How to save powershell hyper-v errors to file - powershell

Question

I am making a script to turn virtual machines on and off in hyper-v.
Sometimes the Stop-VM command fails and I need to save the bug or reflect it in some way in a log file
I tried putting the command in a trycath but it didn't work.
Command:
Stop-VM $VMapagar
Sometimes the command gives me this error and does not turn off the machine
Stop-VM: Could not stop.
I would like to be able to reflect the failure in some way in a log.txt
Thanks!

Answer 1

Use Try..Catch to trap the error by telling PS to treat it as a terminating error, then process it as you require:
# Rest of your script
Try {
# Run your command, but tell PS to stop if it find an error
# You can explore the effects of the other possible values for -ErrorAction in PS documentation.
Stop-VM $VMpagar -ErrorAction Stop
# If it's got this far, then there can't have been an error so write a success message to console
Write-Host "OK"
}
Catch {
# This code will process if there was an error in the "Try" block
# By default, within the "Catch" block, the "$_" variable contains the error message
Write-Host "Error: $_"
# Write the error to a log file - "`n" tells PS to write a newline before the subsequent text
Add-Content -Path 'c:\temp\log.txt' -Value "`n$_"
# You could stop the script here using "Throw" or "Exit" commands if you want the whole script to stop on ANY error
}
# Your script will continue from this point if you haven't stopped it

Answer 2

Scepticalist's helpful answer shows how to capture a terminating error, by using the common -ErrorAction (-ea) parameter with value 'Stop' in order to promote non-terminating errors (the most common kind) to terminating ones, which allows them to be trapped with a try/ catch / finally statement.
Note that this approach limits you to capturing the first non-terminating error (whereas a single cmdlet call may emit multiple ones), because it - thanks to -ErrorAction Stop - then instantly terminates the statement and transfers control the catch block (where the automatic $_ variable reflects the triggering error in the form of an [ErrorRecord] instance).
Also note that execution continues after a catch block by default - unless you explicitly use throw to re-throw the terminating error (or use a statement such as exit to exit the script).
To capture - potentially multiple - non-terminating errors you have two options:
Redirect them directly to a file, using the redirection operator > with the number of the error stream, 2:
Stop-Vm $vms 2>errs.txt
This sends any errors quietly to file errs.txt; that is, you won't see them in the console. If no errors occur, an empty file is created.
Note: This technique is the only option for directly redirecting an external program's errors (stderr output); however, using redirection 2>&1 you can capture success output (stdout) and errors (stderr) combined, and split them by their source stream later - see the bottom section of this answer.
Use the common -ErrorVariable (-ev) parameter to collect any non-terminating errors in a variable - note that the target variable must be specified without the $:
Stop-Vm $vms -ErrorVariable errs
By default, the errors are still output as well and therefore print to the console (host) by default, but you can add -ErrorAction SilentlyContinue to prevent that. Caveat: Do not use -ErrorAction Ignore, as that will categorically suppress errors and prevent their collection.
You can then inspect the $errs array (list), which is empty if no errors occurred and otherwise contains one or more [ErrorRecord] instances, and send the collected errors to a file on demand; e.g.:
if ($errs) { $errs > errs.txt }
See also:
This answer for information about PowerShell's two fundamental error types.
GitHub docs issue #1583 for a comprehensive overview of PowerShell's surprisingly complex error handling.