Using Invoke-Command with Variables in ScriptBlock

Question

New to PowerShell and learning through writing random scripts using the help info. I've tried the following 3 ways to properly get variables into the ScriptBlock(along with way too many small variations to list) with listed error message wrapped in **:

do
{
try {
[ValidateRange(1,7)][int]$days = Read-Host "Let's pull up some Warning event logs. How many days back would you like to go back? (1-7)"
} catch {}
} until ($?)

do
{
try {
[ValidateSet('desktop','documents',IgnoreCase)]$location = Read-Host "Would you like me to save the log on your Desktop or in your Documents?"
} catch {}
} until ($?)
$filename = Read-Host "What would you like to name the file?"
$DaysAgo = [datetime]::Now.AddDays(-$days)
Invoke-Command -AsJob -Jobname JobEventLog -ScriptBlock {Get-EventLog -logname system | Where-Object EntryType -eq Warning | where TimeGenerated -ge $DaysAgo | Out-File $HOME\$location\$filename.txt}

Invoke-Command : Parameter set cannot be resolved using the specified named parameters.

---

do
{
try {
[ValidateRange(1,7)][int]$days = Read-Host "Let's pull up some Warning event logs. How many days back would you like to go back? (1-7)"
} catch {}
} until ($?)

do
{
try {
[ValidateSet('desktop','documents',IgnoreCase)]$location = Read-Host "Would you like me to save the log on your Desktop or in your Documents?"
} catch {}
} until ($?)
$filename = Read-Host "What would you like to name the file?"
$DaysAgo = [datetime]::Now.AddDays(-$days)
Invoke-Command -AsJob -Jobname JobEventLog -ScriptBlock {Get-EventLog -logname system | Where-Object EntryType -eq Warning | where TimeGenerated -ge $Using:$DaysAgo | Out-File $Using:HOME\$Using:location\$Using:filename.txt}

Invoke-Command : Parameter set cannot be resolved using the specified named parameters.

---

do
{
try {
[ValidateRange(1,7)][int]$days = Read-Host "Let's pull up some Warning event logs. How many days back would you like to go back? (1-7)"
} catch {}
} until ($?)
do
{
try {
[ValidateSet('desktop','documents',IgnoreCase)]$location = Read-Host "Would you like me to save the log on your Desktop or in your Documents?"
} catch {}
} until ($?)
$filename = Read-Host "What would you like to name the file?"
Write-Host "Processing..."
$DaysAgo = [datetime]::Now.AddDays(-$days)
$parameters = @{
    ScriptBlock = { Param ($Arg1,$Arg2,$Arg3) Invoke-Command -AsJob -Jobname JobEventLog -ScriptBlock {Get-EventLog -logname system | Where-Object source -eq DCOM | where TimeGenerated -ge $Arg1 | Out-File "$HOME\$Arg2\$Arg3.txt"}}
    JobName = "DCOM"
    ArgumentList = ($DaysAgo,$location,$filename)
}
Invoke-Command @parameters

Invoke-Command : Cannot validate argument on parameter 'ScriptBlock'. The argument is null. Provide a valid value for the argument, and then try running the command again.

I'm just looking to have user input how far back they want to view Event Logs, where to save it, and what to name it. I've been able to work my way through everything so far until I hit the Invoke-Command line and haven't been able to get through it. I prefer the one line style of 1 and 2 over the parameters style, however after spending way too much time using the help_Invoke-Command-full and googling I'm throwing in the towel over what I'm sure is a simple error on my syntax.

Answer 1

You can use $args inside the scriptblock, see an example:

$DaysAgo = [datetime]::Now.AddDays(-$days)
Invoke-Command -AsJob -Jobname JobEventLog -ScriptBlock {
Get-EventLog -logname system | Where-Object EntryType -eq Warning | 
where TimeGenerated -ge $args[0] | 
Out-File $HOME\$location\$filename.txt
} -ArgumentList $DaysAgo

Add the arguments at the end of the Invoke-Command like in the example and use $args[0] for the first argument, $args[1] for the second and so on...

Answer 2

This works for me. The computer is localhost as a test, at an elevated prompt, which you would need for the system log anyway. Level 3 is warning. If it was for the same computer you wouldn't need invoke-command at all.

$location = 'foo'
$filename = 'myfile'
$date = get-date
$daysago = $date.adddays(-1) 
invoke-command localhost { param($daysago, $location, $filename)
  get-winevent @{logname = 'system'; level = 3; starttime = $daysago} |
  out-file $home\$location\$filename.txt } -args $daysago,$location,$filename

Answer 3

In order to use Invoke-Command's -AsJob switch, you must execute code remotely, such as by targeting a different computer with the -ComputerName or -Session arguments.

In the absence of such arguments, your command would run locally, but it fails due to the syntactic restriction described above.

If you want to run a job locally, use Start-Job directly:

$job = Start-Job -Name JobEventLog -ScriptBlock {
    Get-EventLog -logname system | 
      Where-Object EntryType -eq Warning | 
        Where-Object TimeGenerated -ge $using:DaysAgo | 
          Out-File $HOME\$using:location\$using:filename.txt
  }

Note: Since your background script block references variables from the caller's scope, they must be referenced via the $using: scope (as you've also done in your last Invoke-Command-based attempt). This requirement also applies to script blocks executed remotely, such as via Invoke-Command -ComputerName - see this answer for background information. The alternative is to pass arguments to the script block, via the -ArgumentList (-Args) parameter, though the $using: approach is usually simpler.

Start-Job returns a job-information object (System.Management.Automation.Job), which you can use to monitor the progress of and obtain output from the background job, using the various *-Job cmdlets, notably Wait-Job and Receive-Job - see the about_Jobs conceptual help topic.

Generally, using Invoke-Command for local code execution, while technically supported, is rarely necessary.
For direct, synchronous invocation (not as a job) of a command or script block, use &, the call operator (not needed for single commands, as long as the command name isn't quoted or specified via a variable), or, for execution directly in the caller's scope, ., the dot-sourcing operator (. { ... }).

Answer 4

You have a couple of options. Since you're only running this on your local machine, you can use Start-Job instead of Invoke-Command.

That being said, the problem that you're running into is 2-fold. First, if you're running the Invoke-Command cmdlet, you'll need to specify the ComputerName parameter. Even though it's an optional parameter, you'll need to use it to tell Powershell which parameter set you're using, otherwise it's going to get confused.

Secondly, you'll need to pass the arguments into the scriptblock. This is because Start-Job and Invoke-Command are part of PSRemoting and will actually look for environment variables on the specified computer instead of variables that you've declared in your script.

Here's what worked for me:

Invoke-Command -ComputerName $(hostname) -AsJob -JobName "TestJob" -ScriptBlock { Get-EventLog -logname system | Where-Object EntryType -eq Warning | Where-Object -property TimeGenerated -ge $args[0] | Out-File "$HOME\$($args[1])\$($args[2]).txt" } -ArgumentList $DaysAgo, $location, $filename

The Invoke-Command option is powerful if you're wanting to get this information from other devices on your network.

And here's the Start-Job version:

Start-Job -Name "TestJob2" -ScriptBlock { Get-EventLog -logname system | Where-Object EntryType -eq Warning | Where-Object -property TimeGenerated -ge $args[0] | Out-File "$HOME\$($args[1])\$($args[2]).txt" } -ArgumentList $DaysAgo, $location, $filename