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Our Error Handling, Ourselves - time to fully understand and properly document PowerShell's error handling #1583

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Our Error Handling, Ourselves - time to fully understand and properly document PowerShell's error handling#1583
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area-error-handlingArea - Error handling
@mklement0

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@mklement0
mklement0
opened on Aug 15, 2017

The existing help topics that touch on error handling (about_Throw,
about_CommonParameters,
about_Preference_Variables, about_Trap, about_Try_Catch_Finally
):

  • have sown longstanding confusion due to conflating the two distinct types of terminating errors:

    • statement-terminating errors, as reported by cmdlets in certain non-recoverable situations (via the .ThrowTerminatingError() method) and by expressions in which a .NET exception / a PS runtime error occurs.
    • script-terminating errors (fatal errors), as either triggered by Throw or by escalating one of the other error types via error-action preference-variable / parameter value Stop.

  • have always contained the incorrect claim that the error-action preference / parameter values only pertain to non-terminating errors - which is true for the -ErrorAction parameter, but not the $ErrorActionPreference preference variable - see Clarify the intended behavior/documentation/terminology of terminating errors PowerShell/PowerShell#4292

It's time to:

Below is my understanding of how PowerShell's error handling actually works as of Windows PowerShell v5.1 / PowerShell Core v7.3.4, which can serve as a starting point for about_Error_Handling, along with links to issues to related problems.

Do tell me if and where I got things wrong.
The sheer complexity of PowerShell's current error handling is problematic, though I do realize that making changes in this area is a serious backward-compatibility concern.

  • Types of errors:

    • True PowerShell errors:

      • Non-terminating errors are issued by cmdlets or functions to signal failure with respect to specific inputs while continuing to process further (pipeline) input by default.

        • This allows potentially long-running commands to run to completion, despite partial failure. The errors reported can then be inspected via the error records collected in automatic variable $Error, allowing reprocessing of only the failed objects later (see below).

        • Note, however, that is possible for all input objects to cause nonterminating errors, amounting to complete failure overall.

        • Most cmdlet errors are non-terminating errors; e.g.: '/NoSuch', '/' | Get-Item reports an error for non-existent path /NoSuch, but continues processing with valid path /.

      • Terminating errors:

        • Important: In the context of remoting - whether explicit (e.g., via Invoke-Command) or implicit (e.g., via modules using implicit remoting) - terminating errors (of either kind) are converted to non-terminating ones.
        • Types of terminating errors:

          • Script-terminating (fatal) errors:

            • By default, they abort the entire enclosing script as well as any calling scripts. On the command line, a single statement or a list of statements submitted together can be thought of as an implicit script.
              • Only a try / catch or Trap statement can prevent hat.
            • The only way to directly trigger such an error is with the Throw keyword.
            • Note: Script-terminating errors are in effect fatal errors from the perspective of your code, if unhandled, - see Cmdlet.ThrowTerminatingError() is not affected by -ErrorAction PowerShell/PowerShell#14819 (comment) for a technical discussion.

          • Statement-terminating errors are the (statement-level) counterparts to non-terminating errors: they terminate the enclosing statement (pipeline or expression) and are issued to signal that a statement encountered a problem that doesn't allow it to meaningfully start or continue processing.

            • Important:
              • Statement-terminating errors truly only terminate the statement at hand. By default, the enclosing script continues to run.
              • The statement that is terminated is only the immediately enclosing statement; therefore, for instance, a statement-terminating error that occurs in the (-Process) script block of a ForEach-Object call does NOT terminate the pipeline as a whole - see below for an example.
            • Situations in which statement-terminating errors occur:
              • PowerShell's fundamental inability to even invoke a command generates a statement-terminating runtime error, namely:
                • A non-existent command:
                  nosuch -l # no such command exists
                • A cmdlet or function call with invalid syntax, such as incorrect parameter names or missing or mistyped parameter values:
                  Get-Item -Foo # -Foo is an invalid parameter name
                  Select-Object -First notanumber # -First expects an [int]
                  • Exception: If the mistyped value is bound via the pipeline, the resulting error is non-terminating see @alx9r's example.
                • Attempting to call another script that fails to parse (that is syntactically invalid).
              • While rare, cmdlet invocations that succeed syntactically can themselves generate statement-terminating errors, such as based on the contents of parameter values (not caught by parameter validation) or the state of the environment.
              • Strict-mode violations (such as trying to access a nonexistent variable when Set-StrictMode -Version 1 or higher is in effect).
              • Expressions can trigger statement-terminating errors too, namely:

                • via PowerShell expression runtime errors; e.g.:
                  1 / 0

                • via exceptions thrown by .NET method calls; e.g.:
                  [int]::Parse('foo')

                • via statement-terminating errors that occurs inside a (...) subexpression (but not inside $(...) or @(...), which are independent statement contexts); e.g.: (Get-Item -Foo) + 'hi' is a statement composed of a single expression that is terminated as a whole; by contrast, $(Get-Item -Foo) + 'hi' is a statement composed of an expression and an embedded statement, so only the $(...) part is terminated - + 'hi' is still executed.

