a script that create a *.bak file for target file

bak.ps1

param(
    [Parameter(Position=0, mandatory=$true)]
    [string] $Path
)

function RenameBak {
    param (
        [string] $Path
    )

    if (Test-Path $Path) {
        $BakPath = "$Path.bak"
        RenameBak $BakPath
        Rename-Item -NewName $BakPath $Path
    }
}

function MakeBak {
    param (
        [string] $Path
    )

    $BakPath = "$Path.bak"
    RenameBak $BakPath
    Copy-Item -Path $Path -Destination $BakPath -Recurse
}

MakeBak $Path

rbak.ps1

param(
    [Parameter(Position=0, mandatory=$true)]
    [string] $Path
)

$BakPath = "$Path.bak"

if (Test-Path $BakPath) {
    Remove-Item $Path -Recurse
    Rename-Item -NewName $Path $BakPath
}