Create($using:ScriptBlock)     
        $sb.Invoke() 
    }   
    # Execute Command on remote computer in Same Directory as Local Machine
    Invoke-Command -Computer pv3039 -ScriptBlock $wrapper 
}
nice_invoke -Computer pv3039 -ScriptBlock {
      hostname
      get-location 
      #dir
}

answered Oct 27, 2021 at 17:12

pico

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1 Comment

codewario Over a year ago

Code-only answers are generally considered low-quality on Stack Overflow. You may want to provide an explanation as to what changes you made to the code and why.

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