Lab in Visual Basic .NET

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22.10 Lab
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The scripting in listing 22.8 includes several commands that might cause an error, either because a file doesn t exist or because a computer can t be contacted. Add error handling to the script so that it ignores file not found errors, and so that it logs the names of any computers that can t be contacted. You can use either kind of error handling, but I suggest sticking with the Try construct. Note that this is slightly tricky because there are two calls to WMI involved. Here s a hint: you can safely assume that if the first WMI command works, the second one will also work.
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Listing 22.8 A script for you to add error handling to
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function Get-Inventory { BEGIN { Remove-Item c:\retries.txt } PROCESS { $os = Get-WmiObject Win32_OperatingSystem -comp $_ $bios = Get-WmiObject Win32_BIOS -comp $_ $obj = New-Object -TypeName PSObject $obj | Add-Member -MemberType NoteProperty -Name ComputerName
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Trapping and handling errors
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-Value
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($_) $obj | Add-Member -MemberType NoteProperty -Name OSBuild -Value ($os.buildnumber) $obj | Add-Member -MemberType NoteProperty -Name SPVersion -Value ($os.servicepackmajorversion) $obj | Add-Member -MemberType NoteProperty -Name BIOSSerial -Value ($bios.serialnumber) Write-Output $obj } } 'localhost','server-r2' | Out-File c:\names.txt Get-Content names.txt | Get-Inventory | Export-CSV c:\inventory.csv
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22.11 Ideas for on your own
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Make a short list of other errors that you might anticipate, and the commands that might cause them. As you start writing your own scripts in the future, refer to that list. When you find yourself using a command that you think might generate an error, build in your error-handling right from the start.
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Debugging techniques
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Anytime you ve typed more than two letters on your keyboard, you have created an opportunity for mistakes to creep in. In the programming world, those mistakes are called bugs. Although using PowerShell isn t necessarily programming, we PowerShell jockeys use the word bug, too. Fun story: The word bug, as applied to computers behaving incorrectly, actually came from a real insect (a moth) that got trapped inside Harvard University s Mark II Aiken Relay Calculator. The moth got stuck in one of the computer s relays, causing the computer to generate incorrect results. The whole story is told at http:// www.jamesshuggins.com/h/tek1/first_computer_bug.htm.
23.1 An easy guide to eliminating bugs
There are two broad categories of bugs in the PowerShell world (actually, this applies to software in general, but we ll stick with PowerShell). The first category is syntax errors, and the second is logic errors.
23.1.1 Syntax errors
Syntax errors are by far the easiest to deal with, and we won t spend much time on them in this chapter. A syntax error simply means you typed something wrong. It might be a command name that you misspelled (Gte-Content instead of GetContent, for example), or it might be that you got the actual syntax of the command wrong (forgetting the hyphen between the verb and noun of a cmdlet name, or using a colon instead of a space to separate a parameter s name and value). Whatever the cause, correcting your typing will solve the problem.
Debugging techniques
Best of all, PowerShell will usually tell you, in explicit detail, where the problem is. PowerShell might not always know what the problem is, but it will usually get pretty close to the location of the error. For example, here s a syntactically incorrect command and the resulting error message:
PS C:\> get-content -file names.txt Get-Content : A parameter cannot be found that matches parameter name 'file'. At line:1 char:18
That s pretty clear: I used a parameter named -file, and PowerShell couldn t find one. The error occurred on line 1 of my command, at character position 18. You might notice that character position 18 is where the parameter value, names.txt, is located. It s a bit odd that PowerShell chose to pinpoint that as the location of the error, but once you understand that that s how it works, you ll understand future error messages. Ultimately, the problem here is that I didn t read the help file to find out what parameters were available. Fixing the problem is as simple as reading that help file, and seeing that -path is the parameter I m after, not -file. You can help yourself avoid this kind of error by using a quality third-party PowerShell console or editor, such as SAPIEN PrimalScript (www.primalscript.com), Idera PowerShell Plus (www.idera.com), or PowerGUI (www.powergui.org; there are both free and commercial versions). These products all include a few common features:
Code hinting Reminds you of the available parameters and helps type them for you saving time, and helping to protect against typos. Syntax highlighting Colors valid syntax elements in a specific way, with invalid syntax often getting a different color. That helps to visually alert you to a potential problem. Live syntax checking Works a bit like the spell-check feature in Microsoft Word: the product puts a red underline underneath bits it doesn t think are correct, such as invalid parameter names.
You might not believe this, but I regularly see students struggling with simple syntax errors, mostly because they won t take the time to read the actual error message. I understand where they re coming from. Frankly, I freak out a little bit when all that red text starts spilling across the screen. I think it reminds me of grade school, when my teachers were ruthless with that red pen. But if you slow down, read the error message, and think about what it s saying, you can usually point your eyes directly at the problem. Read the help, and look at some of the examples in the help to see if you can figure out the correct way to proceed. Double-check your punctuation in particular. Here s a checklist:
Make sure you typed the cmdlet name correctly. If you used an alias, make sure it s typed correctly, and that it points to the cmdlet you think it does. You can run Get-Alias alias (insert your alias name for alias) to double-check which cmdlet an alias points to.
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