                  • As an aside: Better names for operators $() and @() would therefore have been [array] substatement operators rather than [array] subexpression operators.

    • Failures signaled by external utilities (command-line / console applications such as findstr.exe on Windows and terminal-based utilities such as awk on Unix) via their exit codes are non-terminating - in fact, external utilities reporting nonzero exit codes are not errors in a PowerShell sense.

      • Note that the only standardized information that a utility's exit code carries is whether it succeeded overall (exit code 0) or failed overall (nonzero exit code), with no further distinction.
        • The exit code of the most recently executed external utility is stored in the automatic $LASTEXITCODE variable - see below.
      • Stderr output from external utilities is NOT considered error output - see below.

  • Default error actions, logging and success indicators:

    • Default actions when errors occur:

      • a non-terminating error:

      • a statement-terminating error:

        • terminates the current statement, but the script continues processing.
          • Important: The statement that is terminated is only the immediately enclosing statement; therefore, for instance, a statement-terminating error that occurs in the (-Process) script block of a ForEach-Object call does NOT terminate the pipeline as a whole - it only terminates the script-block invocation at hand.
            • Example: 1, 2 | ForEach-Object -Process { Get-Item -Foo } -End { 'pipeline ran to completion' }
              • The Get-Item -Foo calls cause statement-terminating errors, but the statements they terminate are the instances of the -Process script blocks, so pipeline processing continues, and pipeline ran to completion prints.
        • is output to PowerShell's error stream.
        • is logged in the automatic $Error collection.

      • a script-terminating error:

        • terminates the entire script
        • is output to PowerShell's error stream
        • is logged in the automatic $Error collection (which only matters if the session isn't terminated as a whole)

      • an external utility signaling failure by exit code / producing stderr output:

        • does not terminate the current statement (pipeline).
        • While stderr output is sent to PowerShell's error stream, it is not formatted like an error in the console, and by itself does not imply that an error occurred.
        • Stderr output is NOT logged in the automatic $Error collection

    • Default success indicators:

      • Automatic Boolean variable $? reflects whether the most recent statement, including calls to external utilities, experienced any error:

        • $True, if none, and $False, if at least one error occurred (because, in the case of non-terminating errors, multiple errors may have occurred).

        • In short: $? returning $False tells you only that some error occurred:

          • You cannot infer how many inputs experienced failure, which can range from 1 to all of them.
          • You cannot infer (from $? alone) whether a terminating error occurred or not.

        • Caveat: Because $? is set by every statement, its value is only meaningful immediately after the statement of interest.

        • $? is NOT set / set as expected in the following cases:

          • If a (by definition terminating) error was handled with a Try/Catch or Trap statement - unless the Catch handler is empty. (Non-terminating errors cannot be caught that way.)
            • With a Catch or Finally block present, the success of whatever statement appears last inside of them, if any, determines what $? is set to, with a (non-empty) Finally block taking precedence.
          • If the error is a non-terminating error and the command originating the error is enclosed in (...) - which turns the command into an expression - it is then the expression's own success that is reflected in $?:
            (Get-Item /NoSuch); $? yields $True, because the expression itself - whose sole purpose was to wrap the command - technically succeeded.
            (Get-Item -Foo); $? yields $False, because the statement-terminating error terminated the expression as a whole.

      • When calling external utilities, automatic variable $LASTEXITCODE complements $? by containing the specific exit code set by the most recently executed external utility.

        • $? is set to $True if the utility's exit code was 0, and to $False otherwise.

        • Note that while $? is only meaningful immediately after the statement of interest, $LASTEXITCODE remains relevant until the next external-utility call is made; either way, however, it is preferable to save the value in another variable if it needs to be inspected later.

