What does $_ mean in PowerShell?

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

$_

Solution 1:

This is the variable for the current value in the pipe line, which is called $PSItem in Powershell 3 and newer.

1,2,3 | %{ write-host $_ } 

or

1,2,3 | %{ write-host $PSItem } 

For example in the above code the %{} block is called for every value in the array. The $_ or $PSItem variable will contain the current value.

Solution 2:

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

Solution 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 $_ }

Solution 4:

$_ 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.