Windows PowerShell $_. Variable

Introduction to the PowerShell Dollar Underscore Variable

Constructions such as ... | Where {$_.name -Match "win"} are incredibly useful in PowerShell.  Definitions such as: '$_ means in this pipeline', are a bit stuffy.  The best way to understand $_ is to work through examples.

Examples of the PowerShell $_ Variable

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PowerShell's $_ Variable

The first point to remember is that $_ is a variable or placeholder. 

# PowerShell $_ Variable Example
Get-Service | Where-Object {$_.name -Match "win"}

What we want to say here is
Get the services, where the service name matches 'Win'.  Did the repetition of 'service' seem a little verbose?  Well, script writing is brilliant at eliminating extraneous words?

In a nutshell $_ saves us repeating Get-Service, or whatever else may be to the left of ... | Where{$_

The Significance of the Dot in PowerShell's $_.PowerShell Dollar Variable

Here is a similar dollar underscore example but featuring .DisplayName instead of .Name.  My point is I want to illustrate how the .dot command introduces a property.

If you use PowerShell's ISE GUI, then as soon as you type that period (.) you get a drop-down list of properties.

# PowerShell $_ Variable Example
Get-Service | Where {$_.DisplayName -Match "win"}

Challenge:  Research more properties with Get-Service | Get-Member.

$_ Example to Filter WmiObjects

The key to remembering the syntax is to breakdown the construction in to:  $dollar / _underscore / dot.property.  The commonest example would be: $_.name.

# PowerShell script to find Network WMI Objects
Get-WmiObject -List | Where-Object {$_.name -Match 'Network'}

Note 1: The real-life task is to research for network type WMI objects.  Without the where clause it would be like looking for a needle in a haystack.

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 The PowerShell Variable $_

Another way of looking at PowerShell's $_ is purely as a variable.  After all, the dollar sign is PowerShell's way of introducing any variable, not just this special 'In this pipeline' item.

#PowerShell Script to List Dll files
$Path = "C:\Windows\System32\"
Get-ChildItem $Path | Where {$_.extension -eq '.DLL'}

Note 2: The point of this example is to compare the special $_ variable with an ordinary variable called $Path.

Significance of the Where {Evaluation}

Most of the $_ examples feature the 'where' filter.  While many scripters also like the alias ?, the underlying cmdlet is: Where-Object.  Let us take the time to research its properties.

Clear-Host
Get-Help Where-Object -full

Note 3: Remember that Where-Object is a filter; therefore to perform its job Where needs a ScriptBlock to evaluate the test.  Actually, the two characters '$ and _' play a small but important role; $_'s job is to shorten the evaluation by saying, 'In this pipeline', rather than explicitly mentioning the test for a second time.

See more on measuring the speed of Where-Object

PowerShell $_ with ForEach

Most of my $_ examples are found in Where-Object clauses, but here is a different use for this special pipeline variable: ForEach.  Once again, observe that $_ is the first item inside the curly brackets, but this time the underscore is followed by the -replace parameter rather than a .property.

The purpose of this script is to remove and duplicate "the the" in documents stored in the ProofRead folder.

Clear-Host
$file = gci "D:\ProofRead\*.doc"
$file
ForEach ($str in $file)
{
$cont = Get-Content -path $str
$cont
$cont | ForEach {$_ -replace "the , "the"} | Set-Content $str
}

Note 4: In the ForEach example the syntax is purely $_ there is no (.) property.

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More of PowerShell's Built-In Variables

You can enumerate PowerShell's variables with this command:

Get-Variable | Format-Table name, value -auto

Variable Name Description
$_ The current pipeline object; used in script blocks, filters, the process clause of functions, where-object, ForEach-object and switch
$Args  Used in creating functions that require parameters
$Env:Path Environmental Path to files.
$Error  If an error occurred, the object is saved in the $error PowerShell variable
$ForEach Refers to the enumerator in a ForEach loop.
$HOME The user's home directory; set to %HOMEDRIVE%\%HOMEPATH%
$Input Input piped to a function or code block
$Match A hash table consisting of items found by the -Match operator.
$Host Information about the currently executing host
$LastExitCode The exit code of the last native application to run
$true Boolean TRUE
$false Boolean FALSE
$null A null object
$ShellID The identifier for the shell.  This value is used by the shell to determine the ExecutionPolicy and what profiles are run at startup.
$StackTrace  contains detailed stack trace information about the last error

See also PowerShell's Get-PSProvider ยป

Summary of PowerShell $_ Variable

Perhaps the key to understanding this construction is to look at what follows $_.  For example, $_.Name leads us to think, 'Name is a property in the current ScriptBlock, or PowerShell pipeline.  Further reflection shows that $_ saves scripting space by substituting two characters for a whole extra construction.

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See more Windows PowerShell  examples of variables

Syntax   • PowerShell Variables   • Get-PSProvider   • PowerShell Dollar Variable

PowerShell Functions   • Get-PSDrive   • PowerShell New-PSDrive   • Remove-PSDrive

PowerShell Home   • Foreach loops   • PowerShell Foreach   • Foreach-Object cmdlet

Please email me if you have a better example script.  Also please report any factual mistakes, grammatical errors or broken links, I will be happy to  correct the fault.

 

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