L5. Control Flow and Functions

Control Flow

PowerShell supports familiar conditional and loop patterns:

if (Test-Path .\config.json) {
  'Config found'
} else {
  'Config missing'
}
foreach ($service in Get-Service) {
  if ($service.Status -eq 'Stopped') {
    $service.Name
  }
}

switch is useful when several cases share the same input:

switch ($env:OS) {
  'Windows_NT' { 'Windows host' }
  default { 'Non-Windows or unknown host' }
}

Functions

Functions turn repeated command sequences into reusable tools:

function Get-ProcessHealth {
  param(
    [int]$Top = 5
  )
  Get-Process |
    Sort-Object CPU -Descending |
    Select-Object -First $Top Name, Id, CPU
}
Get-ProcessHealth -Top 10

Parameter Validation

function Test-PortRange {
  param(
    [Parameter(Mandatory)]
    [ValidateRange(1, 65535)]
    [int]$Port
  )
  [pscustomobject]@{
    Port = $Port
    Valid = $true
  }
}

Validation catches bad input at the boundary of your function instead of letting the error appear several steps later.

Output Rule

Write objects to the pipeline. Avoid Write-Host for data that another command might need. Use Write-Verbose, Write-Warning, or Write-Information for messages.

Lab

1. Write a function that accepts a path and returns file count and total size.
2. Add [Parameter(Mandatory)] to the path.
3. Add [ValidateScript({ Test-Path $_ })].
4. Return a [pscustomobject], not formatted text.