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SystemManagement Automation ErrorCategoryInfo CategoryInfo {get;} SystemManagement AutomationErrorDetails ErrorDetails {get;set;} SystemException Exception {get;} SystemString FullyQuali edErrorId {get;} SystemManagement AutomationInvocationInfo InvocationInfo {get;} SystemObject TargetObject {get;}
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22 THE COMMAND PROMPT AND POWERSHELL
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PS D:\> $errException Cannot nd path D:\nothere because it does not exist PS D:\> $errtargetobject D:\nothere
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This assignment works great, except you have to rely on redirecting the error stream so the errors are no longer displayed Another option is available via the ErrorVariable parameter, which allows a variable to be passed for the storage of errors related only to a particular command:
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PS D:\> Get-ChildItem nothereeither -ErrorVariable Get-ChildItem : Cannot nd path D:\nothereeither does not exist At line:1 char:14 + Get-ChildItem <<<< nothereeither -ErrorVariable PS D:\> $errcount 1 PS D:\> $err[0]targetobject D:\nothereeither PS D:\> $err[0]exception Cannot nd path D:\nothereeither because it does err because it
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Notice that the error was still displayed onscreen; however, it was also stored in the err variable You did not use $ in front of the variable name because the ErrorValue parameter works by storing the data directly into the variable These are sophisticated error-collection techniques, but just as
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in other languages and scripts, all you might want to do is see if an error occurred To view the last error code, view the content of $LASTEXITCODE To see if there was an error in the last command, view the content of $ This contains a Boolean TRUE/FALSE simply showing if the last command ran without error; a value of TRUE means no error occurred
PS D:\> get-childitem temp Directory: MicrosoftPowerShellCore\FileSystem::D:\temp PS D:\> write-host $ True PS D:\> get-childitem notfound Get-ChildItem : Cannot nd path D:\notfound because it does not exist At line:1 char:14 + get-childitem <<<< notfound PS D:\> write-host $ False
Error-Handling Methods
Methods also exist to set what to do when an error occurs for an entire session via the setting of the $ErrorActionPreference variable (Continue, which is the default to output error and move on; SilentlyContinue, which outputs nothing to screen but still writes to $error and sets $ ; and Continues and Stop, which aborts an error) You can also set this on a command-by-command basis using the ea parameter, for example:
PS D:\> get-childitem notfound -ea silentlycontinue PS D:\> write-host $ False PS D:\> get-childitem notfound Get-ChildItem : Cannot nd path D:\notfound because it does not exist At line:1 char:14 + get-childitem <<<< notfound
Summary
In the example, when the command listing the invalid object was set to
silentlycontinue, no message was written, but $ was still set to error If you run the same command with no -ea parameter, the error appears PowerShell also exhibits sophisticated error handling by using the trap and throw statements in addition to the Set-PSDebug cmdlet to enable
22 THE COMMAND PROMPT AND POWERSHELL
debugging by stepping through your code; however, this is beyond the scope of this chapter
Summary
Huge amounts of information are available, both at Microsoft and on the Web, about command-line usage, including batch les, VBScript, and PowerShell This book looks at using the environments as they relate to the various components of the OS With PowerShell s great built-in help, logical verb-noun syntax, and consistent usage, you will be amazed with what is possible after a little practice, thanks to the piping of one cmdlet output to another and then to another, as needed Why would you ever need to use a command line or script with Windows, especially with the advanced remote management facilities available Consider installations of Windows Server 2008 such as Core, which has a limited graphical interface, as well as the WS-Management capabilities of Windows, which are command-line driven The only way to perform sequences of actions in an unattended fashion, such as in a scheduled process, is via a script; because of this, the powerful command-line and scripting capabilities of Windows Server 2008 are a blessing