How to Compare Two Arrays in PowerShell (Matches and Differences)

To compare two arrays in PowerShell, use Compare-Object. It lists the items that are only in one array: <= means only in the first (reference) array, and => only in the second:

$approved  = 'WEB01', 'WEB02', 'SQL01'
$installed = 'web02', 'SQL01', 'FILE01'

Compare-Object -ReferenceObject $approved -DifferenceObject $installed

Output:

InputObject SideIndicator
----------- -------------
FILE01      =>
WEB01       <=

WEB02 and web02 matched, because Compare-Object ignores case and order. I ran every example in PowerShell 7.6 and Windows PowerShell 5.1, with the same results.

Find matches and missing items

Add -IncludeEqual -ExcludeDifferent to list the matches. For “what’s missing”, a -notin filter reads more clearly than side indicators:

$approved  = 'WEB01', 'WEB02', 'SQL01'
$installed = 'web02', 'SQL01', 'FILE01'

"In both:              $((Compare-Object $approved $installed -IncludeEqual -ExcludeDifferent).InputObject -join ', ')"
"Missing from install: $(($approved | Where-Object { $_ -notin $installed }) -join ', ')"
"Not approved:         $(($installed | Where-Object { $_ -notin $approved }) -join ', ')"

Output:

In both:              WEB02, SQL01
Missing from install: WEB01
Not approved:         FILE01

-in and -notin covers these operators in detail.

Case-sensitive comparison

Add -CaseSensitive when WEB02 and web02 should count as different items:

$approved  = 'WEB01', 'WEB02', 'SQL01'
$installed = 'web02', 'SQL01', 'FILE01'

"Ignoring case: $((Compare-Object $approved $installed -IncludeEqual -ExcludeDifferent).InputObject -join ', ')"
"-CaseSensitive: $((Compare-Object $approved $installed -IncludeEqual -ExcludeDifferent -CaseSensitive).InputObject -join ', ')"

Output:

Ignoring case: WEB02, SQL01
-CaseSensitive: SQL01

A $null array breaks Compare-Object

An empty array works, but $null doesn’t. That happens when a command finds nothing and you store its output:

$installed = 'WEB01', 'SQL01'
$found = Get-ChildItem -Path C:\psfaqs\NoSuchFolder -ErrorAction SilentlyContinue | ForEach-Object Name

try { Compare-Object -ReferenceObject $found -DifferenceObject $installed -ErrorAction Stop }
catch { "Failed. $($_.Exception.Message)" }

"With @(): $(@(Compare-Object -ReferenceObject @($found) -DifferenceObject $installed).Count) differences"

Output:

Failed. Cannot bind argument to parameter 'ReferenceObject' because it is null.
With @(): 2 differences

Wrap both sides in @() whenever they come from a command, so an empty result is still an array.

Check if two arrays are equal

-eq doesn’t compare two arrays. With an array on the left, it filters, and here it returned nothing. Use Compare-Object, which returns nothing when the arrays match:

$a = 1, 2, 3
$b = 1, 2, 3
$c = 3, 2, 1

"`$a -eq `$b returns: [$($a -eq $b)]"
"a and c, any order:  $(-not (Compare-Object $a $c))"
"a and c, same order: $(-not (Compare-Object $a $c -SyncWindow 0))"
"a and b, same order: $(-not (Compare-Object $a $b -SyncWindow 0))"

Output:

$a -eq $b returns: []
a and c, any order:  True
a and c, same order: False
a and b, same order: True

-SyncWindow 0 compares position by position, so it also checks the order. Comparison operators explains why -eq filters arrays.

Duplicates count

Compare-Object matches items one for one, so an extra copy shows up as a difference:

$expected = 'WEB01', 'WEB01', 'SQL01'
$actual   = 'WEB01', 'SQL01'

Compare-Object $expected $actual | ForEach-Object { "$($_.InputObject) $($_.SideIndicator)" }

Output:

WEB01 <=

Remove duplicates first if you only care about which values appear. Removing duplicates shows how.

Compare arrays of objects

For objects, name the properties to compare. Here the BITS service changed state between two snapshots:

$yesterday = @(
    [pscustomobject]@{ Name = 'Spooler'; Status = 'Running' }
    [pscustomobject]@{ Name = 'BITS';    Status = 'Running' }
)
$today = @(
    [pscustomobject]@{ Name = 'Spooler'; Status = 'Running' }
    [pscustomobject]@{ Name = 'BITS';    Status = 'Stopped' }
)

Compare-Object $yesterday $today -Property Name, Status |
    ForEach-Object { '{0,-8} {1,-8} {2}' -f $_.Name, $_.Status, $_.SideIndicator }

Output:

BITS     Stopped  =>
BITS     Running  <=

Without -Property, Compare-Object compares the objects’ text form, which rarely works. The Compare-Object documentation covers -PassThru too.

Compare large arrays quickly

Compare-Object and -notin slow down with tens of thousands of items. A HashSet removes one list from another almost instantly:

$all    = 1..20000 | ForEach-Object { "PC$_" }
$online = 1..20000 | Where-Object { $_ % 4 } | ForEach-Object { "PC$_" }

$offline = [System.Collections.Generic.HashSet[string]]::new([string[]]$all, [StringComparer]::OrdinalIgnoreCase)
$offline.ExceptWith([string[]]$online)
"Offline computers: $($offline.Count), first three: $(($offline | Select-Object -First 3) -join ', ')"

Output:

Offline computers: 5000, first three: PC4, PC8, PC12

ExceptWith keeps items only in the first set, IntersectWith keeps matches. Microsoft documents them under HashSet.ExceptWith.

Frequently Asked Questions

Related ways to work with two lists:

• Check if an array contains a value
• Remove duplicates
• Remove items from an array
• Hashtables for fast lookups