On Preferences and Scopes

James O'Neill

Progress in PowerShell: a tale of Verbosity and other preferences with lessons in Scopes and Proxies thrown in

It started, as these things often do, with someone complaining. In PowerShell Version 7.2 the output of Invoke-WebRequest -Verbose and Invoke-RestMethod -Verbose look like this:

VERBOSE: GET with 0-byte payload

In all the earlier versions they look like the version below, which is more helpful when you’re trying to debug code that builds URIs:

VERBOSE: GET https://google.com/ with 0-byte payload

A proxy function will fix that. If two commands have the same name an alias beats a function, which beats a cmdlet, which beats an external program. You can specify the full path to an external program or cmdlet – for example Microsoft.PowerShell.UtilityInvoke-RestMethod so an Invoke-RestMethod function can act as a proxy for the cmdlet, anything which calls Invoke-RestMethod will go to the function, which calls the cmdlet with its fully qualified name. PowerShell even has a mechanism to create the function’s code:

$cmd      = Get-Command Invoke-RestMethod
$MetaData = New-Object System.Management.Automation.CommandMetaData $cmd
[System.Management.Automation.ProxyCommand]::create($MetaData) | clip

(I don’t carry those 3 lines in my head, when I need them I refer to a script Jeffrey Snover wrote long ago, a newer version is on the PowerShell Gallery.)
I added extra Write-Verbose calls and tidied up the autogenerated code and posted the result as a gist.
A module I’m working has lots of calls to Invoke-RestMethod but the proxy function wouldn’t see that I’d specified -Verbose. So I needed to investigate.

The -Verbose switch sets the value of $VerbosePreference in the function being called; if you thought setting the global $VerbosePreference to continue was the same as a running everything with -Verbose, trying the following might surprise you:

Invoke-WebRequest "https://google.com" -OutFile delete.me
Copy-Item delete.me delete.too

Invoke-WebRequest heeds the preference, but Copy-Item only prints a message if the -Verbose switch is specified.

My proxy function would print a verbose message if run with -Verbose, it would heed the global preference-variable but not the switch passed to the function that called it. The -Confirm, -Debug, -ErrorAction, -InformationAction, -Verbose, -WarningAction, and -WhatIf switches all set the corresponding preference-variable inside a function but that wasn’t being inherited when the functions in my module called the proxy function.
I could reproduce this with the two functions below. First, I loaded them from a single .PSM1 file (and things were the same if I pasted them in at a PowerShell prompt)

function one {
  Write-Verbose "One calls two"
  Write-Verbose "Two returned"

function two {
  Write-Verbose "Two has a message"

If I load this, I get three messages:

VERBOSE: One calls two
VERBOSE: Two has a message
VERBOSE: Two returned

Things change if One is in its own module

Screen shot showing the difference if the calling function is a module

One -verbose now just produces two messages:

VERBOSE: One calls two
VERBOSE: two returned

Setting the global preference-variable at the prompt returns all 3 – because function two sees it.
It is common to assume that a function inherits the variables from whatever called it; we can see that working with simpler functions

function three {

function four {

If I set $e and run three from the prompt

> $e = "eye"  
> three

the assumption holds; and a lot of documentation stops there, but if I import three from a module

> three

The inheritance assumption is qualified by a rule that says what happens in a module stays in the module.
The question is what can we do about it?

Before it dawned on me that this was a scopes thing and not just -Verbose, a search brought me a clue; the solution is to change function two (or my proxy function) as follows:

function two {
  param($VerbosePreference = $PSCmdlet.GetVariableValue('VerbosePreference')
  Write-Verbose "Two has a message"

We can only use $PSCmdlet if we have either [cmdletBinding()] or a [parameter()] decoration – just a side note on that, if you paste in

function five { param ($p) }
function six  { param ([parameter()]$p) }

when you try to tab-complete parameters for five and six, you’ll see six gets all the common parameters but five does not – [parameter()] is an implicit [cmdletBinding()], although it’s still good to write the latter explicitly.

