Multiple variables in path - powershell

Question

I am planning on using the following script by looping through a text file to set variables for the location of the source PDF, and designate the path to create a new folder (with week number) to move the source PDF to.
$pdfSource = 'C:\path\in\text\file'
$newFolder = 'C:\path\to\newfolder\in\text\file'
Get-ChildItem $pdfSource '*.pdf' -Recurse | foreach {
$x = $_.LastWriteTime.ToShortDateString()
$new_folder_name = Get-Date $x -UFormat %V
$des_path = "C:\path\to\newfolder\$new_folder_name"
if (Test-Path $des_path) {
Move-Item $_.FullName $des_path
} else {
New-Item -ItemType Directory -Path $des_path
Move-Item $_.FullName $des_path
}
}
I can't seem to figure out the syntax for the line below to include the $newFolder path variable along with the existing $new_folder_name I'm creating.
$des_path = "C:\path\to\newfolder\$new_folder_name"

Answer 1

Option-1:
$des_path = "${newFolder}\${new_folder_name}"
Option-2:
$des_path = "${0}\${1}" -f $newFolder, $new_folder_name
Option-3:
$des_path = $newFolder + $new_folder_name
Option-4:
$des_path = Join-Path -Path $newFolder -ChildPath $new_folder_name

Answer 2

There's nothing wrong with your approach to string expansion (interpolation):
$new_folder_name = 'foo' # sample value
$des_path = "C:\path\to\newfolder\$new_folder_name" # use string expansion
yields string literal C:\path\to\newfolder\foo, as expected.
Adam's answer shows you alternatives to constructing file paths, with Join-Path being the most robust and PowerShell-idiomatic, albeit slow.
Another option is to use [IO.Path]::Combine():
[IO.Path]::Combine('C:\path\to\newfolder', $new_folder_name)
The way you calculate the value for $new_folder_name should be problematic if your current culture is not en-US (US-English), but due to a bug actually isn't[1]; either way, it should be simplified:
Instead of:
$x = $_.LastWriteTime.ToShortDateString()
$new_folder_name = Get-Date $x -uformat %V
use:
$new_folder_name = Get-Date $_.LastWriteTime -uformat %V
That is, pass $_.LastWriteTime directly to Get-Date, as a [datetime] instance - no detour via a string representation needed.
[1] .ToShortDateString() returns a culture-sensitive string representation, whereas PowerShell typically uses the invariant culture to ensure cross-culture consistency; therefore, if you pass a string to a parameter that accepts a [datetime] instance, it is the invariant culture's formats that should (only) be recognized, not the current culture's. While that is true for functions written in PowerShell, in compiled cmdlets (typically C#-based), the current culture is unexpectedly applied; while this is a bug, a decision was made not to fix it for the sake of backward compatibility - see this GitHub issue