Variables & Scope

Variables are the building blocks of every script. In PowerShell, a variable is a named container for any .NET object.

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Declaring Variables

Variables begin with $. You don't need to declare a type — PowerShell infers it:

$name    = "Alice"
$age     = 30
$pi      = 3.14159
$today   = Get-Date
$running = $true
$nothing = $null

Multiple assignment

$a = $b = $c = 0          # all three start at 0
$x, $y, $z = 1, 2, 3      # destructuring assignment
$first, $rest = 1..10     # $first = 1, $rest = 2..10

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Reading and Writing

$greeting = "Hello"
$greeting             # output: Hello
$greeting = "Hi"      # reassign
"${greeting}, World!" # interpolate in a string

String interpolation

Double-quoted strings expand variables; single-quoted strings do not:

$name = "Bob"
"Hello, $name"        # → Hello, Bob
'Hello, $name'        # → Hello, $name  (literal)
"Today is $(Get-Date -Format 'yyyy-MM-dd')"  # expression inside $()

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Variable Types

PowerShell uses dynamic typing but you can declare a type to enforce it:

[int]    $count   = 42
[string] $label   = "items"
[bool]   $enabled = $true
[double] $rate    = 0.07
[datetime] $start = "2024-01-01"

Type Accelerator	.NET Type	Example
[string]	System.String	"hello"
[int]	System.Int32	42
[long]	System.Int64	1000000000000
[double]	System.Double	3.14
[bool]	System.Boolean	$true / $false
[datetime]	System.DateTime	Get-Date
[array]	System.Object[]	@(1,2,3)
[hashtable]	System.Collections.Hashtable	@{a=1}
[PSCustomObject]	System.Management.Automation.PSCustomObject	[PSCustomObject]@{}

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Special Variables

PowerShell provides many built-in automatic variables:

Variable	Meaning
$_ / $PSItem	Current pipeline object
$?	$true if last command succeeded
$LASTEXITCODE	Exit code of last native command
$Error	Array of recent errors
$null	Null / nothing
$true / $false	Boolean constants
$env:PATH	Environment variable access
$PSVersionTable	PowerShell version info
$HOME	User home directory
$PWD	Current directory (object)
$args	Arguments passed to a script
$MyInvocation	Info about the current script/function
$PSScriptRoot	Directory containing the running script
$PSBoundParameters	Parameters explicitly passed to a function

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Environment Variables

Access environment variables through the env: drive:

$env:USERNAME          # current Windows username
$env:COMPUTERNAME      # machine name
$env:PATH              # the PATH variable
$env:TEMP              # temp directory

# Set an environment variable (current session only)
$env:MY_VAR = "hello"

# Set it permanently (user scope)
[System.Environment]::SetEnvironmentVariable("MY_VAR","hello","User")

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Variable Scope

Scope controls where a variable is visible:

Scope	Description
Local (default)	Visible only in the current scope (function, script block)
Script	Visible throughout the current .ps1 file
Global	Visible in the entire session
Private	Visible only in the current scope; not inherited by child scopes

$x = 10           # local scope

function Show-X {
    Write-Host $x   # can READ parent scope variables
    $x = 99         # creates a NEW local $x — does NOT change the parent
}
Show-X             # outputs: 10
$x                 # still 10

# Use $script: or $global: to write to outer scopes
function Update-X {
    $global:x = 99
}
Update-X
$x   # now 99

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Arrays

$fruits = @("apple", "banana", "cherry")
$fruits[0]          # apple (0-indexed)
$fruits[-1]         # cherry (last element)
$fruits[0..1]       # apple, banana (slice)
$fruits.Count       # 3

# Add an element (creates a new array)
$fruits += "date"

# Iterate
foreach ($fruit in $fruits) {
    Write-Host $fruit
}

Typed arrays

[int[]] $numbers = 1, 2, 3, 4, 5
$numbers | Measure-Object -Sum

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Hash Tables

$person = @{
    Name = "Alice"
    Age  = 30
    City = "Boston"
}

$person.Name          # Alice  (dot notation)
$person["Age"]        # 30     (index notation)
$person.Keys          # Name, Age, City
$person.Values        # Alice, 30, Boston

# Add or update
$person.Email = "alice@example.com"
$person["Age"] = 31

# Remove
$person.Remove("City")

# Check for key
$person.ContainsKey("Name")  # $true

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PSCustomObject

For structured data, PSCustomObject is more powerful than a hashtable because it preserves property order and supports methods:

$user = [PSCustomObject]@{
    Name  = "Alice"
    Age   = 30
    Email = "alice@example.com"
}

$user.Name   # Alice
$user | Format-Table

# Add a property after creation
$user | Add-Member -MemberType NoteProperty -Name Role -Value "Admin"

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Checking and Removing Variables

Test-Path variable:\myVar      # $true if variable exists
Get-Variable myVar             # get the variable object
Remove-Variable myVar          # delete the variable
Clear-Variable myVar           # set to $null without deleting