You want to compare the output of two commands.

To compare the output of two commands, store the output of each command in variables, and then use the Compare-Object cmdlet to compare those variables:

PS > notepad
PS > $processes = Get-Process
PS > Stop-Process -ProcessName Notepad
PS > $newProcesses = Get-Process
PS > Compare-Object $processes $newProcesses

InputObject                           SideIndicator
-----------                           -------------
System.Diagnostics.Process (notepad)  <=

The solution shows how to determine which processes have exited between the two calls to Get-Process. The SideIndicator of <= tells us that the process was present in the left collection ($processes) but not in the right ($newProcesses). To work with the actual object that was different, access the InputObject property:

PS > $diff = @(Compare-Object $processes $newProcesses)[0]
PS > $process = $diff.InputObject
PS > $process.Handles
55

By default, the Compare-Object cmdlet uses the comparison functionality built into most .NET objects. This works as expected most of the time, but some times you might want to override that comparison behavior. In the example above, for example, you might want two processes to be considered different if their memory usage changes. In that case, use the -Property parameter.

PS > Compare-Object $processes $newProcesses -Property Name,WS | Sort Name

Name                                           WS SideIndicator
----                                           -- -------------
dwm                                      31358976 <=
dwm                                      29540352 =>
explorer                                 37969920 <=
explorer                                 38023168 =>
lsass                                     1548288 =>
lsass                                     1372160 <=
notepad                                   5701632 <=
notepad                                   2891776 =>
powershell                               44281856 =>
powershell                               44290048 <=
SearchIndexer                            13606912 =>
SearchIndexer                            13619200 <=
svchost                                  56061952 <=
svchost                                  43982848 <=
svchost                                  56037376 =>
svchost                                  44048384 =>
svchost                                  12193792 <=
svchost                                  12201984 =>
taskeng                                   9220096 <=
taskeng                                   9228288 =>

When you use the -Property parameter, the Compare-Object cmdlet outputs custom objects that have only the properties you used in the comparison. If you still want access to the original objects used in the comparison, also use the -PassThru parameter. In that case, PowerShell instead adds the SideIndicator property to the original objects.

For more information about the Compare-Object cmdlet, type Get-Help Compare-Object.

You want to determine the differences between two files.

To determine simple differences in the content of each file, store their content in variables, and then use the Compare-Object cmdlet to compare those variables:

PS > "Hello World" > c:\temp\file1.txt
PS > "Hello World" > c:\temp\file2.txt
PS > "More Information" >> c:\temp\file2.txt
PS > $content1 = Get-Content c:\temp\file1.txt
PS > $content2 = Get-Content c:\temp\file2.txt
PS > Compare-Object $content1 $content2

InputObject                              SideIndicator
-----------                              -------------
More Information                         =>

The primary focus of the Compare-Object cmdlet is to compare two unordered sets of objects. Although those sets of objects can be strings (as in the content of two files), the output of Compare-Object when run against files is usually counterintuitive due to the content losing its order.

When comparing large files (or files where the order of comparison matters), you can still use traditional file comparison tools such as diff.exe or the WinDiff application that comes with both the Windows Support Tools and Visual Studio.

For more information about the Compare-Object cmdlet, type Get-Help Compare-Object.

You want to determine whether any files have been modified or damaged in a set of files.

To verify the integrity of file sets, use the Get-FileHash script provided in the section called “Program: Get the MD5 or SHA1 Hash of a File” to generate the signatures of those files in question. Do the same for the files on a known good system. Finally, use the Compare-Object cmdlet to compare those two sets.

To generate the information from the files in question, use a command like:

dir C:\Windows\System32\WindowsPowerShell\v1.0 | Get-FileHash |
    Export-CliXml c:\temp\PowerShellHashes.clixml

This command gets the hash values of the files from C:\Windows\System32\ WindowsPowerShell\v1.0, and uses the Export-CliXml cmdlet to store that data in a file.

Transport this file to a system with files in a known good state, and then import the data from that file.

$otherHashes = Import-CliXml c:\temp\PowerShellHashes.clixml

net use x: \\lee-desk\c$\Windows\System32\WindowsPowerShell\v1.0
$otherHashes = dir x: | Get-FileHash

Generate the information from the files you know are in a good state:

$knownHashes = dir C:\Windows\System32\WindowsPowerShell\v1.0 |
    Get-FileHash

Finally, use the Compare-Object cmdlet to detect any differences:

Compare-Object $otherHashes $knownHashes -Property Path,HashValue

If there are any differences, the Compare-Object cmdlet displays them in a list, as shown in Example 22.1, “The Compare-Object cmdlet showing differences between two files”.

PS > Compare-Object $otherHashes $knownHashes -Property Path,HashValue
Path                     HashValue                SideIndicator
----                     ---------                -----------------------
system.management.aut... 247F291CCDA8E669FF9FA... =>
system.management.aut... 5A68BC5819E29B8E3648F... <=


PS > Compare-Object $otherHashes $knownHashes -Property Path,HashValue |
>> Select-Object Path
>>

Path
----
system.management.automation.dll-help.xml
system.management.automation.dll-help.xml

For more information about the Compare-Object cmdlet, type Get-Help Compare-Object. For more information about the Export-CliXml and Import-CliXml cmdlets, type Get-Help Export-CliXml and Get-Help Import-CliXml, respectively.