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PowerShell Compare-Object

Compare-Object to Find the Differences Between Files

Scenario: I had a folder of files.  Many of those files were missing a phrase, I wanted a list of files with these deficiencies.  Stage 1 was to create a text file containing a directory listing.  Stage 2 was to create another file listing only those files containing a particular phrase.  Stage 3 was to employ Compare-Object, process the two files, and then create a third file with a list of all the files that were missing my particular phrase.

Topics for PowerShell Compare-Object Cmdlet

• Mission to Generate a List of Differences
• Example 1: To Compare Two Files, and List Differences
• Example 2: To Compare Two Files, with -Parameters
• Example 3: How to Research Windows Services
• Compare-Object - PowerShell Diff Alias
• Summary of Compare-Object

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Mission to Generate a List of Differences

In this mission I had a directory of files.  Some of those files were missing a phrase, I wanted a list of files with these deficiencies.  Stage one was to create a text file containing a directory listing.  Stage two was to create another file listing only those files containing a particular phrase.  Stage three was to launch Windows PowerShell and then call for Compare-Object.  Using this technique you can process the two files and create a third file with a list of all the files that were missing my particular phrase.  In these ezine I just want to focus on stage three, generating a list of differences.

Preparation: Use PowerShell's Built-In Help

This script is a useful reminder to see Compare-Object's parameters.

Note 1:  With the Compare-Object cmdlet, there are two required parameters -referenceObject and -differenceObject.  For most examples -referenceObject controls the master content, the file which holds ALL the information, while -differenceObject has the secondary or 'after the event' file.   Also note what help says about these parameters, what we learn is that that the position of these parameters is important, reference list first, difference list second.

Note 2:  In a production script we employ Get-Content to open the file, read the text and assign the content to a variable.  For example $strReference = Get-Content "c:\scripts\list.txt"

Note 3: Please investigate more parameters such as -IncludeEqual and -syncWindow.  Incidentally, there is a PowerShell diff alias.

Example 1: To Compare Two Files, and List Their Differences

For the sake of clarity, and so that we focus on Compare-Object, I have scaled down our mission to tackle only comparing two files.  The best way to understand this example is if you pretend that you have a shopping list.

Preparation
We need two files, the first is for reference and the second for comparison.  Let us use a shopping list for our test scenario.  The file called ShopList.txt is a list of goods we need to buy at the supermarket.  Checkout.txt, is a list of what we actually purchased.

ShopList: Save these 10 items into a text file, I called mine C:\temp\ShopList.txt

Oranges
Bananas
Cucumbers
Beer
Burgers
Onions
Sausages
Steak
Chips

Checkout: Save these 7 items into a text file, I called mine C:\temp\Checkout.txt

Oranges
Bananas
Beer
Sausages
Steak
Chips
Ice-Cream

Now we are ready to execute this Compare-Object script.  My favoured method is to save these instructions into a cmdlet with a .ps1 extension, then call the file from the PowerShell command line.  Alternatively, copy the following lines and then paste into the command line.

Note 1: You could add the explicit parameters: -referenceObject -differenceObject and thus substitute this for the last line of Example 1:
Compare-Object -referenceObject $strReference -differenceObject $strDifference

The reason that I omit these positional parameters is that PowerShell can deduce what to compare from the sequence.

Note 2: If your script fails, then check a) The names of the two files, b) They are in the folder referenced by your script.

Challenge 1: You could add the -IncludeEqual parameter and thus display all the items that you bought that were on the original list.

SideIndicator

=> Means the InputObject is present in the difference (second) file, but not in the first file.  In this scenario it means we bought stuff, which was not on the original list.

<= Present in reference (first) file, but not in the second file.  In my analogy, we forgot to buy this item on our shopping list.

== Present in both files.  Try my challenge and add the -IncludeEqual parameter, then you will see this double-Equals underneath 'SideIndicator' in the output.

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Example 2: To Compare Two Files, with -Parameters

Note 1: Observe how Compare-Object relies on Get-Content to provide the stream of data

Note 2: Even though it's not necessary with such a short list, I have included my favourite switch - syncWindow because this is how to find extra differences.  The default for -syncWindows is 5.

Because the resulting command would not fit on one line, added the tiny backtick ` to tell PowerShell that the command wrapped onto the next line.

Note 3: You can use the PowerShell alias diff instead of Compare-Object.

See more about PowerShell Parameters

Example 3: How to Research Windows Services

The purpose of this example is to give you ideas for using Compare-Object on a production server.

One classic task for Compare-Object is to investigate Windows services, for example, generate a list of services that have stopped.  One complication is that firstly we need to master Get-WmiObject win32_service.  The second complication is that you need to have objectives and data; for example, a list services that were running yesterday, but have now stopped.  A solution would be to take regular snap-shots of which services are running, save the data into a file, then use Compare-Object to highlight differences, i.e. services that are no longer running.

This mini-project illustrates two factors about scripting in general and PowerShell in particular.  Firstly build-up your scripts gradually, secondly, there are always at least three ways of achieving any scripting goal, in this instance, Get-WmiObject win32_service may have enough properties and methods for you not to need Compare-Object at all.

Note 4:  Examine the file referenced by $strPath, Excel would be an ideal program to open this csv file.

Note 5:  Research deeper with Get-Member: for example:
Get-WmiObject win32_service |gm

A Big and Advanced Challenge:  I really have left a big task for you apply what you have learned in Example 1, and apply the technique to Windows services.  If you accept this challenge you need to create a difference file, then stop one or more services, finally, run Example 1, but with a modified $strReference and $strDifference.

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 Compare-Object - PowerShell Diff Alias

Compare-Object has two aliases, Diff and Compare.

Note 6: Expected result
Alias Diff
Alias Compare
See more aliases in PowerShell v3.

Summary of Compare-Object

If you have a scenario where you want to automate processing of two files, and come up with a list of differences; then research Compare-Object and its parameters.  Incidentally, you could use the PowerShell Alias 'Diff'.

See more Microsoft PowerShell file tutorials:

• PowerShell Home   • Add-Content   • Get-Content   • Set-Content  • PowerShell -Filter   • Test-Path

• PowerShell Get-ChildItem   • Get-ChildItem -Include   • Get-ChildItem -Exclude   • Compare-Object

• PowerShell Registry  • Get-Credential  • PowerShell ItemProperty  • PowerShell ItemPropery GCI

Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.

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