Powershell的管道参数问题

Question

I ran into a weird issue that I do not know how to explain. I hope someone could shed some light for me.

I would like to create a function which takes a parameter of an array, let's say $scriptnames and users have an option to pipe it from previous results should they prefer. I stripped away the unrelated stuff to better illustrate my confusion.

In the process of the module, I read each item from the array and just print the item.

My function:

function Get-Test
{
 [CmdletBinding()]
 param
 (
 [parameter(mandatory=$true,
             ValueFromPipeline=$True,
             ValueFromPipelineByPropertyName=$true)]
  [string[]]$scriptNames
  )
  BEGIN
  {
  }
  PROCESS
  {
    foreach ($scriptName in $scriptNames)
    {
        Write-Verbose "Executing: $scriptname" 
    }    
  }
  END{}

Here was what confused me:

Scenario 1:

I used this command to get the list of the file in my directory:

get-childitem | Select-Object {$_.BaseName}

The list of file was return correctly without extension:

However, when I piped the results to my function, I got this print out:

Notice the unwanted $_.BaseName= literal was added.

Scenario 2:

However, if I issue this command:

get-childitem | Select-Object $_.BaseName

The results did not really filter out the basename only

but when piping to my function:

get-childitem | Select-Object $_.BaseName|
Get-Test -Verbose 

the $_basename literal was not included:

However, the basename included the extension, which really confused me.

Can someone see something flaring up at you? and why did the following things happen :

1) Why was $_.BaseName literal tagged in the print out in scenario 1, which I did not ask for?

2) Why the select did not seem to work in scenario 2, but the print-out did not have $_.baseName with the exact same script?

3) What do I need to correct in my script to print out the filename only without the extension and without the literal $_.BaseName?

Answer 1

When you include Select-Object BaseName , your returning an array of objects with 1 property, BaseName . Switch it to Select-Object -ExpandProperty BaseName to do what you're looking for, or you can use (Get-ChildItem).BaseName to achieve the same result.

Whenever you see something expected as a string (ie Filename ) that's instead returned as an object/hashtable (ie @{$_.BaseName=Filename} ). If you want to have it return the extension as well, use Name instead of BaseName . If you want the full path to the file, use FullName in that case.

To answer your scenario questions...

1. That behaved as expected, as you only specified a single property to be returned from the objects, but are still returning objects. The object property is named $_.BaseName because you defined it that way with how you called it (usually you would just need to specify the property name without including the $_. in front to indicate the pipeline object, ie Select-Object BaseName ). It's also working because of the surrounding curly braces in Scenario 1

2. The verbose text appears to be defaulting to the Name property, even though you're passing full objects to it. The first example in Scenario 2 returns the full object since you're calling Select-Object incorrectly.

3. My initial answer should cover the solution, I think

Answer 2

You did not specify how you are calling your function, so cannot really tell what's wrong with #1, it seems like you are returning an array instead of string

#2 is easy. You should have simply wrote

gci |select-object basename

instead of $_.basename.

#3 Probably related to 2.

Also, do not use Select-Object if you are trying to get Basename and do not want the header. You can instead do:

(get-childitem).basename | whatever

Here is how it prints with the example function you provided above

PS C:\> (gci).basename |get-test -ver
VERBOSE: Executing: batch
VERBOSE: Executing: DB
VERBOSE: Executing: Modules
VERBOSE: Executing: nix
VERBOSE: Executing: py
VERBOSE: Executing: utils
VERBOSE: Executing: Microsoft.PowerShellISE_profile
VERBOSE: Executing: Microsoft.PowerShell_profile
VERBOSE: Executing: PowerShellTranscripts

Oh you really should remove Mandatory, if you are expecting to pipe stuff to it or you will get a binding error:

Get-Test : Cannot bind argument to parameter 'scriptNames' because it is an empty string