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AUTHOR S NOTE
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While it s tempting to think that you could just set $MaximumErrorCount to some very large value and never have to worry about capturing errors, in practice this is not a good idea. Rich error objects also imply fairly large error objects. If you set $MaximumErrorCount to too large a value, you won t have any memory left. In practice, there usually is no reason to set it to anything larger than the default, though you may want to set it to something smaller if you want to make more space available for other things.
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Let s explore using the $error variable. We ll start with the same error as before.
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PS (1) > dir nosuchfile Get-ChildItem : Cannot find path 'C:\working\book\nosuchfile' be cause it does not exist. At line:1 char:4 + dir <<<< nosuchfile
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We didn t explicitly capture it, but it is available in $error[0]
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PS (2) > $error[0] Get-ChildItem : Cannot find path 'C:\working\book\nosuchfile' be cause it does not exist. At line:1 char:4 + dir <<<< nosuchfile
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with all of the error properties. For example, here is the exception object:
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PS (3) > $error[0].exception Cannot find path 'C:\working\book\nosuchfile' because it does no t exist.
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ERRORS, EXCEPTIONS, AND SCRIPT DEBUGGING
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and here s the target object that caused the error:
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PS (4) > $error[0].targetobject C:\working\book\nosuchfile
Now let s do something that will cause a second error.
PS (5) > 1/$null Attempted to divide by zero. At line:1 char:3 + 1/$ <<<< null
Here we have a division by zero error.
AUTHOR S NOTE
The example here uses 1/$null. The reason for doing this instead of simply 1/0 is because the PowerShell interpreter does something called constant expression folding. It looks at expressions that contain only constant values. When it sees one, it evaluates that expression once at compile time so it doesn t have to waste time doing it again at runtime. This means that impossible expressions, such as division by zero, are caught and treated as parsing errors. Parsing errors can t be caught and don t get logged when they re entered interactively, so they don t make for a good example. (If one script calls another script and that script has one of these errors, the calling script can catch it, but the script being parsed cannot.)
Let s verify that the second error is in $error[0]. We ll do so by looking at the exception member
PS (6) > $error[0].exception Attempted to divide by zero.
Yes, it is. We ll also verify that the previous error, the file not found error, is now in position 1.
PS (7) > $error[1].exception Cannot find path 'C:\working\book\nosuchfile' because it does no t exist.
Again, yes it is. As you can see, each new error shuffles the previous error down one element in the array. The key lesson to take away from this is that when you are going to try to diagnose an error, you should copy it to a working variable so it doesn t get accidentally shifted out from under you because you made a mistake in one of the commands you re using to examine the error. The $error variable is a convenient way to capture errors automatically, but there are two problems with it. First, as we discussed earlier, it only captures a limited number of errors. The second problem is that it mixes all of the errors from all commands together in one collection. The first problem can be worked around using redirection to capture all the errors, but that still doesn t address mixing all the errors together. To deal with this second issue when you want to capture all the errors from a specific command, you use a standard parameter on all commands called -ErrorVariable. This
ERROR HANDLING
parameter names a variable to use for capturing all the errors that the command generates. Here s an example. This command generates three error objects, since the files nofuss , nomuss , and nobother don t exist.
PS (1) > dir nofuss,nomuss,nobother -ErrorVariable errs Get-ChildItem : Cannot find path 'C:\Documents and Settings\bruc epay\nofuss' because it does not exist. At line:1 char:4 + dir <<<< nofuss,nomuss,nobother -ErrorVariable errs Get-ChildItem : Cannot find path 'C:\Documents and Settings\bruc epay\nomuss' because it does not exist. At line:1 char:4 + dir <<<< nofuss,nomuss,nobother -ErrorVariable errs Get-ChildItem : Cannot find path 'C:\Documents and Settings\bruc epay\nobother' because it does not exist. At line:1 char:4 + dir <<<< nofuss,nomuss,nobother -ErrorVariable errs
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