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PowerShell Compare-Object: Find Missing and Changed Files Between Two Folders

#powershell #scripting #windows #beginners

You copied a folder to a backup drive, a USB stick, or a new laptop. Now you need to know one thing: is the copy actually the same? Not "roughly the same size". The same files, with the same content.

Windows Explorer can't answer that. Compare-Object plus Get-FileHash can, in a short script you can rerun any time.

The one-liner: which file names differ?

Start cheap. Compare just the file names in two folders:

Compare-Object (Get-ChildItem C:\Photos -Name) (Get-ChildItem D:\Backup\Photos -Name)
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InputObject        SideIndicator
-----------        -------------
IMG_2041.jpg       =>
IMG_1987.jpg       <=
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Reading the SideIndicator column:

  • <= means the item exists only in the first (reference) set, here C:\Photos.
  • => means it exists only in the second (difference) set, the backup.
  • Matching items are hidden. Add -IncludeEqual to see them marked ==.

So IMG_1987.jpg never made it to the backup, and IMG_2041.jpg is only on the backup.

That catches missing files. It does not catch a file that exists on both sides but has different content, like a half-copied video or a document you edited after the backup. For that you need hashes.

The real job: compare names and content, recursively

This script builds a fingerprint for every file (relative path plus SHA256 hash), then compares the two lists:

$left  = (Resolve-Path 'C:\Photos').Path
$right = (Resolve-Path 'D:\Backup\Photos').Path

function Get-FileFingerprint {
    param([string]$Root)
    Get-ChildItem -LiteralPath $Root -Recurse -File | ForEach-Object {
        [pscustomobject]@{
            RelPath = $_.FullName.Substring($Root.Length).TrimStart('\')
            Hash    = (Get-FileHash -LiteralPath $_.FullName -Algorithm SHA256).Hash
        }
    }
}

$a = @(Get-FileFingerprint -Root $left)
$b = @(Get-FileFingerprint -Root $right)

Compare-Object -ReferenceObject $a -DifferenceObject $b -Property RelPath, Hash |
    Sort-Object RelPath
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Why each piece matters:

  • RelPath strips the root, so C:\Photos\2025\a.jpg and D:\Backup\Photos\2025\a.jpg both become 2025\a.jpg and can actually match.
  • -Property RelPath, Hash tells Compare-Object an item is only "equal" when both the path and the content hash match.
  • @(...) forces an array even when a folder is empty. More on that below.

Identical files produce no output. What's left is the interesting part:

RelPath                Hash          SideIndicator
-------                ----          -------------
2025\IMG_1987.jpg      7427D152...   <=
2025\trip.mp4          3BB2ABB6...   <=
2025\trip.mp4          73CB3858...   =>
IMG_2041.jpg           A4FB6214...   =>
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Notice trip.mp4 shows up twice, once per side with different hashes. That's the signature of a file that exists in both places but changed.

Turn it into a readable report

Two rows per changed file is accurate but annoying. Group by path and label each case:

Compare-Object $a $b -Property RelPath, Hash |
    Group-Object -Property RelPath |
    ForEach-Object {
        $status = if ($_.Count -eq 2) { 'Changed' }
                  elseif ($_.Group[0].SideIndicator -eq '<=') { 'Missing in backup' }
                  else { 'Only in backup' }
        [pscustomobject]@{ Status = $status; File = $_.Name }
    } |
    Sort-Object Status, File
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Status             File
------             ----
Changed            2025\trip.mp4
Missing in backup  2025\IMG_1987.jpg
Only in backup     IMG_2041.jpg
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Need to send it to someone or keep a record? Pipe the same result to Export-Csv:

... | Export-Csv -Path .\backup-diff.csv -NoTypeInformation
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An empty report means the two folders match, file for file and byte for byte.

Gotchas that bite on real folders

1. An empty folder throws a terminating error. If Get-ChildItem finds nothing, it returns $null, and Compare-Object refuses it:

Cannot bind argument to parameter 'ReferenceObject' because it is null.
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Wrapping the call in @(...) gives you an empty array instead, and the comparison simply reports every file on the other side as missing. That's why the script above uses $a = @(...).

2. Comparison is case-insensitive by default. Report.docx and report.docx count as the same name, which matches how NTFS behaves. If you're comparing against a Linux share or a Git checkout where case matters, add -CaseSensitive.

3. Hashing big folders is slow, so filter first. Hashing reads every byte. For a 200 GB media folder, compare cheap properties first and only hash what's left:

function Get-QuickFingerprint {
    param([string]$Root)
    Get-ChildItem -LiteralPath $Root -Recurse -File |
        Select-Object @{ n = 'RelPath'; e = { $_.FullName.Substring($Root.Length).TrimStart('\') } }, Length
}

Compare-Object @(Get-QuickFingerprint $left) @(Get-QuickFingerprint $right) -Property RelPath, Length
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A different Length proves a file changed. The same length doesn't prove it's identical, so run the hash version when you need certainty.

4. Don't compare timestamps to verify a copy. CreationTime resets when a file is copied, and not every copy tool or sync client preserves LastWriteTime, so a time-based diff can flag thousands of false "changes". Content hashes don't care how the file got there.

5. It's not a line-by-line text diff. Compare-Object (Get-Content a.txt) (Get-Content b.txt) tells you which lines exist on only one side. It won't give line numbers, and a line that merely moved won't be reported. For code, use git diff. For "did this config gain or lose a line?", it's perfect.

Quick reference

Goal Command
Names only Compare-Object (gci A -Name) (gci B -Name)
Show matches too add -IncludeEqual
Compare on chosen properties -Property RelPath, Hash
Case-sensitive names add -CaseSensitive
Content check Get-FileHash -LiteralPath $file -Algorithm SHA256
Save report `\

Practice it before you need it

The time you most need this is right after a backup or migration, which is the worst time to learn the syntax. If you want to get the pipeline habit into your fingers first, practice PowerShell pipelines like Get-ChildItem and Compare-Object in a free browser terminal on CMD Master. No install, and nothing on your real disk gets touched.

Official docs

Related in this series: sort files by size, date, or name with Sort-Object and mirror a folder safely with Robocopy.

Learning Windows command line from scratch? CMD Master's interactive CMD, PowerShell, and Bash lessons are free and run entirely in your browser.

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