CODEWITHSUNDEEP
powershellfunctional-programming
LudvigH
LudvigH

Reputation: 4802

How to decorate an existing function or passing function objects in Powershell?

Lets say I want to save the output of a Powershell command to file. I would do this like ls | out-file "path.txt". I make this call a few times per day and am worried that the function call (ls in this case) produce bad data ruining my file. I feel like I need a backup!

Next step for me would be decorating the out-file call so that it automatically backs up the data in a separate file. One backup per day would be sufficient. This could be achieved by a custom out-bak function as per below. Suddenly I get automated backups with ls | Out-Bak "path.txt".

function Out-Bak {
  [cmdletbinding()]

  Param (
  [parameter(ValueFromPipeline)]
  [string]$inputObject,

  [parameter(Mandatory=$false)]
  [string]$myPath
  )

  Begin {
   $backupPath = [System.IO.Path]::ChangeExtension($myPath,".bak_$([DateTime]::Now.ToShortDateString())")
   Remove-Item $myPath
   Remove-Item $backupPath
  }

  Process {
    Out-File -InputObject $input -FilePath $myPath -Append
    Out-File -InputObject $input -FilePath $backupPath -Append
  }
}

This solves my problem fine, but I would like to be able to use exactly the same pattern for Out-csv and similar filewriting fucntion. Is there a way to pass the Out-File command as a parameter to the Out-Bak so that I can use the function as a somewhat generic decorator for output commands?

Upvotes: 2

Views: 385

Answers (1)

woxxom
woxxom

Reputation: 73746

Let the backup function do only what its name suggests: backup the file.

  •  ls | Out-File $path | Backup

  • ........ | Out-File foo.txt | Backup

  • ........ | Out-File -FilePath "$path\$name" | Backup

  • ........ | Export-Csv -NoTypeInformation bar.csv | Backup

The backup cmdlet will simply copy the file once the pipeline finishes.
To find the file path from a previous pipeline command we'll have to use arcane stuff like AST parser:

function Backup {
    end {
        $bakCmdText = (Get-PSCallStack)[1].Position.text
        $bakCmd = [ScriptBlock]::Create($bakCmdText).
            Ast.EndBlock.Statements[0].PipelineElements[-2].CommandElements
        $bakParamInfo = if (!$bakCmd) { @{} }
            else { @{} + (Get-Command ($bakCmd[0].value)).Parameters }
        $bakSource = ''; $bakLiteral = $false; $bakPos = 0
        while (!$bakSource -and ++$bakPos -lt $bakCmd.count) {
            $bakToken = $bakCmd[$bakPos]
            if ($bakToken.ParameterName) {
                if ($bakToken.ParameterName -match '^(File|Literal)?Path$') {
                    $bakLiteral = $bakToken.ParameterName -eq 'LiteralPath'
                } elseif (!$bakParamInfo[$bakToken.ParameterName].SwitchParameter) {
                    $bakPos++
                }
                continue
            }
            $bakSource = if ($bakToken.StringConstantType -in 'SingleQuoted', 'BareWord') {
                $bakToken.value
            } else {
                [ScriptBlock]::Create($bakToken.extent.text).
                    InvokeWithContext(@{}, (Get-Variable bak* -scope 1))
            }
        }
        if (!$bakSource) {
            Write-Warning "Could not find file path in pipeline emitter: $bakCmdText"
            return
        }
        $backupTarget = "$bakSource" + '.' + [DateTime]::Now.ToShortDateString() + '.bak'
        $bakParams = @{ $(if ($bakLiteral) {'LiteralPath'} else {'Path'}) = "$bakSource" }
        copy @bakParams -destination $backupTarget -Force
    }
}

Warning: it fails with $() like ... | out-file "$($path)" | backup because Get-PSCallStack for some reason returns the expression contents as the callee, and right now I don't know other methods of getting the parent invocation context.

Upvotes: 3

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