What is conceptually wrong with get-date|Write-Host($_)

Question

I'm trying to understand Powershell, but find somethings not so intuitive. What I understand of it is that in the pipeline objects are passed, instead of traditionally text. And $_ refers to the current object in the pipeline. Then, why is the following not working:

get-date|Write-Host "$_"

The errormessage is:

Write-Host : The input object cannot be bound to any parameters for the command either because the command does not take pipeline input or the input and its properties do not matc
h any of the parameters that take pipeline input.
At line:1 char:10
+ get-date|Write-Host $_
+          ~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (10-9-2014 15:17:00:PSObject) [Write-Host], ParameterBindingException
    + FullyQualifiedErrorId : InputObjectNotBound,Microsoft.PowerShell.Commands.WriteHostCommand

Answer 1

$_ is the current single item in the pipeline. To write each item in the pipeline you would write

get-data | foreach { Write-Host $_ }

Or in the short form

get-data |% { Write-Host $_ }

Conceptually, Foreach is a cmdlet that receives a function parameter, a pipeline input and applies the function on each item of the pipeline. You can't just write code with $_ - you need to have a function explicitly states that it agrees to receive pipeline input

Answer 2

And $_ refers to the current object in the pipeline

Indeed, the automatic $_ variable refers to the current pipeline input object, but only in script blocks ({ ... }), notably those passed to the ForEach-Object and Where-Object cmdlets.

Outside of script blocks it has no meaningful value.

Therefore, the immediate fix to your command is the following:

# Note: 
# * Get-Date *alone* is preferable; its output *implicitly* prints.
# * If you want to use Write-Host, this is better: Get-Date | Write-Host
Get-Date | ForEach-Object { Write-Host $_ }

However, note that:

• Write-Host is is often the wrong tool to use, unless the intent is to write to the display only, bypassing the success output stream and with it the ability to send output to other commands, capture it in a variable, or redirect it to a file.

• To output a value (which prints to the display by default), simply don't capture it, if it is a command's output (e.g., Get-Date) or, if it is stored in a variable, use the latter by itself; e.g, $value, instead of Write-Host $value (or use Write-Output $value); see this answer.
  To explicitly print only to the display (instead of outputting data), but with rich formatting, use Out-Host.

Therefore, if merely outputting each pipeline input object is the goal, Get-Date | ForEach-Object { $_ } would do, where the ForEach-Object call is redundant if each input object is to simply be passed through (without transformation); that is, in the latter case just Get-Date would do.

---

As for what you tried:

get-date|Write-Host "$_"

As noted, the use of $_ in this context is pointless, but the reason for the error message you saw is unrelated to that problem:

Instead, the reason for the error is that you're mistakenly trying to provide input to Write-Host both via the pipeline (Get-Date | Write-Host ...) and by way of an argument (... | Write-Host "...").

• Given that the argument ("$_") (positionally) binds to the -Object parameter, the pipeline input then has no parameter left to bind to, which causes the error at hand.