Debugging Monad Scripts, Part 2: $error

PowerShell Team

Did your command or script fail and/or report an error?  We hope to have a proper script debugger in a future version, but until then, MSH has some handy features to help you figure out what went wrong.  In this series of blog entries, I will present some of those features.  Thanks to Jim Truher [MSFT], Bruce Payette [MSFT], and Jeff Jones [MSFT] for their help in putting this together.

See the Windows “Monad” Shell Beta 2 Documentation Pack ( for general information about Monad.

Part 1: (Terminating vs. Non-Terminating Errors, ErrorRecord)

Jon Newman [MSFT]



Errors occurring in MSH commands or scripts will be stored in $error. $error is always on, so you can use it even for errors which you can’t reproduce (or would rather not repeat).

$error is an ArrayList containing the most recent errors: $error[0] is the most recent error, $error[1] the one before that, etc. $MaximumErrorCount (default 256) is the maximum number of errors which will be stored before the oldest is discarded.

So, when you see errors, you should have a look at $error[0]. Note that any new error will move $error[0] to $error[1], so you’ll probably want to copy $error[0] to another shell variable before investigating it, e.g.

MSH C:\temp> start-service $servicename
start-service : Cannot bind argument to parameter ‘ServiceName’ because it is null.
At line:1 char:14
+ start-service  <<<< $servicename
MSH C:\temp> $savethis = $error[0]
MSH C:\temp> $savethis.GetType().Name
MSH C:\temp> $savethis.InvocationInfo.MyCommand

CommandType     Name                                                Definition
———–     —-                                                ———-
Cmdlet          start-service                                       start-service [-ServiceName] String[] [-PassThru…

MSH C:\temp>

Note that Exception objects will in some cases also appear in $error. In particular, the “throw” script command may add both an ErrorRecord and a RuntimeException to $error.

[Edit: Monad has now been renamed to Windows PowerShell. This script or discussion may require slight adjustments before it applies directly to newer builds.]


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