PowerShell Jobs, writing to a file

Having some problems getting a Start-Job script block to output to a file. The following three lines of code work without any problem:

$about_name = "C:\0\ps_about_name.txt"
$about = get-help about_* | select Name,Synopsis
if (-not (Test-
Path $about_name)) { ($about | select Name | sort Name | Out-String).replace("[Aa]bout_", "") > $about_name }

The file is created in C:\0\

But I need to do a lot of collections like this, so I naturally looked at stacking them in parallel as separate jobs. I followed online examples and so put the last line in the above as a script block invoked by Start-Job:

Start-Job { if (-not (Test-Path $about_name)) { { ($about | select Name | sort Name | Out-String).replace("[Aa]bout_", "") > $about_name } }

The Job is created, goes to status Running, and then to status Completed, but no file is created. Without Start-Job, all works, with Start-Job, nothing... I've tried a lot of variations on this but cannot get it to create the file. Can someone advise what I am doing wrong in this please?

Solution

IMO, the simplest way to get around this problem by use of the $using scope modifier.

$about_name = "C:\0\ps_about_name.txt"
$about = get-help about_* | select Name,Synopsis
$sb = { if (-not (Test-Path $using:about_name)) { 
          $using:about.Name -replace '^about_' | Sort-Object > $using:about_name 
        }
      }
Start-Job -Scriptblock $sb

Explanation:

$using allows you to access local variables in a remote command. This is particularly useful when running Start-Job and Invoke-Command. The syntax is $using:localvariable.

This particular problem is a variable scope issue. Start-Job creates a background job with its own scope. When using -Scriptblock parameter, you are working within that scope. It does not know about variables defined in your current scope/session. Therefore, you must use a technique that will define the variable within the scope, pass in the variable's value, or access the local scope from the script block. You can read more about scopes at About_Scopes.

As an aside, character sets [] are not supported in the .NET .Replace() method. You need to switch to -replace to utilize those. I updated the code to perform the replace using -replace case-insensitively.

HCM's perfectly fine solution uses a technique that passes the value into the job's script block. By defining a parameter within the script block, you can pass a value into that parameter by use of -ArgumentList.

Another option is to just define your variables within the Start-Job script block.

$sb = { $about_name = "C:\0\ps_about_name.txt"
        $about = get-help about_* | select Name,Synopsis
        if (-not (Test-Path $about_name)) { 
          $about.Name -replace '^about_' | Sort-Object > $about_name 
        }
      }
Start-Job -Scriptblock $sb