PowerShell Functions & Modules: Build Reusable Code

Key Facts

Functions enable code reuse and reduce repetition in PowerShell scripts
Modules package functions into distributable units (ps1, psd1, dll files)
The PSGallery repository hosts 7,000+ PowerShell modules available for download
Proper parameter validation prevents invalid input from breaking automation

What Are Functions?

Functions are reusable blocks of code that perform a specific task. Instead of writing the same code repeatedly, define a function once and call it many times.

function Get-SystemInfo {
  param([string]$ComputerName = $env:COMPUTERNAME)

  $os = Get-CimInstance -ClassName Win32_OperatingSystem -ComputerName $ComputerName
  $cpu = Get-CimInstance -ClassName Win32_Processor -ComputerName $ComputerName

  [PSCustomObject]@{
    ComputerName = $ComputerName
    OS = $os.Caption
    Memory = [math]::Round($os.TotalVisibleMemorySize / 1MB, 2)
    CPUCores = $cpu.NumberOfCores
  }
}

# Call the function
Get-SystemInfo
Get-SystemInfo -ComputerName "Server01"

Function Parameters and Validation

Parameters allow functions to accept input. Validation ensures the input meets requirements:

function New-User {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateLength(3,20)]
    [string]$Username,

    [Parameter(Mandatory=$true)]
    [ValidatePattern('^[a-zA-Z]')]
    [string]$FirstName,

    [Parameter(Mandatory=$true)]
    [ValidateSet("IT", "HR", "Finance", "Operations")]
    [string]$Department
  )

  # Function body
  Write-Host "Creating user: $Username in $Department"
}

# Valid call
New-User -Username "jdoe" -FirstName "John" -Department "IT"

# Invalid call (too short)
New-User -Username "jd" -FirstName "John" -Department "IT"  # Error!