Pester

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.

Installation

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.

Features

Test runner

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.

Assertions

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.

Mocking

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.

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.

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:
- Visual Studio 2017
- Ubuntu
install:
- ps: Install-Module Pester -Force -SkipPublisherCheck -Scope CurrentUser
build: off
test_script:
- ps: Invoke-Pester -EnableExit