Errors: Terminating and Non-Terminating

PowerShell has two kinds of errors, and knowing the difference is the key to handling them.

• A non-terminating error reports a problem and carries on — Get-Item given three paths, one missing, still returns the other two.
• A terminating error stops the command (and the script, unless caught).

'a.txt', 'c.txt' | ForEach-Object { Set-Content $_ 'x' }
Get-Item a.txt, b.txt, c.txt | Select-Object -ExpandProperty Name
'Still running'

a.txt
Get-Item: C:\demo\script.ps1:2
Line |
   2 |  Get-Item a.txt, b.txt, c.txt | Select-Object -ExpandProperty Name
     |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Cannot find path 'C:\demo\b.txt' because it does not exist.
c.txt
Still running

The error was shown, but the other files were still returned and the script continued.

-ErrorAction

-ErrorAction controls what a command does with non-terminating errors:

$item = Get-Item missing.txt -ErrorAction SilentlyContinue
$null -eq $item

True

$ErrorActionPreference = 'Stop' at the top of a script changes the default for every command — a common choice for automation, so that problems don't go unnoticed.

$Error and -ErrorVariable

Get-Item nope1.txt, nope2.txt -ErrorAction SilentlyContinue -ErrorVariable problems
$problems.Count
$problems[0].Exception.Message
$problems[0].CategoryInfo.Category

2
Cannot find path 'C:\demo\nope1.txt' because it does not exist.
ObjectNotFound

• -ErrorVariable name collects that command's errors (without the $).
• $Error holds every error in the session, newest first.

Writing errors in your own functions

function Get-Square {
  [CmdletBinding()]
  param([Parameter(Mandatory)][object[]]$Value)
  foreach ($v in $Value) {
    if ($v -isnot [int]) {
      Write-Error "'$v' is not a whole number"
      continue
    }
    $v * $v
  }
}

Get-Square -Value 3, 'x', 5 -ErrorAction SilentlyContinue

9
25

• Write-Error — a non-terminating error: report and move on to the next item.
• throw — a terminating error: stop, because continuing makes no sense.