Interpret error actions for Windows PowerShell commands

  • 8 minutes

When a PowerShell command generates an error, that error might be one of two types, either a terminating error or a non-terminating error. A terminating error occurs when Windows PowerShell determines that it's not possible to continue processing after the error and the command stops. A non-terminating error occurs when Windows PowerShell determines that it's possible to continue processing after the error. When a non-terminating error occurs, the script running or pipeline running will continue. Non-terminating errors are more common than terminating errors.

Consider the following command:

Get-WmiObject -Class Win32_BIOS -ComputerName LON-SVR1,LON-DC1

If you assume that the computer LON-SVR1 isn't available on the network, Get-WmiObject generates an error when it tries to query that computer. However, the command could continue with the next computer, LON-DC1. The error is therefore a non-terminating error.

$ErrorActionPreference

Windows PowerShell has a built-in, global variable named $ErrorActionPreference. When a command generates a non-terminating error, the command checks that variable to decide what it should do. The variable can have one of the following four possible values:

  • Continue is the default, and it tells the command to display an error message and continue to run.
  • SilentlyContinue tells the command to display no error message, but to continue running.
  • Inquire tells the command to display a prompt asking the user what to do.
  • Stop tells the command to treat the error as terminating and to stop running.

To set the $ErrorActionPreference variable, use the following syntax:

$ErrorActionPreference = 'Inquire'

Note

Be selective about using SilentlyContinue for $ErrorActionPreference. You might think that this makes your script better for users, but it could make troubleshooting difficult.

If you intend to trap an error within your script so that you can manage the error, commands must use the Stop action. You can trap and manage only terminating errors.

-ErrorAction parameter

All Windows PowerShell commands have the –ErrorAction parameter. This parameter has the alias –EA. The parameter accepts the same values as $ErrorActionPreference, and the parameter overrides the variable for that command. If you expect an error to occur on a command, you use –ErrorAction to set that command’s error action to Stop. Doing this lets you trap and manage the error for that command but leaves all other commands to use the action in $ErrorActionPreference. An example is:

Get-WmiObject -Class Win32_BIOS -ComputerName LON-SVR1,LON-DC1 -ErrorAction Stop

The only time that you'll modify $ErrorActionPreference is when you expect an error outside of a Windows PowerShell command, such as when you're running a method such as the following:

Get-Process –Name Notepad | ForEach-Object { $PSItem.Kill() }

In this example, the Kill() method might generate an error. But because it's not a Windows PowerShell command, it doesn't have the –ErrorAction parameter. You would instead set $ErrorActionPreference to Stop before running the method, and then set the variable back to Continue after you run the method.