Supporting -Whatif, -Confirm, -Verbose – In SCRIPTS! - Windows PowerShell Blog - Site Home

One of the greatest things about PowerShell is that when you use a Cmdlet which is going to have a side effect on the system, you can always type –Whatif, -Confirm or –Verbose. Sadly, you lose this option when it comes to scripts. Over time we want to makes scripts be the semantic equivalent of Cmdlets – that is to say that a Cmdlet is a Cmdlet independent of whether you implement it in C#, VB.NET or PowerShell. That said, it takes us a while to get releases out and it really torques me that most scripts don't support –Whatif, -Confirm, and –Verbose so I decided to do something about it. I wrote a PowerShell fuction, Should-Process, which helps you implement these in your scripts (this is very similar to how we do it in Cmdlets). First let me show you a script using it and a transcript of using that script. This is a script which stops all the running calc programs:

function Stop-Calc ([Switch]$Verbose, [Switch]$Confirm, [Switch]$Whatif)
{
$AllAnswer = $null
foreach ($p in Get-Process calc)
{ if (Should-Process Stop-Calc $p.Id ([REF]$AllAnswer) "`n***Are you crazy?" -Verbose:$Verbose -Confirm:$Confirm -Whatif:$Whatif)
{ Stop-Process $p.Id
}
}
}

Here is a transcript of using that function:

PS> calc;calc;calc
PS> Stop-Calc -Whatif
What if: Performing operation "Stop-Calc" on Target "436"
What if: Performing operation "Stop-Calc" on Target "3776"
What if: Performing operation "Stop-Calc" on Target "4104"
PS>
PS>
PS>
PS> Stop-Calc -Verbose
VERBOSE: Performing "Stop-Calc" on Target "436".
VERBOSE: Performing "Stop-Calc" on Target "3776".
VERBOSE: Performing "Stop-Calc" on Target "4104".
PS> calc;calc;calc
PS>
PS>
PS>
PS> Stop-Calc -Confirm
Confirm
Are you sure you want to perform this action?
Performing operation "Stop-Calc" on Target "4596".
***Are you crazy?
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (defau
lt is "Y"): n
Confirm
Are you sure you want to perform this action?
Performing operation "Stop-Calc" on Target "5492".
***Are you crazy?
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (defau
lt is "Y"): s
PS> gps calc

Handles NPM(K) PM(K) WS(K) VM(M) CPU(s) Id ProcessName
------- ------ ----- ----- ----- ------ -- -----------
55 3 1252 5148 49 0.02 4596 calc
55 3 1248 4936 49 0.06 5492 calc
55 3 1248 5168 49 0.06 5588 calc

PS> exit
Confirm
Are you sure you want to perform this action?
Performing operation "Stop-Calc" on Target "5492".
***Are you crazy?
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (defau
lt is "Y"): l
PS>
PS>
PS>
PS> Stop-Calc -Confirm
Confirm
Are you sure you want to perform this action?
Performing operation "Stop-Calc" on Target "4596".
***Are you crazy?
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (defau
lt is "Y"): ?
Y - Continue with only the next step of the operation.
A - Continue with all the steps of the operation.
N - Skip this operation and proceed with the next operation.
L - Skip this operation and all subsequent operations.
S - Pause the current pipeline and return to the command prompt. Type "exit"
to resume the pipeline.
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (defau
lt is "Y"): a
PS>

Here is the function that makes this all possible:

Function Should-Process ($Operation, $Target, [REF]$AllAnswer, $Warning = "", [Switch]$Verbose, [Switch]$Confirm, [Switch]$Whatif)
{
# Check to see if "YES to All" or "NO to all" has previously been selected
# Note that this technique requires the [REF] attribute on the variable.
# Here is an example of how to use this:
# function Stop-Calc ([Switch]$Verbose, [Switch]$Confirm, [Switch]$Whatif)
# {
# $AllAnswer = $null
# foreach ($p in Get-Process calc)
# { if (Should-Process Stop-Calc $p.Id ([REF]$AllAnswer) "`n***Are you crazy?" -Verbose:$Verbose -Confirm:$Confirm -Whatif:$Whatif)
# { Stop-Process $p.Id
# }
# }
# }
if ($AllAnswer.Value -eq $false)
{ return $false
}elseif ($AllAnswer.Value -eq $true)
{ return $true
}

if ($Whatif)
{ Write-Host "What if: Performing operation `"$Operation`" on Target `"$Target`""
return $false
}
if ($Confirm)
{
$ConfirmText = @"
Confirm
Are you sure you want to perform this action?
Performing operation "$Operation" on Target "$Target". $Warning
"@
Write-Host $ConfirmText
while ($True)
{
$answer = Read-Host @"
[Y] Yes [A] Yes to All [N] No [L] No to all [S] Suspend [?] Help (default is "Y")
"@
switch ($Answer)
{
"Y" { return $true}
"" { return $true}
"A" { $AllAnswer.Value = $true; return $true }
"N" { return $false }
"L" { $AllAnswer.Value = $false; return $false }
"S" { $host.EnterNestedPrompt(); Write-Host $ConfirmText }
"?" { Write-Host @"
Y - Continue with only the next step of the operation.
A - Continue with all the steps of the operation.
N - Skip this operation and proceed with the next operation.
L - Skip this operation and all subsequent operations.
S - Pause the current pipeline and return to the command prompt. Type "exit" to resume the pipeline.
"@
}
}
}
}
if ($verbose)
{
Write-Verbose "Performing `"$Operation`" on Target `"$Target`"."
}

return $true
}

I'm including this as an attachment to this entry just in case you have any copy/paste issues.

Use this often – it works great.

BTW – this all depends upon a technique that is not well documented – the [REF] attribute on parameters. Yes – PowerShell supports reference parameters! This is how we can set a variable in the Should-Process function in a way that that change is seen in the parent's scope. We do that in order to handle the YES-TO-ALL/NO-TO-ALL feature. We need to change state and then see that change the next time we get called. Notice that this requires you to pass a reference variable using a very specific syntax ([REF]$AllAnswer). Then when we want to access this parameter, we have to access it's VALUE – e.g. $AllAnswer.Value . This deserves an entire blog entry for itself but I needed to use it here so I thought a quick explanation was in order.

Jeffrey Snover [MSFT]
Windows PowerShell/MMC Architect
Visit the Windows PowerShell Team blog at: http://blogs.msdn.com/PowerShell
Visit the Windows PowerShell ScriptCenter at: http://www.microsoft.com/technet/scriptcenter/hubs/msh.msp