$PSCmdlet is available when the function is setting up its variables. Inside the function we’d never replace $x with the long-winded $PSCmdlet.GetVariableValue("x"), but in a parameter it is “use the value from the scope that called you – even if that scope is a module”.

Now the called function inherits the preference from its caller. If we specify -Verbose it takes precedence, so nothing breaks copying in $VerbosePreference. The same thing applies to -Confirm and -WhatIf.

function seven {
  Write-Host "Something Safe "

function eight {
  if ($PSCmdlet.ShouldProcess("This","Do you want to do")) {
    Write-Host "Something dangerous"

If seven and eight are loaded from the same psm1 file

seven -Confirm

Are you sure you want to perform this action?
Performing the operation "Do you want to do" on target "This".
[Y] Yes [A] Yes to All [N] No [L] No to All [S] Suspend [?] Help (default is"Y"): n

Something Safe

But, if seven loads from its own module …

seven -Confirm
Something dangerous
Something Safe

Modifying the parameters in eight with the code below restores the confirmation

param($ConfirmPreference = $PSCmdlet.GetVariableValue('ConfirmPreference'))

-confirm sets $confirmPreference to low which triggers $PSCmdlet.ShouldProcess to prompt the user. I treat -Force as an extra preference and my functions typically have
if ($Force -or $PSCmdlet.ShouldProcess(...
to ensure the a command can be run non-interactively, even if its impact is set higher than $ConfirmPreference.

The example above might lead you to think -Confirm and -WhatIf should be inherited, but this would cause a problem with scopes at the script level (which apply module-wide). Suppose the module that contained seven read like this:

$ConfirmPreference = 'None'

function seven {

Any cmdlet that would normally ask for confirmation inside seven will run silently. But what if the module containing eight sets that option to something different? If a function overrides something from its own script-scope, then only things which share that scope see the change, which seems logical. Thinking of functions’ scopes as children of their module’s scope which in turn is usually a child of the global scope means we can say things pass down their branch of the scope “tree” but don’t jump between branches.

Before leaving $ConfirmPreference, I have been think about changing it inside a function, so it drops to low if many items are being updated. I’m not sure that’s a great idea because coming to rely on a prompt and which isn’t there reliably, will lead to trouble.

The title said this is about Progress. PowerShell 7.2 has an optional new way of displaying the progress bar, but the things it relies on are a bit flaky in the Integrated-Shell in Visual Studio Code (things improve if you use Write-Progress -completed) and Invoke-WebRequest downloading 100 log files becomes a mess. There is no -Progress common parameter, but the day before I started on the -Verbose problem, I had found that adding $ProgressPreference as a parameter worked, so it makes sense to do it the same way. I inserted the following parameter into the function that calls Invoke-WebRequest. Unlike the examples above, which might be tagged with [Parameter(DontShow)] I want this to tab-complete the possible values, so it is marked with the ActionPreference type:

[ActionPreference]$ProgressPreference = $PSCmdlet.GetVariableValue('ProgressPreference')

And I will also want that to inherit into my proxy function.
With that in place, I can use the code below to call my function and replace the byte-by-byte progress indicator that Invoke-WebRequest displays with my own file-by-file one:

$count = 0
ForEach ($f in $file) {
  Write-Progress "Downloading" -PercentComplete ($count/$file.Count)
  Myfunction $f -ProgressPreference SilentlyContinue
  $count += 100
Write-Progress "Downloading" -Completed

Quick tip: adding 100 to the counter each time (instead of adding 1) removes the need to multiply by 100 for a percentage.

Since I mentioned ErrorAction above, before finishing I wanted to share one last tip about preferences:

function nine {
  param ($Name)
  if (-not $Name) {throw "Name is required"}
  Write-Host "Deleting $Name*"

The “dangerous” line in the example above never runs if $n is empty, right? Wrong, actually.

Screen shot showing the effect of -ErrorAction on throw

Specifying -ErrorAction prevents the throw statement throwing so

nine "" -ErrorAction SilentlyContinue would mean the dangerous code is run.

When it is acting as a “fence” around dangerous code, it is worth putting a return after throw.


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