Supporting -WhatIf and -Confirm

Any function that changes something should support -WhatIf and -Confirm, exactly like Remove-Item. It takes two small additions.

function Remove-OldLog {
  [CmdletBinding(SupportsShouldProcess)]
  param(
    [string]$Path = '.',
    [int]$Days = 30
  )
  $cutoff = (Get-Date).AddDays(-$Days)
  Get-ChildItem -Path $Path -Filter *.log |
    Where-Object LastWriteTime -lt $cutoff |
    ForEach-Object {
      if ($PSCmdlet.ShouldProcess($_.Name, 'Delete log')) {
        Remove-Item -LiteralPath $_.FullName
      }
    }
}

'old.log', 'older.log', 'today.log' | ForEach-Object { New-Item $_ | Out-Null }
(Get-Item old.log).LastWriteTime = (Get-Date).AddDays(-45)
(Get-Item older.log).LastWriteTime = (Get-Date).AddDays(-90)

Remove-OldLog -WhatIf
(Get-ChildItem -Filter *.log).Count
Remove-OldLog
Get-ChildItem -Filter *.log -Name

What if: Performing the operation "Delete log" on target "old.log".
What if: Performing the operation "Delete log" on target "older.log".
3
today.log

1. [CmdletBinding(SupportsShouldProcess)] adds the -WhatIf and -Confirm parameters.
2. Wrap each change in if ($PSCmdlet.ShouldProcess(target, action)). With -WhatIf, it prints a message and returns $false; with -Confirm, it asks; otherwise, it returns $true.

-WhatIf also flows into cmdlets you call — the Remove-Item inside would respect it even without the if. The explicit check gives you a clear message and lets you skip other work, like logging.

ConfirmImpact

For dangerous operations, add ConfirmImpact = 'High': PowerShell then asks for confirmation by default, unless the caller passes -Confirm:$false.

# illustrative
function Reset-Database {
  [CmdletBinding(SupportsShouldProcess, ConfirmImpact = 'High')]
  param([string]$Name)
  if ($PSCmdlet.ShouldProcess($Name, 'Drop and recreate')) {
    # ...
  }
}

Common mistakes

• Declaring SupportsShouldProcess but never calling ShouldProcess — -WhatIf then silently does nothing useful. PSScriptAnalyzer warns about this.