Powershell Passing Function to remote command

Question

I am trying to pass local function to remote code but not getting any success. Below is my code.

#variables defined over here
...
function comparehash ($source, $destination)
{
    $sourcefiles = @{}
    ...
}

$securePassword = ConvertTo-SecureString -AsPlainText -Force $Password 
$cred = New-Object System.Management.Automation.PSCredential $Username, $securePassword
$session = New-PSSession -ComputerName $srv -port 22 -Credential $cred  –Authentication CredSSP
Invoke-Command -Session $session -ScriptBlock { 
    param( $source, $destination, $application)
    #Write-Host "This is" $source
    # Take backup of the site first
    Copy-Item $source\$application $destination -Recurse -force
    ${function:comparehash}
}  -ArgumentList $site_path_local, $backup_path, $app
Remove-PSSession -Session $session

This code is copying source files to destination. Function has been created to validate md5 sum of the copied files. When I run the script script runs fine but it doesn't call the function code. Is there anything additional need to be done to call function?

Update

Invoke-Command -Session $session -ScriptBlock { 
    param( $source, $destination, $application, $fundef, $comparefunc )
    Write-Host "This is" $source, $destination
    # Take backup of the site first
    #Copy-Item $source\$application $destination -Recurse -force
    [ScriptBlock]::($comparefunc).Invoke($source,$destination)
    #comparehash $site_path_local $backup_path
}  -ArgumentList $site_path_local, $backup_path, $app, ${function:comparehash}
Remove-PSSession -Session $session

Above code is throwing following error:

PS C:\Windows\system32> C:\Users\vijay.patel\Documents\myscripts\Env_Refresh.ps1
This is F:\inetpub F:\Env_Backup\Refresh_Backup\
You cannot call a method on a null-valued expression.
    + CategoryInfo          : InvalidOperation: (Invoke:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull
    + PSComputerName        : 172.16.82.124

Mathias R. Jessen · Accepted Answer · 2015-05-21 11:44:45Z

4

Since the function is defined in your own local scope, you'll have to pass it along to the remote session as well. Through testing, it seems that it gets passed as a string, but you can use [scriptblock]::Create() to "recreate" it in the remote session:

Invoke-Command -Session $session -ScriptBlock { 
    param( $source, $destination, $application, $comparefunc)
    # do stuff
    # Now invoke the function that was provided
    [ScriptBlock]::Create($comparefunc).Invoke($source,$destination)
} -ArgumentList $site_path_local, $backup_path, $app, ${function:comparehash}

edited May 21, 2015 at 11:44

answered May 18, 2015 at 13:42

Mathias R. Jessen

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10 Comments

Vijay Over a year ago

Thanks for your time. Now at least it is doing something. I am getting error now.

You cannot call a method on a null-valued expression.     + CategoryInfo          : InvalidOperation: (Invoke:String) [], RuntimeException     + FullyQualifiedErrorId : InvokeMethodOnNull     + PSComputerName        : 172.16.82.124

Mathias R. Jessen Over a year ago

@Vijay function comparehash needs to be defined at the calling scope first: gist.github.com/IISResetMe/75a16fe7dc40934d3fb4

Vijay Over a year ago

Yeah I have made those changes suggested by you and after that I am getting this error. Do you know why it is throwing null valued expression?

Mathias R. Jessen Over a year ago

@Vijay either the function can't be passed to a remote session, or the function itself contains a method call that depends on a variable/object that exists only in the caller scope

Mathias R. Jessen Over a year ago

@Vijay when I run it against a remote session, the function gets passed as a string, see update

|

Brain2000 · Accepted Answer · 2018-12-11 04:10:31Z

3

I found the $Using command can be your friend here. If you keep the remote PSSession open, you can transfer all the functions you will need one at a time, then invoke the final scriptblock that calls/uses those functions.

function comparehash($source, $destination)
{
    Write-Output "I like hash";
    if( $source -eq $destination) { PackThePipe $source; }
}

function PackThePipe($hash)
{
    Write-Output "Pushing $hash through the pipe";
}


$session = New-PSSession -ComputerName $srv -port 22 -Credential $cred  –Authentication CredSSP

#create the functions we need remotely
Invoke-Command $session { Invoke-Expression $Using:function:comparehash.Ast.Extent.Text; }
Invoke-Command $session { Invoke-Expression $Using:function:PackThePipe.Ast.Extent.Text; }

#call the scriptblock that utilizes those functions
Invoke-Command $session { $s = "12345"; $d="12345"; comparehash $s $d; }

Remove-PSSession $session;

edited Dec 11, 2018 at 4:10

answered Dec 11, 2018 at 3:10

Brain2000

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4 Comments

Dennis Over a year ago

Wow. This is so much better than the way I used to create functions remotely :)

PainElemental Over a year ago

How do you have to do the Invoke-Expression, when the functions have an '-' character, like 'compare-hash' and 'Pack-ThePipe'? I get an error message from Invoke-Expression in these cases.

Brain2000 Over a year ago

@PainElemental ${Using:function:compare-hash}

PainElemental Over a year ago

@Brain2000 Thx for pointing me in the right direction! Is has to be ${Using:function:compare-hash}.Ast.Extent.Text

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