Advanced Practice Event

by Don Jones


I want to direct your attention to this forums post, which I think is worth anyone’s time to look through. I’ve left a pretty long reply with some comments on the entry that would also be worth a read.

I find that a LOT of folks – like the gentleman who posted his script – have a really good approach to PowerShell scripts. They want to use parameters. They want verbose output. They want to proactively check for errors. Where I think folks get lost is in the fine points of how PowerShell enables these features. I see folks working harder than they need to, coding functionality that the shell will actually give them for free. I also see some not-entirely-perfect approaches to things like parameters and error handling, and some occasional mis-use of advanced features (I often see SupportsShouldProcess declared but not actually implemented).

Sometimes, this simply happens because a lot of these advanced features aren’t well-documented in one convenient spot – they’re all spread out – and because folks are learning from blog posts, which may themselves have been written by someone with an incomplete understanding. Or, they’re pasting bits together without really knowing what they’re doing. That’s cool – what you have to sometimes do is take a whack at something like this poster did, and get some feedback. I’m really glad he did, because it offers an opportunity to clear up some misunderstandings, which will just make his scripts even better in the future.

I hope everyone’s looking at the Games as a learning opportunity. I hope everyone will vote on folks’ entries and leave comments when they do; I hope as many people as possible spend some time blogging about what  they see, what they’ve learned, and what they don’t understand. That’s how we’ll all improve.

Let me give you a perfect example (we’re no longer discussing the forums post, here – I’m moving on to a new topic):

Try {
  $continue = $true
  $bios = Get-WmiObject -class Win32_BIOS -computername $computer -EA Stop
} Catch {
  $continue = $false
  $computer | Out-File errors.txt -append
}
if ($continue) {
 $os = Get-WmiObject -class Win32_OperatingSystem -computername $computer
 # and so on...
}

This is how I used to code for error handling when querying multiple WMI classes. I’d set a “flag” variable, $continue, to $false if the first WMI call failed, so that I didn’t waste time on subsequent calls. Note that this is just a snippet; it isn’t an entire script. Then I had a student who coded it this way:

Try {
  $bios = Get-WmiObject -class Win32_BIOS -computername $computer -EA Stop
  $os = Get-WmiObject -class Win32_OperatingSystem -computername $computer
  # and so on...
} Catch {
  $computer | Out-File errors.txt -append
}

Much more concise, and same effect. If the first WMI call fails, I jump into the Catch block, and skip the remaining code anyway. So there are constantly learning opportunities in seeing someone else’s approach. For me, I learn new approaches that are sometimes better than what I’ve been doing. I also learn how to better teach PowerShell to people, by seeing common mistakes and misunderstandings. It’s great to share your failures – that’s how we grow!

Update: Someone dropped me a line and made a couple of points, which I want to address:

In the reply to the blog post you say: “Please consider properly setting -ErrorAction on the command (Get-WmiObject, in your case) and using a Try/Catch construct to actually handle errors, not just hide them.” The example shown does the exact opposite. Any terminating error is caught, logged to a file, but not re-thrown effectively hiding the exception.

I disagree. First, handling an error may still involve suppressing the error message. But I’m suppressing it for just one command, not the entire script; I’m also handling the error by, in my case, logging it to a file. How you choose to handle may differ. What I don’t want to do is toss a terminating exception – I’m in a loop, and want my command to continue processing the next object.

Also the $os  = … part is missing the -errorAction STOP.

That’s deliberate. If there’s going to be an anticipated error – lack of connectivity, bad credentials, etc., I’m going to get an error on the first WMI call ($bios). I’ll trap it, log it, and move on to the next computer (one presumes those snippets of mine are running in a loop of some kind, processing one computer at a time). If there’s an unexpected error, like a corrupt WMI repository or something, the second WMI call ($os) will explode, generating an error that I very much want to see, because I didn’t anticipate it.

Notice a word that I used a lot there: “I.” I’m coding the script for the way I want to it to run. I want anticipated errors logged, and I want unanticipated errors to continue exploding. You may want your scripts to do different things. I’m not putting these snippets out there as the One True Way To Code, because there’s no such thing. What I am saying is that you need to think about why you’re coding the way you are, and have some justification for it.

Globally suppressing error messages, but not doing anything to handle errors that you do suppress, is a poor practice. Beyond that, do what you need to do. I’m fine with someone suppressing an error they’ve dealt with. But if your code isn’t dealing with it, then the person running the script needs to see something’s gone wrong.

 

About the Author

About Don Jones

Don Jones is a Windows PowerShell MVP, author of several Windows PowerShell books (and other IT books), Co-founder and President/CEO of PowerShell.org, PowerShell columnist for Microsoft TechNet Magazine, PowerShell educator, and designer/author of several Windows PowerShell courses (including Microsoft's). Power to the shell!

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