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Windows PowerShell $_. Variable

Introduction to the PowerShell Dollar 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

• PowerShell's $_ Variable
• The Significance of the Dot in PowerShell's $_.
• $_ Example to Filter WmiObjects
• PowerShell $_ with Foreach  $_.
• More PowerShell Variables

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PowerShell's $_ Variable

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

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 $_.

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

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.

Note:  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.

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.

Note 1: 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.

Note 2:  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.

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 it's followed by the -replace parameter rather than a .property.

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

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More of PowerShell's Built-In Variables

You can enumerate PowerShell's variables with this command:

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.

See more PowerShell examples

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

• Syntax   • Variables   • -whatIf   • -ErrorAction   • Windows PowerShell   • PowerShell 2.0

• PowerShell Functions   • [System.Math]   • Get-Credential   • Windows 7 PowerShell 2.0

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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Author: Guy Thomas Copyright © 1999-2012 Computer Performance LTD All rights reserved. Please report a broken link, or an error to: