Error Handling

PowerShell produces two kinds of errors: terminating errors that stop execution, and non-terminating errors that report a problem but continue running the next statement. Understanding the difference is essential for writing robust scripts.

Terminating vs Non-Terminating Errors

- Non-terminating: Most cmdlet errors (e.g., Get-Item on a file that does not exist). The cmdlet reports the error and moves on.

- Terminating: Errors from the throw keyword, .NET exceptions, or cmdlets that encounter critical failures. These stop the current pipeline or script unless caught.

Controlling Error Behavior

Use $ErrorActionPreference to change how non-terminating errors behave for an entire session or script:

$ErrorActionPreference = "Stop"   # Treat all errors as terminating

Or use the -ErrorAction parameter on individual cmdlets:

Get-Item "C:\nonexistent" -ErrorAction SilentlyContinue

try / catch / finally

Wrap risky code in a try block and handle failures in catch. The finally block always runs, whether or not an error occurred:

try {
    $content = Get-Content "C:\data\config.txt" -ErrorAction Stop
    Write-Output "File loaded successfully."
} catch {
    Write-Output "Error: $_"
} finally {
    Write-Output "Cleanup complete."
}

Inside the catch block, $_ (or $PSItem) holds the ErrorRecord object. You can access details like $_.Exception.Message or $_.ScriptStackTrace.

Write-Error vs throw

These two look similar but behave differently:

Write-Error "Something went wrong"   # Non-terminating by default
throw "Something went wrong"         # Always terminating

- `Write-Error` writes to the error stream but execution continues.

- `throw` immediately stops execution and can be caught by try/catch.

Catching Specific Exceptions

You can filter catch blocks by .NET exception type to handle different errors differently:

try {
    $value = [int]"not_a_number"
} catch [System.FormatException] {
    Write-Output "Invalid number format."
} catch {
    Write-Output "Unexpected error: $_"
}