[powershell] What does $_ mean in PowerShell?

I've seen the following a lot in PowerShell, but what does it do exactly?

$_

$_ is an variable which iterates over each object/element passed from the previous | (pipe).

$_ is a variable created by the system usually inside block expressions that are referenced by cmdlets that are used with pipe such as Where-Object and ForEach-Object.

But it can be used also in other types of expressions, for example with Select-Object combined with expression properties. Get-ChildItem | Select-Object @{Name="Name";Expression={$_.Name}}. In this case the $_ represents the item being piped but multiple expressions can exist.

It can also be referenced by custom parameter validation, where a script block is used to validate a value. In this case the $_ represents the parameter value as received from the invocation.

The closest analogy to c# and java is the lamda expression. If you break down powershell to basics then everything is a script block including a script file a, functions and cmdlets. You can define your own parameters but in some occasions one is created by the system for you that represents the input item to process/evaluate. In those situations the automatic variable is $_.

$_ is an alias for automatic variable $PSItem (introduced in PowerShell V3.0; Usage information found here) which represents the current item from the pipe.

PowerShell (v6.0) online documentation for automatic variables is here.

I think the easiest way to think about this variable like input parameter in lambda expression in C#. I.e. $_ is similar to x in x => Console.WriteLine(x) anonymous function in C#. Consider following examples:

PowerShell:

1,2,3 | ForEach-Object {Write-Host $_}

Prints:

1
2
3

or

1,2,3 | Where-Object {$_ -gt 1}

Prints:

2
3

And compare this with C# syntax using LINQ:

var list = new List<int> { 1, 2, 3 };
list.ForEach( _ => Console.WriteLine( _ ));

Prints:

1
2
3

or

list.Where( _ => _ > 1)
    .ToList()
    .ForEach(s => Console.WriteLine(s));

Prints:

2
3

According to this website, it's a reference to this, mostly in loops.

$_ (dollar underscore) 'THIS' token. Typically refers to the item inside a foreach loop. Task: Print all items in a collection. Solution. ... | foreach { Write-Host $_ }