Putting it All Together

Now that we've looked at all of the error handling tools and identified some potential "gotcha" scenarios, here are some tips and examples of how I approach error handling in my own scripts.

Suppressing errors (Mostly, don't do this)

There are occasions where you might suppress an error without the intention of handling it, but the valid situations for this are few and far between. For the most part, don't set ErrorAction or $ErrorActionPreference to SilentlyContinue unless you intend to examine and respond to the errors yourself later in the code. Using Try/Catch with an empty catch block amounts to the same thing for terminating errors; it's usually the wrong thing to do.

It's better to at least give the user the default error output in the console than it is to have a command fail with no indication whatsoever that something went wrong.

The $? variable (Use it at your own risk)

The $? variable seems like a good idea on paper, but there are enough ways for it to give you bad data that I just don't trust it in a production script. For example, if the error is generated by a command that is in parentheses or a sub-expression, the $? variable will be set to True instead of False:

image015.png

Figure 4.1: Annoying false positives from $?

Determining what types of errors can be produced by a command

Before you can decide how best to handle the error(s) from a particular command, you'll often need to know what kind of errors it might produce. Are they terminating or non-terminating? What are the Exception types? Unfortunately, PowerShell's cmdlet documentation doesn't give you this information, so you need to resort to some trial and error. Here's an example of how you can figure out whether errors from a cmdlet are Terminating or Non-Terminating:

image016.png

Figure 4.2: Identifying Terminating errors.

Ironically, this was a handy place both to use the Trap statement and to set $ErrorActionPreference to SilentlyContinue, both things that I would almost never do in an enterprise script. As you can see in figure 4.2, Get-Acl produces terminating exceptions when the file exists, but the cmdlet cannot read the ACL. Get-Item and Get-Acl both produce non-terminating errors if the file doesn't exist.

Going through this sort of trial and error can be a time-consuming process, though. You need to come up with the different ways a command might fail, and then reproduce those conditions to see if the resulting error was terminating or non-terminating. As a result of how annoying this can be, in addition to this ebook, the GitHub repository will contain a spreadsheet with a list of known Terminating errors from cmdlets. That will be a living document, possibly converted to a wiki at some point. While it will likely never be a complete reference, due to the massive number of PowerShell cmdlets out there, it's a lot better than nothing.

In addition to knowing whether errors are terminating or non-terminating, you may also want to know what types of Exceptions are being produced. Figure 4.3 demonstrates how you can list the exception types that are associated with different types of errors. Each Exception object may optionally contain an InnerException, and you can use any of them in a Catch or Trap block:

image017.png

Figure 4.3: Displaying the types of Exceptions and any InnerExceptions.

Dealing with Terminating Errors

This is the easy part. Just use Try/Catch, and refer to either $_ or $error[0] in your Catch blocks to get information about the terminating error.

Dealing with Non-Terminating Errors

I tend to categorize commands that can produce Non-Terminating errors (Cmdlets, functions and scripts) in one of three ways: Commands that only need to process a single input object, commands that can only produce Non-Terminating errors, and commands that could produce a Terminating or Non-Terminating error. I handle each of these categories in the following ways:

If the command only needs to process a single input object, as in figure 4.4, I use ErrorAction Stop and handle errors with Try/Catch. Because the cmdlet is only dealing with a single input object, the concept of a Non-Terminating error is not terribly useful anyway.

image018.png

Figure 4.4: Using Try/Catch and ErrorAction Stop when dealing with a single object.

If the command should only ever produce Non-Terminating errors, I use ErrorVariable. This category is larger than you'd think; most PowerShell cmdlet errors are Non-Terminating:

image019.png

Figure 4.5: Using ErrorVariable when you won't be annoyed by its behavior arising from Terminating errors.

When you're examining the contents of your ErrorVariable, remember that you can usually get useful information about what failed by looking at an ErrorRecord's CategoryInfo.Activity property (which cmdlet produced the error) and TargetObject property (which object was it processing when the error occurred). However, not all cmdlets populate the ErrorRecord with a TargetObject, so you'll want to do some testing ahead of time to determine how useful this technique will be. If you find a situation where a cmdlet should be telling you about the TargetObject, but doesn't, consider changing your code structure to process one object at a time, as in figure 4.4. That way, you'll already know what object is being processed.

A trickier scenario arises if a particular command might produce either Terminating or Non-Terminating errors. In those situations, if it's practical, I try to change my code to call the command on one object at a time. If you find yourself in a situation where this is not desirable (though I'm hard pressed to come up with an example), I recommend the following approach to avoid ErrorVariable's quirky behavior and also avoid calling $error.Clear():

image020.png

Figure 4.6: Using $error without calling Clear() and ignoring previously-existing error records.

As you can see, the structure of this code is almost the same as when using the ErrorVariable parameter, with the addition of a Try block around the offending code, and the use of the $previousError variable to make sure we're only reacting to new errors in the $error collection. In this case, I have an empty Catch block, because the terminating error (if one occurs) is going to be also added to $error and handled in the foreach loop anyway. You may prefer to handle the terminating error in the Catch block and non-terminating errors in the loop; either way works.

Calling external programs

When you need to call an external executable, most of the time you'll get the best results by checking $LASTEXITCODE for status information; however, you'll need do your homework on the program to make sure it returns useful information via its exit code. There are some odd executables out there that always return 0, regardless of whether they encountered errors.

If an external executable writes anything to the StdErr stream, PowerShell sometimes sees this and wraps the text in an ErrorRecord, but this behavior doesn't seem to be consistent. I'm not sure yet under what conditions these errors will be produced, so I tend to stick with $LASTEXITCODE when I need to tell whether an external command worked or not.