How to Successfully Pass Variables to ForEach-Object -Parallel in PowerShell's Start-Job

Learn how to correctly pass variables from a PowerShell script when using `ForEach-Object -Parallel` within `Start-Job`, overcoming common errors to effectively manage EC2 instances.
---

Visit these links for original content and any more details, such as alternate solutions, comments, revision history etc. For example, the original title of the Question was: Passing variable to foreach-object -parallel which is with in start-job

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Challenge: Passing Variables in PowerShell Jobs

PowerShell is a powerful scripting language that enables system administrators and developers to manage configurations and automate tasks. However, when dealing with parallel processing using Start-Job, one may encounter specific challenges, especially in passing variables effectively between script blocks.

In this guide, we will focus on a common scenario where users of PowerShell are trying to pass profile names and region variables to ForEach-Object -Parallel while within a Start-Job block. A user encountered a specific error stating:

"ForEach-Object: The value of the using variable '$using:profilename' cannot be retrieved because it has not been set in the local session."

To address this issue, let’s dive deeper into the solution.

Breakdown of the Issue

When using parallel processing in PowerShell, the scope of variables becomes crucial. The $using: scope modifier is intended for this purpose, but often users experience difficulties getting their variables recognized within nested commands.

Key Points:

The error typically arises from attempting to access variables that weren't correctly passed into the parallel execution context.

The proposed solution is to ensure that you are setting the variable outside the Start-Job block and passing it through the param() setup inside the job.

Solution Overview

Step 1: Setup Variables Before the Job

To fix the error and ensure that your script runs smoothly, it’s essential to declare your variables outside the Start-Job. Here is how you can achieve this:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update Your Job ScriptBlock

Implement the following within your job's script block to ensure variables are used correctly:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Alternative Methods

If you're still facing difficulties, consider these alternatives:

Using ForEach-Object with -AsJob Parameter

Another method you can use simplifies the job management process. You can run ForEach-Object -Parallel as a job directly:

[[See Video to Reveal this Text or Code Snippet]]

Utilizing Start-ThreadJob

PowerShell 7 introduces Start-ThreadJob, which is a more efficient alternative:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Using ForEach-Object -Parallel within a PowerShell Start-Job might seem challenging at first, especially when managing variables across different scopes. By following the steps outlined in this post, you can effectively tackle any issues related to variable passing, ensuring smooth execution of your scripts.

By mastering these techniques, you will be well on your way to leveraging PowerShell's full capabilities for managing AWS EC2 instances, especially across multiple accounts with numerous instances.

Now, get out there and harness the power of parallel processing in PowerShell!