Error Handling in PowerShell - inspect error output from external programs

Question

I have a script that runs a batch file. The batch file is throwing some errors, but runs successfully. But when the batch file throws errors, the powershell script throws an error.

Included in the output is "0 failed". Is there a way to check to check to see if the code contains this message, and if it does then to ignore all errors?

Here is some psuedocode that might be able to describe what I am trying to accomplish.

$output = ./testrunner.bat 
if ($output -Like "(0 failed)") { 
    # ignore errors
} else {
    # don't ignore errors 
}

mklement0 · Accepted Answer · 2022-10-03 15:51:15Z

Unfortunately, it is not easy to selectively capture stderr output from an external utility (console application) in a variable.

The most concise, though somewhat obscure approach is (PSv4+):

# First, make sure that $ErrorActionPreference is at its default,
# 'Continue'

$stdout, $stderr = (./testrunner.bat 2>&1).Where({
  $_ -is [string]
}, 'Split')

Note: If you're content to collect the combined stdout/stderr output in a single variable, the task simplifies to:

$output = ./testrunner.bat 2>&1

Note that the variables in both scenarios will contain an array of objects representing the collection of the individual output lines: stdout lines become (as onw would expect) [string] array elements, whereas PS converts stderr lines to [System.Management.Automation.ErrorRecord] instances.

You can then inspect the $stderr (or $output) variable for the success/failure indicator substring - note the use of enclosing wildcard characters *, because -like always matches against the entire string (unlike the regex-matching -match operator):

if ($stderr -like '*(0 failed)*') {
    # ignore errors
} else {
    # don't ignore errors 
}

As for what you experienced:

when the batch file throws errors, the powershell script throws an error.

External programs, including batch files cannot throw errors in a PowerShell sense; they integrate as follows:

Their exit code is recorded in $LASTEXITCODE, and $? is set to $True if $LASTEXITCODE is 0, and $False otherwise.
In the regular console, stderr output is passed through to the console - not colored in red - and not logged in $Error (the in-memory log of errors). In other words: stdout and stderr lines are formatted the same.
Unfortunately, the ISE behaves differently and does print stderr lines as if they were PowerShell errors and does log them in $Error - see this GitHub issue.

Therefore, it sounds like you were running from the ISE.
Additionally, if $ErrorActionPreference = 'Stop' is in effect when you run your code from the ISE, not only would the first stderr line present as a PowerShell error, but it would trigger termination of your entire PowerShell script.

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