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PS (5) > $err | gm -type property TypeName: System.Management.Automation.ErrorRecord Name ---CategoryInfo ErrorDetails Exception FullyQualifiedErrorId InvocationInfo TargetObject MemberType ---------Property Property Property Property Property Property Definition ---------System.Management.Automation... System.Management.Automation... System.Exception Exception {... System.String FullyQualified... System.Management.Automation... System.Object TargetObject {...
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Although this shows you all of the properties and their definitions, some of the property names are a little tricky to figure out, so further explanation is in order. Table 9.1 lists all of the properties, their types, and a description of what the property is.
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Table 9.1 Name CategoryInfo ErrorDetails ErrorRecord properties and their descriptions Type ErrorCategoryInfo ErrorDetails Description This string breaks errors into a number of broad categories. This may be null. If present, ErrorDetails can specify additional information, most importantly ErrorDetails.Message, which (if present) is a more exact description and should be displayed instead of Exception.Message. This is the underlying .NET exception corresponding to the error that occurred. This identifies the error condition more specifically than either the ErrorCategory or the Exception. Use FullyQualifiedErrorId to filter highly specific error conditions. This is an object that contains information about where the error occurred typically the script name and line number. This is the object that was being operated on when the error occurred. It may be null, as not all errors will set this field.
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Exception FullyQualifiedErrorId
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System.Exception System.String
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System.Management.Automation.InvocationInfo System.Object
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TargetObject
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Let s look at the content of the properties for this error:
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PS (10) > $err | fl * -force Exception : System.Management.Automation.ItemNotFoun dException: Cannot find path 'C:\files\n osuchfile' because it does not exist. at System.Management.Automation.Sessi onStateInternal.GetChildItems(String pat
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h, Boolean recurse, CmdletProviderContex t context) at System.Management.Automation.Child ItemCmdletProviderIntrinsics.Get(String path, Boolean recurse, CmdletProviderCon text context) at Microsoft.PowerShell.Commands.GetC hildItemCommand.ProcessRecord() TargetObject : C:\files\nosuchfile CategoryInfo : ObjectNotFound: (C:\files\nosuchfile:Str ing) [Get-ChildItem], ItemNotFoundExcept ion FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Comman ds.GetChildItemCommand ErrorDetails : InvocationInfo : System.Management.Automation.InvocationI Nfo
In this output, you can see the exception that caused the error was ItemNotFoundException. The TargetObject member contains the full path the cmdlet used to locate the item. This overall error is placed in the broader category of ObjectNotFound. There are no additional error details for this object. Let s take a closer look at the InvocationInfo member. This member provides information about where the error occurred. Here s what it looks like:
PS (6) > $err.InvocationInfo MyCommand ScriptLineNumber OffsetInLine ScriptName Line PositionMessage : : : : : : Get-ChildItem 1 11 $err = dir nosuchfile 2>&1
At line:1 char:11 + $err = dir <<<< nosuchfile 2>&1 InvocationName : dir PipelineLength : 1 PipelinePosition : 1
Since we entered this command on the command line, the script name is empty and the script line is 1. The offset is the offset in the script line where the error occurred. There is also other information available, such as the number of commands in the pipeline that caused an error, as well as the index of this command in the pipeline. This message also includes the line of script text where the error occurred. Finally, there is the PositionMessage member. This member takes all of the other information and formats it into what you see in PowerShell errors. Clearly, there is a lot of information in these objects that can help you figure out where and why an error occurred. The trick is to make sure that we have the right error objects available at the right time. It simply isn t possible to record every error that occurs as it would take up too much space and be impossible to manage. If we ERROR HANDLING 255
limit the set of error objects that are preserved, we want to make sure that we keep those we care about. Obviously, having the wrong error objects doesn t help. Sometimes we re interested only in certain types of errors or only in errors from specific parts of a script. To address these requirements, PowerShell provides a rich set of tools for capturing and managing errors. The next few sections cover these tools and the techniques for using them. 9.1.2 The $error variable and ErrorVariable parameter The point of rich error objects is that you can examine them after the error has occurred and possibly take remedial action. Of course, to do this, you have to capture them first. In the previous section, we looked at how we can do this by redirecting the error stream, but the problem with doing so is that you have to think of it beforehand. Since you don t know when errors occur, in practice you d have to do it all the time. Fortunately, PowerShell performs some of this work for you. There is a special variable $error that contains a collection of the errors that occurred. This is maintained as a circular bounded buffer. As new errors occur, old ones are discarded. The number of errors that are retained is controlled by the $MaximumErrorCount variable. The collection in $error is an array (technically an instance of System.Collections.ArrayList) that buffers errors as they occur. The most recent error is always stored in $error[0].
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