PowerShell Testing with Pester and PSScriptAnalyzer

This skill provides expertise in PowerShell testing using the Pester framework and code quality analysis with PSScriptAnalyzer.

Pester Testing Framework

Test Structure

Use the standard Pester structure with Describe, Context, and It blocks:

powershell

Describe "FunctionName" {
    Context "When specific condition exists" {
        It "Should produce expected behavior" {
            # Arrange
            $expected = "value"

            # Act
            $result = FunctionName -Parameter "value"

            # Assert
            $result | Should -Be $expected
        }
    }
}

Setup and Teardown

Use lifecycle hooks for test setup and cleanup:

powershell

BeforeAll {
    # Runs once before all tests in the block
    . $PSScriptRoot/../script.ps1
}

BeforeEach {
    # Runs before each test
    $testData = @{ Key = "Value" }
}

AfterEach {
    # Runs after each test
    Remove-Item -Path $testPath -ErrorAction SilentlyContinue
}

AfterAll {
    # Runs once after all tests in the block
    # Cleanup resources
}

Pester Assertions

Common assertion patterns:

•Should -Be - Exact equality
•Should -BeExactly - Case-sensitive string equality
•Should -Match - Regex pattern matching
•Should -Contain - Collection contains item
•Should -BeNullOrEmpty - Null or empty check
•Should -Throw - Exception testing
•Should -Exist - File/path existence
•Should -BeOfType - Type checking
•Should -Invoke - Mock verification (with -Times, -Exactly, -ParameterFilter)

Mocking

Mock external dependencies to isolate tests:

powershell

BeforeAll {
    Mock Get-Content { return "mocked content" }

    Mock Get-ChildItem {
        return @(
            [PSCustomObject]@{ Name = "file1.txt"; Length = 100 }
        )
    } -ParameterFilter { $Path -eq "C:\temp" }
}

It "Should call Get-Content" {
    $result = Get-SomeData
    Should -Invoke Get-Content -Times 1 -Exactly
}

Test Organization

•Name test files with .Tests.ps1 suffix
•Group related tests in the same Describe block
•Use Context to group tests by scenario or condition
•Keep test names descriptive and action-oriented
•One assertion per It block when possible

PSScriptAnalyzer Best Practices

PSScriptAnalyzer is a static code analyzer that enforces PowerShell best practices and catches common issues.

Running PSScriptAnalyzer

powershell

# Analyze a single file
Invoke-ScriptAnalyzer -Path script.ps1

# Analyze recursively with severity filter
Invoke-ScriptAnalyzer -Path . -Recurse -Severity Error,Warning

# Use specific rules
Invoke-ScriptAnalyzer -Path script.ps1 -IncludeRule PSAvoidUsingPositionalParameters

Critical Rules to Follow

PSAvoidUsingPositionalParameters

powershell

# Bad
Get-ChildItem C:\temp *.txt

# Good
Get-ChildItem -Path C:\temp -Filter *.txt

PSUseShouldProcessForStateChangingFunctions

powershell

function Remove-CustomItem {
    [CmdletBinding(SupportsShouldProcess)]
    param($Path)

    if ($PSCmdlet.ShouldProcess($Path, "Remove")) {
        Remove-Item -Path $Path
    }
}

PSAvoidUsingCmdletAliases

powershell

# Bad
gci | ? { $_.Length -gt 1MB } | % { $_.Name }

# Good
Get-ChildItem | Where-Object { $_.Length -gt 1MB } | ForEach-Object { $_.Name }

PSUseDeclaredVarsMoreThanAssignments

•Ensure variables are used after assignment
•Remove unused variables

PSAvoidUsingWriteHost

powershell

# Bad - can't be captured or redirected
Write-Host "Information"

# Good - proper output streams
Write-Output "Information"    # Success stream
Write-Error "Error message"   # Error stream
Write-Warning "Warning"       # Warning stream
Write-Verbose "Details"       # Verbose stream

PSAvoidUsingPlainTextForPassword

powershell

# Bad
param([string]$Password)

# Good
param([SecureString]$Password)

Cmdlet Development Guidelines

Follow Microsoft's official cmdlet development guidelines from https://learn.microsoft.com/en-us/powershell/scripting/developer/cmdlet/cmdlet-development-guidelines

Naming Conventions:

•Use approved verbs: Get, Set, New, Remove, Add, Clear, etc.
•Use singular nouns: Get-Process, not Get-Processes
•Follow Verb-Noun pattern

Parameter Guidelines:

•Use [CmdletBinding()] for advanced functions
•Support common parameters via [CmdletBinding()]
•Use [Parameter()] attributes to define required/optional parameters
•Use ValueFromPipeline and ValueFromPipelineByPropertyName appropriately
•Validate parameters with [ValidateSet()], [ValidateRange()], etc.

Integrating PSScriptAnalyzer in Tests

powershell

Describe "PSScriptAnalyzer" {
    Context "Script quality" {
        It "Should pass PSScriptAnalyzer rules" {
            $results = Invoke-ScriptAnalyzer -Path $PSScriptRoot/../script.ps1
            $results | Should -BeNullOrEmpty
        }

        It "Should have no high severity issues" {
            $results = Invoke-ScriptAnalyzer -Path $PSScriptRoot/.. -Recurse -Severity Error
            $results | Should -BeNullOrEmpty
        }
    }
}

Test Execution

Running Pester Tests

powershell

# Run all tests in current directory
Invoke-Pester

# Run specific test file
Invoke-Pester -Path .\MyFunction.Tests.ps1

# Run with code coverage
Invoke-Pester -CodeCoverage .\script.ps1

# Output to CI-friendly format
Invoke-Pester -OutputFormat NUnitXml -OutputFile test-results.xml

# Run with detailed output
Invoke-Pester -Output Detailed

CI/CD Integration

For GitHub Actions or other CI systems:

powershell

$config = New-PesterConfiguration
$config.Run.Path = './tests'
$config.Run.Exit = $true
$config.TestResult.Enabled = $true
$config.TestResult.OutputPath = 'test-results.xml'
$config.CodeCoverage.Enabled = $true

Invoke-Pester -Configuration $config

Common Patterns

Testing Functions with Pipeline Input

powershell

It "Should accept pipeline input" {
    $input = @("file1.txt", "file2.txt")
    $result = $input | Process-CustomFunction
    $result.Count | Should -Be 2
}

Testing Error Handling

powershell

It "Should throw when path doesn't exist" {
    { Get-CustomData -Path "C:\nonexistent" } | Should -Throw
}

It "Should write error for invalid input" {
    Get-CustomData -Path "invalid" -ErrorVariable err
    $err | Should -Not -BeNullOrEmpty
}

Testing Private Functions

powershell

InModuleScope MyModule {
    It "Should test private function" {
        PrivateFunction | Should -Be $expected
    }
}

Quick Reference

When to use this skill

•Writing new Pester tests for PowerShell scripts or modules
•Debugging failing PowerShell tests
•Analyzing PowerShell code quality with PSScriptAnalyzer
•Refactoring PowerShell code to meet best practices
•Setting up PowerShell test infrastructure
•Integrating PowerShell tests into CI/CD pipelines