Lines like $MyInvocation.MyCommand.Definition can be useful to determine the folder in which the current script is stored, i.e. to access other resources located in the same folder.

However, ever since PowerShell 3, there have been easy alternatives to find out the current script name and/or the path to the folder that contains the current script. Run the following code to test for yourself:

$MyInvocation.MyCommand.Definition
$PSCommandPath

Split-Path -Path $MyInvocation.MyCommand.Definition
$PSScriptRoot

If you run these lines interactively (or in an “Untitled” script), they all return nothing. Once you save the script and run the script file, though, the first two lines return the script file path, and the second two lines return the folder path in which the script is located.

The good thing about $PSCommandPath and $PSScriptRoot is that they always contain the same information. $MyInvocation, in contrast, can change, and will change once this variable is read from within a function:

function test
{
    $MyInvocation.MyCommand.Definition
    $PSCommandPath

    Split-Path -Path $MyInvocation.MyCommand.Definition
    $PSScriptRoot
}

test 

Now, $MyInvocation turns out to be useless because it always returns information about who invoked the current script block.

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Anonymous