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
- PowerShell’s $_ Variable
- The Significance of the Dot in PowerShell’s $_.
- $_ Example to Filter WmiObjects
- PowerShell $_ with ForEach $_.
- More PowerShell Variables
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 $_.
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 »
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.
