#PSTip Taking control of verbose and debug output, part 5

Note: This tip requires PowerShell 3.0 or later.

In the final part of the series, we will combine everything we’ve found out so far and create a tool that will identify any command that needs to be wrapped by advanced function in order to make access to verbose and debug messages easy and natural.

Note: You can find a function here.

Our function will take only one parameter, (-Name), but will validate it extensively using knowledge we already have. First of all, it will check if command exists. Then, it will check if command is already advanced and if it is, there is no need to wrap it. Next we check command’s Abstract Syntax Tree (AST) to see if Write-Verbose or Write-Debug are present. If these commands are absent, we also won’t proceed. Entire param() block:

param (
    [Parameter(Mandatory = $true)]
    [ValidateScript({

        $command = Get-Command -Name $_ -ErrorAction Stop

        if ($command.CmdletBinding) {
            throw 'This is already an advanced command.'
        }

        $astVerboseOrDebug = $command.ScriptBlock.Ast.FindAll(
            {
                $args[0] -is [System.Management.Automation.Language.CommandAst] -and
                $args[0].CommandElements[0].Value -match '^Write-(Verbose|Debug)$'
            },
            $true
        )

        if (-not $astVerboseOrDebug) {
            throw 'No need to turn into advanced: Write-Debug and Write-Verbose not used.'
        }

        $true
    })]
    [String]$Name
)

We filtered out commands that require additional processingĀ and it is time to apply a fix. Depending on the command type, we will either prefix whole name (for scripts) or noun (for functions). Also, AST check for pipeline-friendliness will be different depending on the command type. We will use a Switch statement for that.

$commandInfo = Get-Command @PSBoundParameters

switch ($commandInfo.CommandType) {
    ExternalScript {
        $isPipelineFriendly = [bool]$commandInfo.ScriptBlock.Ast.ProcessBlock
        $newName = $commandInfo.Name -replace '^', 'Invoke-'
    }

    Function {
        $isPipelineFriendly = [bool]$commandInfo.ScriptBlock.Ast.Body.ProcessBlock
        $newName = $commandInfo.Name -replace '-', '-Advanced'
    }
}

If a wrapped command is marked as ‘pipeline-friendly’, we have to modify parameter block.

$paramBlock = [System.Management.Automation.ProxyCommand]::GetParamBlock($commandInfo)

if ($isPipelineFriendly) {

    $inputParameter = @'
[Parameter(ValueFromPipeline = $true)]
[System.Object]
${InputObject}
'@

    if ($paramBlock) {
        $paramBlock = $paramBlock, $inputParameter -join ','
    } else {
        $paramBlock = $inputParameter
    }
}

We also need to define body of the function. This will be different depending on whether the command is pipeline-friendly or not. We will use formatting operator (-f). Therefore, we have to be careful with curly brackets (we have double them whenever they should be taken literally):

if ($isPipelineFriendly) {
    $body = @'
begin {{
    Write-Verbose 'Removing InputObject from PSBoundParameters'
    $PSBoundParameters.Remove('InputObject') | Out-Null
}}

process {{
    Write-Verbose 'Running {0} with $InputObject as pipeline input and PSBoundParameters'
    $InputObject | {0} @PSBoundParameters
}}
'@

} else {

    $body = @'
    Write-Verbose "Calling {0} with parameters passed."
    {0} @PSBoundParameters
'@

}

We have all building blocks and we just have to generate our command. Because we want to use created command after function completes, we will create it in ‘global’ scope:

$body = $body -f $commandInfo.Name

$scriptText = @"
[CmdletBinding()]
param (
$paramBlock
)
$body
"@

$scriptBlock = [scriptblock]::Create($scriptText)
New-Item -Path function:\Global:$newName -Value $scriptBlock -Force

This function can be used to wrap any function or script to enable -Verbose and -Debug parameters. We just need to specify the name of the target command and we will get updated, advanced function. No more secret verbose and debug outputs!

About the author: Bartek Bielawski

Bartek is a busy IT Admin working for an international company, Optiver. He loves PowerShell and automation. That love got him the honors of a Microsoft MVP. He shares his knowledge on his blog. You can also find him on Twitter: @bielawb.

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