Powershell – Compare-Object gives false differences

powershell

I have some problem with Compare-Object. My task is to get difference
between two directory snapshots made at different times. First
snapshot is taken like this:

ls -recurse d:\dir | export-clixml dir-20100129.xml

Then, later, I get second snapshot and load both of them:

$b = (import-clixml dir-20100130.xml) 
$a = (import-clixml dir-20100129.xml)

Next, I'm trying to compare with Compare-Object, like that:

diff $a $b

What I get is in some places files that were added to $b since $a, but
in some — files that were in both snapshots, and some files, that
were added to $b, are not given in Compare-Object output. Puzzling,
but $b.count – $a.count is EXACTLY the same as (diff $a $b).count. Why
is that?

Ok, Compare-Object has -property param. I try to use that:

diff -property fullname $a $b

And I get the whole mess of differences: it shows me ALL the files.
For example, say $a contains:

A\1.txt 
A\2.txt 
A\3.txt

And $b contains:

X\2.mp3 
X\3.mp3 
X\4.mp3 
A\1.txt 
A\2.txt 
A\3.txt

diff output is something like that:

X\2.mp3 => 
A\1.txt <= 
X\3.mp3 => 
A\2.txt <= 
X\4.mp3 => 
A\3.txt <= 
A\1.txt => 
A\2.txt => 
A\3.txt =>

Weird. I think I don't understand something crucial about Compare-
Object usage, and manuals are scarce… Please, help me to get the
DIFFERENCE between two directory snapshots. Thanks in advance

UPDATE: I've saved data as plain strings like that:

> import-clixml dir-20100129.xml | % { $_.fullname } | out-file -enc utf8 a.txt

And results are the same. Here're excerpts of both snapshots (top 100-something lines, a.txt and b.txt), output of compare-object, and output of UNIX diff (unified). All files are UTF-8: http://dl.dropbox.com/u/2873752/compare-object-problem.zip

Best Answer

One can solve the problem by using -SyncWindow parameter. The value should be half a length of a smallest collection in comparison:

> Compare-Object -SyncWindow ($a.length / 2) $a $b

Source: http://dmitrysotnikov.wordpress.com/2008/06/06/compare-object-gotcha/

Related Solutions

Powershell – Comparing two specific properties of a CSV using Compare-Object isn’t giving the expected results

Given the example CSV file "domain1.xxx.org.csv", with contents as:

"name","samaccountname","description","distinguishedname","enabled","lastlogondate" 
"ADAUser01","ADAUser01",,"CN=ADAUser01,OU=Users,OU=ADA,DC=phl,DC=xxx,DC=ORG","True","8/7/2012 2:28:37 PM"

We can see how Import-Csv converts the text into a PSCustomObject using the headings:

Import-Csv domain1.xxx.org.csv | Select-Object -First 1 | Get-Member

   TypeName: Selected.System.Management.Automation.PSCustomObject

Name              MemberType   Definition
----              ----------   ----------
Equals            Method       bool Equals(System.Object obj)
GetHashCode       Method       int GetHashCode()
GetType           Method       type GetType()
ToString          Method       string ToString()
description       NoteProperty System.String description=
distinguishedname NoteProperty System.String distinguishedname=CN=ADAUser01,OU=Users,OU=ADA,DC=phl,DC=xxx,DC=ORG
enabled           NoteProperty System.String enabled=True
lastlogondate     NoteProperty System.String lastlogondate=8/7/2012 2:28:37 PM
name              NoteProperty System.String name=ADAUser01
samaccountname    NoteProperty System.String samaccountname=ADAUser01

Filtering this via Select-Object SAMAccountName results in the following:

Import-Csv domain1.xxx.org.csv | Select-Object -First 1 SamAccountName

   TypeName: Selected.System.Management.Automation.PSCustomObject

Name           MemberType   Definition
----           ----------   ----------
Equals         Method       bool Equals(System.Object obj)
GetHashCode    Method       int GetHashCode()
GetType        Method       type GetType()
ToString       Method       string ToString()
samaccountname NoteProperty System.String samaccountname=ADAUser01

If you want to compare the NoteProperty across two PSCustomObjects, because we're not just evaluating a String, you'll just need to tell Compare-Object which Property you want to compare with:

Compare-Object $domain1 $domain2 -Property SamAccountName

Powershell – How to export the results of the script to file

Here's a little function called Out-ExcelReport. Just pipe anything to it, and it will open in Microsoft Excel - in case it is installed on your system:

function Out-ExcelReport
{
  param
  (
    $Path = "$env:temp\$(Get-Random).csv"
  )

  $Input | Export-Csv -Path $Path -Encoding UTF8 -NoTypeInformation -UseCulture
  Invoke-Item -Path $Path
}
Get-Process | Out-ExcelReport

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