      • Note: Currently, PowerShell lacks operators that allow chaining of commands based on whether they indicate success or not, such as the && and || control operators in Bash:

    • Default logging:

      • Errors are logged in memory (for the duration of the session) in the global automatic $Error variable in reverse chronological order (most recent error first; i.e., $Error[0] refers to the most recent error):

        • $Error is a collection of type [System.Collections.ArrayList] and the errors are stored as error records of type [System.Management.Automation.ErrorRecord] that wrap the underlying .NET exception, which all errors ultimately are (instances of [System.Exception] or a derived type).

        • To inspect $Error items, pipe them to Format-List -Force (direct output would result in the same format as when the error originally occurred); e.g.: $Error[0] | Format-List -Force

        • You can clear the collection anytime with $Error.Clear()

        • Errors are NOT logged in the following circumstances:

          • When non-terminating errors occur in a cmdlet / advanced function to which -ErrorAction Ignore was passed (see below).

          • What external utilities print to stderr is not considered error output, therefore it is NOT logged in $Error (see below).

  • Modifying the default error actions and logging behavior:

    • PowerShell commands and expressions:

      • Via common parameter -ErrorAction or preference variable $ErrorActionPreference:

        • Per the documentation as of this writing, specifying an error action should only affect non-terminating errors.

        • In the context of remoting, terminating errors (of either type) are converted to non-terminating ones.

          • Caveat: When invoking functions from a module that uses implicit remoting, neither -ErrorAction nor $ErrorActionPreference work as expected:

        • The supported action values are:

          • Continue ... non-terminating errors only: output errors and log them, but continue processing the current statement (non-terminating errors).
          • Stop ... non-terminating errors and statement-terminating errors via $ErrorActionPreference only: escalate the error to a script-terminating one.
          • SilentlyContinue ... like Continue, but silence error output, while still logging errors. Via $ErrorActionPreference only, also applies to both types of terminating errors (processing continues).
          • Ignore (-ErrorAction parameter only) ... non-terminating errors only: like SilentlyContinue, but without logging errors in $Error. Due to Ignore not being supported via $ErrorActionPreference, n/a to terminating errors.
          • Inquire ... prompt the user for the desired action, including the option to temporarily enter a nested session for debugging. Via $ErrorActionPreference only, also applies to both types of terminating errors.
          • Suspend (workflows only) ... automatically suspends a workflow job to allow for investigation.

        • Ad-hoc, when calling cmdlet/advanced functions, you can pass common parameter -ErrorAction to modify the behavior of non-terminating errors (only!).

          • As stated, the -ErrorAction parameter has no impact on terminating errors - in line with documented, but debatable behavior.

        • Scope-wide (including descendant scopes, unless overridden there), you can set preference variable $ErrorActionPreference, which sets the scope's default behavior for all occurrences of non-terminating behaviors, and - against documented behavior - also for terminating errors.

        • The -ErrorAction parameter takes precedence over the $ErrorActionPreference variable.

        • Non-terminating errors:

          • may alternatively be silenced with 2>$null, analogous to silencing stderr output from external utilities (see below). Terminating errors ignore 2>$null
          • may additionally be collected command-scoped in a user variable, via common parameter -ErrorVariable, unless -ErrorAction Ignore is also used. Note that use of -ErrorVariable does not affect the error output behavior; e.g., with error action Continue in effect, non-terminating errors still print to the console, whether or not you use -ErrorAction.

      • Catching terminating errors with Try / Catch or Trap statements:

        • Important: Only terminating errors (of either type) can be caught this way, and Try / Catch and Trap are effective irrespective of the current $ErrorActionPreference value (and any -ErrorAction common parameter, which fundamentally only applies to non-terminating errors).

        • Inside a Catch block:

          • Automatic variable $_ contains the [System.Management.Automation.ErrorRecord] instance representing the terminating error at hand.
          • You can use just Throw (without an argument) to re-throw the error at hand.

        • You may define multiple Catch blocks by applying optional filters based on the underlying .NET exception types - see Get-Help about_Try_Catch_Finally.

        • Error logging: Errors caught this way are still logged in the $Error collection, but there is debate around that - see Why do handled exceptions show in ErrorVariable? PowerShell/PowerShell#3768

    • External utilities:

      • Given that an external utility signaling failure by returning a nonzero exit code is not a true PowerShell error:

        • A failure signaled this way cannot be escalated to a script-terminating error via the $ErrorActionPreference variable.

        • Error messages (printed to stderr) are formatted like regular output and are NOT logged in the automatic $Error collection variable, nor do they affect how automatic variable $? is set (that is solely based the utility's exit code).

