Test-Path - Variable Part of FileName - Variable doesn't get substituted - powershell

Question

SourceFile.txt has the below
<somepath>\filename_$tminus1.csv
The below is the script I've written.
$date = [DateTime]::ParseExact("21-Aug-18", "dd-MMM-yy", $null)
if ($date.DayOfWeek -eq "Monday") {
$tminus1 = $date.AddDays(-3).ToString('yyyyMMdd')
$tminus2 = $date.AddDays(-4).ToString('yyyyMMdd')
Write-Host "Date vaulues set for Monday"
} else {
$tminus1 = $date.AddDays(-1).ToString('yyyyMMdd')
$tminus2 = $date.AddDays(-2).ToString('yyyyMMdd')
Write-Host "Date vaulues set for Non-Monday"
}
$files = Get-Content -Path $PSScriptRoot\SourceFile.txt
foreach ($file in $files) {
Convert-Path $file
if (-not (Test-Path $file)) {
echo ""
Write-Host "$file doesn't exist"
} else {
echo ""
Write-Host "$file exists"
}
}
The Test-Path cmdlet doesn't get the variable value substituted while looking for the file. It prints the below.
<somepath>\filename_$tminus1.csv doesn't exist
I expect the $tminus1 value gets substituted as 20180823 so that the else part prints the below.
<somepath>\filename_$tminus1.csv exists

Answer 1

The problem is when you pull the text in from your CSV it is treated as a literal, as if it as was a single quoted string. You can force PowerShell to re-evalueate the string like so:
$file = $ExecutionContext.InvokeCommand.ExpandString($file)
Or an alternative method would be to change the text in the CSV to this:
<somepath>\filename_{0}.csv
You can then use the format string to apply the variable like so:
$file = $file -f $tminus1

Answer 2

When you read strings from a file, PowerShell does not perform the normal interpolation it does for strings in your code. One way to get around this is to use Invoke-Expression.
Say you have a file (paths.txt) with these paths in it:
C:\Data\$dir1
C:\Data\$dir2
You can get the real paths like this:
$dir1 = "Directory1"
$dir2 = "Directory2"
Get-Content paths.txt |
ForEach-Object {
Invoke-Expression """$_"""
}
This will output:
C:\Data\Directory1
C:\Data\Directory2
Obviously, you can capture these to a variable, send them along the pipeline, etc, instead of just outputting them.