An easy guide to eliminating bugs in Visual Basic .NET

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An easy guide to eliminating bugs
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Make sure parameter names are preceded by a dash and are followed by a space. Make sure you re using the correct parameter name (read the help!), and if you re abbreviating the parameter name, make sure you re providing enough characters to uniquely identify that parameter. Most of PowerShell s punctuation comes in pairs: single quotes, double quotes, square brackets, curly braces, and parentheses are all good examples. Make sure that you end every set that you start, and that you properly nest them. Improper nesting, like ({this)}, means you re ending a pair before ending the pair it encloses. In that example, I closed the parentheses before the curly braces, which is the opposite of the correct order. Watch your spaces. In PowerShell, spaces are special characters that indicate a separation between command elements. PowerShell isn t that case-sensitive (meaning that upper- and lowercase are usually the same to the shell), but it s very space-sensitive. There s a space after a cmdlet name and before any parameters or values. There s a space in between parameter names and values. There s a space after one parameter and before the next. Don t forget those.
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I m not kidding about the red text freaking me out. It s especially embarrassing when someone is looking over my shoulder. Half the time, I ll just run Cls (which is an alias to Clear-Host) to make it all go away, rather than reading the message. Try getting an error in the middle of a conference session demonstration, with a thousand people watching you! No pressure! So here s a trick I use: I ll change the color of the error message text to green. Seriously, I do.
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(Get-Host).PrivateData.ErrorForegroundColor = 'green'
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That only lasts for the duration of the shell session, so I ll either put that in a profile script (more on those in chapter 24), or I ll just remember to do it before I start working in the shell. The green text makes me feel a lot better, and it doesn t look so aggressive when it pops up in the middle of a demonstration. You can also change the ErrorBackgroundColor, WarningForegroundColor, WarningBackgroundColor, and other colors. TechNet has a nice Modifying Message Colors article on the available options (http://mng.bz/1037). I ll also cover these in more detail in chapter 24.
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That s really everything you need to know about syntax errors. They re a pain in the neck, but they shouldn t be that difficult to fix. Just pay close attention to what you re typing.
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23.1.2 Logic errors
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Logic errors mean that your script or command isn t doing what you want it to do, but it s not necessarily generating an error.
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Debugging techniques
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Some logic errors will produce straightforward errors. You should know what to do with a file not found error, for example, or an access denied message, but sometimes errors aren t always so clear. Get-WmiObject, for example, can produce an RPC server not found error if it s not able to locate a remote computer, or if that computer can t accept the WMI connection (perhaps because of a firewall or a permissions issue). But the most vexing logic errors are the ones that don t produce any error at all they just prevent your script or command from working properly. This next listing is an example go ahead and open this script in the PowerShell ISE and run it, or just run it from the PowerShell console host.
Listing 23.1 A short script containing logic errors
$name = Read-Host "Enter computer name" if (test-connection $name) { get-wmiobject win32_bios -computername $nmae }
Logic errors, like syntax errors, can come from typos, and one of the errors in listing 23.1 is a simple typo. Logic errors also come from what I call a bad assumption: you re assuming that a particular variable, cmdlet output, or property contains one thing, when, in fact, it contains something entirely different. Although listing 23.1 is pretty short, it manages to contain a bad assumption as well as a typo. Debugging causes a lot of frustration for a lot of administrators. I ll try and make it simpler by telling you exactly what you need to know to debug any script or command, no matter how complicated it is:
You can t debug a script or command unless you have a clear expectation of what it s going to do. You must execute your script and examine its reality (what it actually does), and compare that reality to your expectations. When reality and your expectations differ, you have found the bug. While executing the script and examining it, you need to read very, very carefully, so that you can spot typos. Sometimes using a different font can help.
In the next three sections, I ll use listing 23.1 as an example, and show you different ways to debug it.
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