Powershell Style Guide

PowerShell Style Guide

Scope Boundaries

• Use this skill when the task matches the trigger condition described in description.
• Do not use this skill when the primary task falls outside this skill's domain.

Use this skill to write and review PowerShell scripts that are predictable, secure, and maintainable for local automation and CI.

Trigger And Co-activation Reference

• If available, use references/trigger-matrix.md for canonical co-activation rules.
• If available, resolve style-guide activation from changed files with python3 scripts/resolve_style_guides.py <changed-path>....
• If available, validate trigger matrix consistency with python3 scripts/validate_trigger_matrix_sync.py.

Quality Gate Command Reference

• If available, use references/quality-gate-command-matrix.md for CI check-only and local autofix mapping.

Quick Start Snippets

Script skeleton with strict mode and fail-fast behavior

Set-StrictMode -Version Latest
$ErrorActionPreference = 'Stop'

function Write-Failure {
    param(
        [string]$Message,
        [System.Exception]$Exception
    )

    Write-Error "$Message`n$($Exception.Message)"
}

try {
    # Main logic
    Write-Host 'Script started'
}
catch {
    Write-Failure -Message 'Unhandled failure' -Exception $_.Exception
    exit 1
}

Required environment variable check

$apiToken = $env:API_TOKEN
if ([string]::IsNullOrWhiteSpace($apiToken)) {
    throw 'API_TOKEN is required.'
}

Parameter validation and approved verbs

function Get-ReleaseArtifact {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [string]$ArtifactId,

        [ValidateSet('dev', 'staging', 'prod')]
        [string]$Environment = 'dev'
    )

    # Implementation
    "artifact=$ArtifactId env=$Environment"
}

Bounded retry with backoff

function Invoke-WithRetry {
    param(
        [Parameter(Mandatory)]
        [scriptblock]$Operation,
        [int]$MaxAttempts = 5,
        [int]$DelaySeconds = 2
    )

    for ($attempt = 1; $attempt -le $MaxAttempts; $attempt++) {
        try {
            return & $Operation
        }
        catch {
            if ($attempt -eq $MaxAttempts) { throw }
            Start-Sleep -Seconds $DelaySeconds
        }
    }
}

Safe external command invocation

$arguments = @(
    '--fail'
    '--silent'
    '--show-error'
    '--header'; "Authorization: Bearer $env:API_TOKEN"
    'https://example.com/health'
)

& curl @arguments

Structure And Readability

1. Use approved PowerShell verb-noun naming (Get-, Set-, Invoke-, Test-).
2. Prefer functions with explicit parameters over global mutable state.
3. Keep scripts orchestration-focused and move reusable logic to functions/modules.
4. Replace magic numbers with named constants including units.
5. Add short comments only where intent is non-obvious.

Parameters, Types, And Data Handling

1. Use [CmdletBinding()] for advanced functions where appropriate.
2. Validate input with ValidateNotNullOrEmpty, ValidateSet, or explicit checks.
3. Use typed parameters/returns for non-trivial data.
4. Avoid implicit string parsing when structured objects are available.
5. Prefer splatting for complex command arguments.

Error Handling And Control Flow

1. Set $ErrorActionPreference = 'Stop' for fail-fast scripts.
2. Use try/catch/finally for boundary error handling and cleanup.
3. Throw specific, actionable errors.
4. Do not suppress failures without explicit rationale.
5. Return non-zero exit codes for automation-visible failures.

Security And Operational Safety

1. Treat all external input as untrusted.
2. Avoid command injection by separating command and arguments.
3. Never print secrets; redact sensitive values in logs.
4. Use least privilege and avoid unnecessary elevation.
5. Validate file paths and destructive command targets.

Performance And Scalability

1. Prefer pipeline/object operations over repeated text parsing when feasible.
2. Avoid unnecessary process spawning in loops.
3. Use streaming (Get-Content -ReadCount, pipeline) for large inputs.
4. Use bounded retries with explicit constants.

Testing And Verification

1. Add Pester tests for critical behavior and failure paths.
2. Cover edge cases: missing env vars, invalid parameters, timeout, transient errors.
3. Document manual verification where environment dependencies exist.
4. Validate idempotency for repeatable automation scripts.

Minimal Pester example

Describe 'Get-ReleaseArtifact' {
    It 'throws when ArtifactId is empty' {
        { Get-ReleaseArtifact -ArtifactId '' } | Should -Throw
    }
}

CI Required Quality Gates (check-only)

1. Run Invoke-ScriptAnalyzer in check mode with fail-on-issue policy.
2. Run formatting/lint check (Invoke-Formatter -Check or repository equivalent).
3. Run Invoke-Pester.
4. Reject changes that rely on implicit failures or silent fallbacks.

Optional Autofix Commands (local)

1. Run Invoke-Formatter (or repository formatter autofix command).
2. Apply safe Invoke-ScriptAnalyzer -Fix results, then rerun checks.