PowerShell - parse beginning characters out of filenames, so they can be compared properly

Question

Sample of file names in our naming convention using PowerShell:

SYSTEM_20201019-01_PRE folder

• SYSTEM_20201118_file1.txt
• SYSTEM_20201118_file2.csv
• SYSTEM_20201118_file3_and_file4.txt

SYSTEM_20201019-01_POST folder

• SYSTEM_20201119_file1.txt
• SYSTEM_20201119_file2.csv
• SYSTEM_20201119_file3_and_file4.txt

We use this convention and it worked well as simply "SYSTEM_file1.txt" before we added the date. If we include the date in there, our script will only compare them if the date matches (which makes sense). This works mostly because the filenames are typically the same in the before and after. However, sometimes the dates will change and we simply have to manually rename all the files in the before, and then it works properly. We want to alleviate this manual work for others.

Is there a way I can take the PRE and POST and redefine them parsing out the first XX characters to only compare based on the name after the second _ character? The filenames, filename lengths, and extensions vary so I need to start from the beginning and not the end.

Here's the full code:

if($changeStatus -eq "POST"){

Write-Host "`n`n*********************************************"
Write-Host "`nComparing all PRE/POST files for differences."

$folderPath = $changeID + "_" + $env:COMPUTERNAME + "_"
$postFolderPath = $folderPath + "POST"
$preFolderPath = $folderPath + "PRE"
$comparisonFile = "$postFolderPath\Output_Comparison.txt"

if(!(Test-Path $preFolderPath)){
Write-Host "Missing PRE folder with the associated ChangeID. Unable to perform comparison."
Write-Host "Press any key to exit......"
Read-Host
exit
}

else{

$fileName = Get-ChildItem -Path $postFolderPath | Select-Object -ExpandProperty Name

ForEach ($file in $fileName){

$preFile = Get-Content ($preFolderPath + "\" + $file)
$postFile = Get-Content ($postFolderPath + "\" + $file)
$comparisonOutput = Compare-Object $preFile $postFile | Where-Object {($_.SideIndicator -eq "=>") -and (($_.InputObject -replace '"',"") -ne $dateRan)}  | Select-Object -ExpandProperty InputObject

if($comparisonOutput -ne $null){
    if(Test-Path $comparisonFile){
        "`r`n`r`nDifference in file $file" | Add-Content -Path $comparisonFile
        $comparisonOutput | Add-Content -Path $comparisonFile
    }

    else{
        "The following are the diffences found in POST files:" | Out-File -FilePath $comparisonFile
        "`r`nDifference in file $file" | Add-Content -Path $comparisonFile
        $comparisonOutput | Add-Content -Path $comparisonFile

    }
}

}


} 


}

---

Answer 1

You are not showing any code, and you should, in order to avoid assumptions and guessing from us and better get guidance from us here; but if I get your use case, there are several ways to address this.

# Break the string into an array
'SYSTEM_20201118_file1.txt'.Split('_')
# Results
<#
SYSTEM
20201118
file1.txt
#>

# Grab the first array item and change it, then join them back together
'SYSTEM_20201118_file1.txt'.Split('_')[0]
# Results
<#
SYSTEM
#>

# Or just replace a specific string
'SYSTEM_20201118_file1.txt' -replace 'SY','SX'
# Results
<#
SXSTEM_20201118_file1.txt
#>

Update as per your stated comment below

Refactor without using split but regex instead.

Clear-Host
(Get-ChildItem -Path 'D:\Temp' -Filter 'System_*').FullName
# Results
<#
D:\Temp\SYSTEM_20201119_file1.txt
D:\Temp\SYSTEM_20201119_file2.csv
D:\Temp\SYSTEM_20201119_file3_and_file4.txt
#>

Clear-Host
Get-ChildItem -Path 'D:\Temp' -Filter 'System_*' |
# Use regex to get the last the string after the last underscore 
ForEach-Object {[regex]::Matches($PSItem.BaseName, '[^_]+$').Value}
# Results
<#
file1
file2
file4
#>

Then do as you wish with those results.