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Pester is the ubiquitous test and mock framework for PowerShell.

# your function
function Get-Planet ([string]$Name='*')
  $planets = @(
    @{ Name = 'Mercury' }
    @{ Name = 'Venus'   }
    @{ Name = 'Earth'   }
    @{ Name = 'Mars'    }
    @{ Name = 'Jupiter' }
    @{ Name = 'Saturn'  }
    @{ Name = 'Uranus'  }
    @{ Name = 'Neptune' }
  ) | foreach { [PSCustomObject]$_ }

  $planets | where { $_.Name -like $Name }

# Pester tests
Describe 'Get-Planet' {
  It "Given no parameters, it lists all 8 planets" {
    $allPlanets = Get-Planet
    $allPlanets.Count | Should -Be 8

  Context "Filtering by Name" {
    It "Given valid -Name '<Filter>', it returns '<Expected>'" -TestCases @(
      @{ Filter = 'Earth'; Expected = 'Earth' }
      @{ Filter = 'ne*'  ; Expected = 'Neptune' }
      @{ Filter = 'ur*'  ; Expected = 'Uranus' }
      @{ Filter = 'm*'   ; Expected = 'Mercury', 'Mars' }
    ) {
      param ($Filter, $Expected)

      $planets = Get-Planet -Name $Filter
      $planets.Name | Should -Be $Expected

    It "Given invalid parameter -Name 'Alpha Centauri', it returns `$null" {
      $planets = Get-Planet -Name 'Alpha Centauri'
      $planets | Should -Be $null

This code example lies a tiny bit, find it annotated and production ready here.

Learn more about the usage and syntax on our wiki.


Pester is compatible with Windows PowerShell 2.x - 5.x on Windows 10, 8, 7, Vista and even 2003. Since version 4.0.9 Pester is compatible also with PowerShell Core 6.x on Windows, Linux, macOS but with some limitations.

Pester comes pre-installed with Windows 10, but we recommend updating, by running this PowerShell command as administrator:

Install-Module -Name Pester -Force -SkipPublisherCheck

Not running Windows 10 or facing problems? See the full installation and update guide.


Test runner

Pester runs your tests and prints a nicely formatted output to the screen.

test run output

Command line output is not the only output option, Pester also integrates with Visual Studio Code, Visual Studio, and any tool that can consume nUnit XML output.


Pester comes with a suite of assertions that cover a lot of common use cases. Pester assertions range from very versatile, like Should -Be, to specialized like Should -Exists. Here is how you ensure that a file exists:

Describe 'Notepad' {
    It 'Exists in Windows folder' {
        'C:\Windows\notepad.exe' | Should -Exist

Learn more about assertion on our wiki.


Pester has mocking built-in. Using mocks you can easily replace functions with empty implementation to avoid changing the real environment.

function Remove-Cache {
    Remove-Item "$env:TEMP\cache.txt"

Describe 'Remove-Cache' {
    It 'Removes cached results from temp\cache.text' {
        Mock -CommandName Remove-Item -MockWith {}


        Assert-MockCalled -CommandName Remove-Item -Times 1 -Exactly

Learn more about Mocking here.

Code coverage

Pester can measure how much of your code is covered by tests and export it to JaCoCo format that is easily understood by build servers.

JaCoCo code coverage report

Learn more about code coverage here.

Build server integration

Pester integrates nicely with TFS, AppVeyor, TeamCity, Jenkins and other CI servers.

Testing your scripts, and all pull requests on AppVeyor is extremely simple. Just commit this appveyor.yml file to your repository, and select your repository on the AppVeyor website:

version: 1.0.{build}
image: WMF 5
- ps: choco install pester
build: off
- ps: Invoke-Pester -EnableExit

See it in action here!

Pester itself is build on the community build server and Travis CI, and distributed mainly via PowerShell gallery.

windows build linux/macos build latest version downloads

Further reading

Do you like what you see? Learn how to use Pester with our wiki guide, and continue with some of the other resources.

Got questions?

Got questions or you just want to get in touch? Use our issues page or one of these channels:

Pester Twitter Pester on StackOverflow Testing channel on Powershell Slack Pester Gitter Pester on PowerShell.org