Powershell PSSession变量上下文

Question

I am using New-PSSession to run a command on a remote machine, via Invoke-Command. As an example, let's say the machine I am running the command FROM is called FROM-MACHINE, and the machine I am running the command TO is called TO-MACHINE.

$session = New-PSSession -ComputerName TO-MACHINE
$script = ScriptBlock::Create("Write-Host Machine name is $env:COMPUTERNAME")
Invoke-Command -session $session -scriptBlock $script

I would expect this to output:

Machine name is TO-MACHINE

However, it actually outputs

Machine name is FROM-MACHINE

This also happens in the case of file paths - if I want to run a program at "C:\\myprogram.exe" powershell will attempt to find myprogram.exe at "\\\\FROM-MACHINE\\c$\\myprogram.exe".

My question is this: is there any way (other than using psexec) to truly run a remote command, variables and all? Am I missing something simple here?

Answer 1

You can avoid this situation by declaring your ScriptBlock directly in your code as a ScriptBlock instead of a String object. You do this by simply surrounding the code in curly braces. You can store the ScriptBlock object in a variable, and then pass it into Invoke-Command just as you were before, with the -ScriptBlock parameter.

$ScriptBlock = {
    $Program = 'c:\MyProgram.exe';
    Start-Process -Wait -FilePath $Program -ArgumentList 'args go here';

    Write-Host -Object "Write-Host Machine name is $env:COMPUTERNAME";
    }

Invoke-Command -ComputerName to-machine -ScriptBlock $ScriptBlock;

Note : You do not have to manually create PowerShell Remoting sessions. Invoke-Command will take care of that for you, unless you specifically want to leave them open in the background.

Answer 2

This is happening because you are using a double quoted string for your script block creation. $env:COMPUTERNAME is being expanded to the local computer name before the script block is created. If you use a single-quoted string, $env:COMPUTERNAME will get passed in the script block as literal text, and expansion won't happen until it gets executed on the remote machine. So, when you're creating script blocks from strings for execution on remote machines, us expandable (double quoted) strings when you want to use local variables in the script block, and non-expandable (single quoted) strings when you want to use variables from the remote machine.

$session = New-PSSession -ComputerName TO-MACHINE
$script = [ScriptBlock]::Create('$env:COMPUTERNAME')
Invoke-Command -session $session -scriptBlock $script

If you want to mix and match in the same script block, use an expandable string, and then escape the $ on all the remote variable references with a backtick:

$session = New-PSSession -ComputerName TO-MACHINE 
$script = [ScriptBlock]::Create(" Write-output $env:Computername; Write-Output `$env:COMPUTERNAME ")
Invoke-Command -session $session -scriptBlock $script