Create a shorthand function for pipelines

[Shayan-To]

I'm using | ConvertTo-Csv | Out-File quite a few times throughout my scripts, and now I want to make a modification to all of them. Is there a way to create a shorthand function for it, so I can refactor my code and do the modifications from one place?

[MartinGC94]

Yes, just define a function that takes pipeline input and also does what you want, for example:

    function DemoFunction
    {
        Param
        (
            [Parameter(Mandatory, ValueFromPipeline)]
            [System.Object]
            $InputObject
        )
        Process
        {
            $InputObject | ConvertTo-Csv | Out-File
        }
    }

Though it's probably worth noting that there already is a wrapper for your use case called Export-Csv which handles both the CSV conversion and exporting the data to a file.

[Shayan-To]

Unfortunately, this does not work.

I change your code a little bit (to include a random file name) to demonstrate:

function DemoFunction
{
    Param
    (
        [Parameter(Mandatory, ValueFromPipeline)]
        [System.Object]
        $InputObject
    )
    Process
    {
        $InputObject | ConvertTo-Csv | Out-File "$(New-Guid).csv"
    }
}

Now, if you run 1..10 | % { @{ k1 = 1; k2 = 2 } } | ConvertTo-Csv | Out-File "$(New-Guid).csv", it will result in a single file, but if you run 1..10 | % { @{ k1 = 1; k2 = 2 } } | DemoFunction, it will result in 10 different files, each having a single row (and the header).

[Shayan-To]

And thanks about mentioning Export-Csv. I need more customizations that it does not provide.

[MartinGC94]

You are right, I didn't think that one through. It creates a new pipeline for each item. You could do it like this though:

function DemoFunction
{
    Param
    (
        [Parameter(Mandatory, ValueFromPipeline)]
        [System.Object]
        $InputObject
    )
    end
    {
        $input | ConvertTo-Csv | Out-File "$(New-Guid).csv"
    }
}

This uses the automatic variable $input which contains all the input data that was passed to the function, see: https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_automatic_variables?view=powershell-7.4#input

[ThomasNieto]

There is a shorthand to create a function with pipeline support.

filter DemoFunction {
  $Input | ConvertTo-Csv | Out-File "$(New-Guid).csv"
}

You can also define a parameter too.

filter DemoFunction {
  param ($Path)
  $Input | ConvertTo-Csv | Out-File $Path
}

[Shayan-To]

@ThomasNieto

Unfortunately, using $Input with Filter does not do what I want here. If you may, run the examples I provided above, you would see that running 1..10 | % { @{ k1 = 1; k2 = 2 } } | DemoFunction and running 1..10 | % { @{ k1 = 1; k2 = 2 } } | ConvertTo-Csv | Out-File "$(New-Guid).csv" yield different results.

The reason is that Filters work like Functions with everything inside a Process block (reference). And behavior of the $Input automatic variable is different in Process and End blocks, as documented in the link) @MartinGC94 provided:

• In the process block, the $input variable contains the current object in the pipeline.
• In the end block, the $input variable enumerates the collection of all input to the function.

So, using $Input inside a Function, or inside the End block of a Function would do what I want (as @MartinGC94 taught me), but putting it inside the Process block would repeat running the code for each element of the pipeline enumeration, and doesn't have the desired behavior I'm looking for.

BTW, using non of the processing method blocks is just like writing everything inside an End block, as is stated here:

If you don't use a named block, then PowerShell puts the code in the end block of the function.