🔥 We recently released Pester 4 and there are breaking changes, please see release notes and migration guide.

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.

Please be aware that PowerShell Core 6.0.0-beta.9 comes with bundle Pester version 3.3.9, but we recommend updating, by running this PowerShell command as administrator:

Install-Module -Name Pester -Force

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

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 {}

        Remove-Cache

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

Learn more about Mocking here.

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.

Learn more about code coverage here.

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
install:
- ps: choco install pester
build: off
test_script:
- ps: Invoke-Pester -EnableExit

See it in action here!

Pester itself is build on the community build server.

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

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