PowerShell proxy cmdlet – function which wraps standard Export-CSV cmdlet but adds handling of the -Append parameter

Export-CSV-Append.ps1

#Requires -Version 2.0

<#
  This Export-CSV behaves exactly like native Export-CSV
  However it has one optional switch -Append
  Which lets you append new data to existing CSV file: e.g.
  Get-Process | Select ProcessName, CPU | Export-CSV processes.csv -Append
  
  For details, see
  http://dmitrysotnikov.wordpress.com/2010/01/19/export-csv-append/
  
  (c) Dmitry Sotnikov
#>

function Export-CSV {
[CmdletBinding(DefaultParameterSetName='Delimiter',
  SupportsShouldProcess=$true, ConfirmImpact='Medium')]
param(
 [Parameter(Mandatory=$true, ValueFromPipeline=$true,
           ValueFromPipelineByPropertyName=$true)]
 [System.Management.Automation.PSObject]
 ${InputObject},

 [Parameter(Mandatory=$true, Position=0)]
 [Alias('PSPath')]
 [System.String]
 ${Path},
 
 #region -Append (added by Dmitry Sotnikov)
 [Switch]
 ${Append},
 #endregion 

 [Switch]
 ${Force},

 [Switch]
 ${NoClobber},

 [ValidateSet('Unicode','UTF7','UTF8','ASCII','UTF32','BigEndianUnicode','Default','OEM')]
 [System.String]
 ${Encoding},

 [Parameter(ParameterSetName='Delimiter', Position=1)]
 [ValidateNotNull()]
 [System.Char]
 ${Delimiter},

 [Parameter(ParameterSetName='UseCulture')]
 [Switch]
 ${UseCulture},

 [Alias('NTI')]
 [Switch]
 ${NoTypeInformation})

begin
{
 # This variable will tell us whether we actually need to append
 # to existing file
 $AppendMode = $false
 
 try {
  $outBuffer = $null
  if ($PSBoundParameters.TryGetValue('OutBuffer', [ref]$outBuffer))
  {
      $PSBoundParameters['OutBuffer'] = 1
  }
  $wrappedCmd = $ExecutionContext.InvokeCommand.GetCommand('Export-Csv',
    [System.Management.Automation.CommandTypes]::Cmdlet)
        
        
	#String variable to become the target command line
	$scriptCmdPipeline = ''

	# Add new parameter handling
	#region Dmitry: Process and remove the Append parameter if it is present
	if ($Append) {
  
		$PSBoundParameters.Remove('Append') | Out-Null
    
  if ($Path) {
   if (Test-Path $Path) {        
    # Need to construct new command line
    $AppendMode = $true
    
    if ($Encoding.Length -eq 0) {
     # ASCII is default encoding for Export-CSV
     $Encoding = 'ASCII'
    }
    
    # For Append we use ConvertTo-CSV instead of Export
    $scriptCmdPipeline += 'ConvertTo-Csv -NoTypeInformation '
    
    # Inherit other CSV convertion parameters
    if ( $UseCulture ) {
     $scriptCmdPipeline += ' -UseCulture '
    }
    if ( $Delimiter ) {
     $scriptCmdPipeline += " -Delimiter '$Delimiter' "
    } 
    
    # Skip the first line (the one with the property names) 
    $scriptCmdPipeline += ' | Foreach-Object {$start=$true}'
    $scriptCmdPipeline += '{if ($start) {$start=$false} else {$_}} '
    
    # Add file output
    $scriptCmdPipeline += " | Out-File -FilePath '$Path' -Encoding '$Encoding' -Append "
    
    if ($Force) {
     $scriptCmdPipeline += ' -Force'
    }

    if ($NoClobber) {
     $scriptCmdPipeline += ' -NoClobber'
    }   
   }
  }
 } 
  

  
 $scriptCmd = {& $wrappedCmd @PSBoundParameters }
 
 if ( $AppendMode ) {
  # redefine command line
  $scriptCmd = $ExecutionContext.InvokeCommand.NewScriptBlock(
      $scriptCmdPipeline
    )
 } else {
  # execute Export-CSV as we got it because
  # either -Append is missing or file does not exist
  $scriptCmd = $ExecutionContext.InvokeCommand.NewScriptBlock(
      [string]$scriptCmd
    )
 }

 # standard pipeline initialization
 $steppablePipeline = $scriptCmd.GetSteppablePipeline($myInvocation.CommandOrigin)
 $steppablePipeline.Begin($PSCmdlet)
 
 } catch {
   throw
 }
    
}

process
{
  try {
      $steppablePipeline.Process($_)
  } catch {
      throw
  }
}

end
{
  try {
      $steppablePipeline.End()
  } catch {
      throw
  }
}
<#

.ForwardHelpTargetName Export-Csv
.ForwardHelpCategory Cmdlet

#>

}