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Say I have the following powershell script:

$hello = "Hello World"
Write-Output $hello

When I run it I get

Hello World

Then I decide that I want my variable to be named $helloWorld:

$helloWorld = "Hello World"
Write-Output $hello

When I run that I get

Hello World

What I want is for that to fail because I have not defined $hello (or at least write out an empty string).

I have seen some examples that try to record off all the variables before running and calling Remove-Variable on all the new ones after. This seems like a lot of work.

I thought I would ask if today's powershell has a better solution. Basically I am looking for a command that will reset the terminal's variable memory back to the initial startup state.

Vaccano
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  • Variable:\ is a drive just like any other. You can get a list of the items prior to script and then just remove the items that don’t exist in the list after. That is far less work then maintaining a list of added variables – Doug Maurer May 06 '22 at 21:03

2 Answers2

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If you call Set-StrictMode with -Version 1 or higher, references to nonexistent variables cause an error, both in the current scope and its descendants:

Set-StrictMode -Version 1

$helloWorld = "Hello World"

# Causes a statement-terminating error, because $hello isn't defined.
Write-Output $hello

Caveats:

  • The error is only a statement-terminating error, which means that script execution will continue by default.

  • If $hello happens to be defined in an ancestral scope, no error will be reported, due to PowerShell's dynamic scoping.

  • IDE caveats:

    • Unfortunately, the obsolescent, Windows PowerShell-only Windows PowerShell ISE invariably runs scripts dot-sourced, i.e. directly in a session's global scope rather than in a child scope, so that previous invocations can leave behind state that affect subsequent ones.

      • Applied to your example: if you initially defined $hello, ran the script, and then changed the variable to $helloWorld, running the script will still succeed even with a strict mode in effect, because the $hello variable is still around.

    • Using Visual Studio Code with the PowerShell extension unfortunately does that too by default, but you can configure it use a new, temporary session for each invocation - see this answer.

mklement0
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0

I am not quite sure of the exact situation of this, however here are some approaches I would use:

  • powershell - create a new instance of powershell
  • Remove-Variable * -ErrorAction SilentlyContinue - see here
NikolaiSch
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