PowerShell: Passing variables to remote commands

If you have started using Invoke-Command, then you have ran into the issue of getting local variables into your remote commands. There is a lot of advice out there on how to do this and some approaches are more clunky than others. First let me show you the problem of scope.

$local = Get-Date
Invoke-Command -ComputerName server01 -ScriptBlock { 
    "Date = {0}" -f $local
}

That script runs in a different scope on the remote system and it is not aware of the variables in the local session. In the example above, $local is actually $null in the remote session because it was never defined in that scope. The good news is that Invoke-Command has a -ArgumentList parameter that we can use.

Invoke-Command -ComputerName server01 -ArgumentList $local -ScriptBlock { 
    "Date = {0}" -f $args[0] 
}

While that works, we are using a legacy variable to access the parameter. I don’t mind that as much but it isn’t intuitive that $args[0] has the value of $local. Easy enough to figure out when you have one argument but that list can get quite large at times. We can address that problem by giving our ScriptBlock a param block.

Invoke-Command -ComputerName server01 -ArgumentList $local -ScriptBlock { 
    param($local)
    "Date = {0}" -f $local
}

Now our variables are a lot easier to work with and this easily scales to as many parameters as we need. There is one more option available to us that I like much better. Starting with Powershell 3.0, they added a $using: scope to the language to address this scenario in workflows. You can also use it with Invoke-Command and with the DSC script resource. It looks like this:

Invoke-Command -ComputerName server01 -ScriptBlock { 
    "Date = {0}" -f $using:local
}

It embeds the variable from the calling scope into the scriptblock. I like this because it is really close to how we started and it is very intuitive once you figure it out. You can do this with any number of variables.