          • The rationale is that external utilities, unlike PowerShell commands, have only two output streams at their disposal: stdout for success (data) output, and stderr for everything else, and while error messages are printed to stderr, other non-data output (warnings, status information) is sent there too, so you cannot make the assumption that all stderr output represents errors.

      • However, you can capture stderr output for later inspection:

        • Using output redirection 2> allows you to suppress stderr output or send it to a file:
          whoami invalidarg 2>$null suppresses stderr output;
          whoami invalidarg 2>err.txt captures stderr output in file err.txt

          • Caveat: Due to a bug still present as of PowerShell Core v6.0.0-beta.5, when you use any 2> redirection (redirection of PowerShell's error stream):

        • There is currently no convenient mechanism for collections stderr lines in a variable, but, as a workaround, you can use redirection 2>&1 to merge stderr output into PowerShell's success output stream (interleaved with stdout output), which allows you to filter out the stderr messages later by type, because PowerShell wraps the stderr lines in [System.Management.Automation.ErrorRecord] instances; e.g.:

  • Reporting custom errors in functions and scripts:

    • Guidelines for when to use non-terminating errors vs. statement-terminating errors in the context of creating cmdlets are in MSDN topic Cmdlet Error Reporting; a pragmatic summary is in this Stack Overflow answer of mine.

    • Non-terminating errors:

    • Terminating errors:

      • Statement-terminating errors:

        • PowerShell has NO keyword- or cmdlet-based mechanism for generating a statement-terminating error.

        • The workaround - available in advanced functions only - is to use $PSCmdlet.ThrowTerminatingError():

          • Note: Despite the similarity in name with the Throw keyword (statement), this truly only generates a statement-terminating error.
          • See example below.

        • On a side note: Sometimes it is desirable to terminate upstream cmdlets, without terminating the pipeline as a whole, the way Select-Object -First <n> does, for instance. As of Windows PowerShell v5.1 / PowerShell Core v6.0.0-beta.5, there is no way to do this, but such a feature has been suggested here: Allow user code to stop a pipeline on demand / to terminate upstream cmdlets. PowerShell/PowerShell#3821

      • Script-terminating errors:

        • Use the Throw keyword.
          • Any object can be thrown, including none. In the context of a Catch handler (as part of a Try / Catch statement), not specifying an object causes the current exception to be re-thrown.
          • Unless the object thrown already is of type [System.Management.Automation.ErrorRecord], PowerShell automatically wraps the object in an instance of that type and stores the object thrown in that instance's .TargetObject property.

Example use of $PSCmdlet.WriteError() in an advanced function so as to create a non-terminating error (to work around the issue that Write-Error doesn't set$? to $False in the caller's context):

# PSv5+, using the static ::new() method to call the constructor.
# The specific error message and error category are sample values.
& { [CmdletBinding()] param()  # quick mock-up of an advanced function

  $PSCmdlet.WriteError([System.Management.Automation.ErrorRecord]::new(
      # The underlying .NET exception: if you pass a string, as here,
      #  PS creates a [System.Exception] instance.
      "Couldn't process this object.",
      $null, # error ID
      [System.Management.Automation.ErrorCategory]::InvalidData, # error category
      $null) # offending object
    )
}

# PSv4-, using New-Object:
& { [CmdletBinding()] param()  # quick mock-up of an advanced function

  $PSCmdlet.WriteError((
    New-Object System.Management.Automation.ErrorRecord "Couldn't process this object.",
      $null,
      ([System.Management.Automation.ErrorCategory]::InvalidData),
      $null
  ))

}

Example use of $PSCmdlet.ThrowTerminatingError() to create a statement-terminating error:

# PSv5+, using the static ::new() method to call the constructor.
# The specific error message and error category are sample values.
& { [CmdletBinding()] param()  # quick mock-up of an advanced function

  $PSCmdlet.ThrowTerminatingError(
    [System.Management.Automation.ErrorRecord]::new(
      # The underlying .NET exception: if you pass a string, as here,
      #  PS creates a [System.Exception] instance.
      "Something went wrong; cannot continue pipeline",
      $null, # error ID
      [System.Management.Automation.ErrorCategory]::InvalidData, # error category
      $null  # offending object
    )
  )

}

# PSv4-, using New-Object:
& { [CmdletBinding()] param()  # quick mock-up of an advanced function

  $PSCmdlet.ThrowTerminatingError((
    New-Object System.Management.Automation.ErrorRecord "Something went wrong; cannot continue pipeline",
      $null, # a custom error ID (string)
      ([System.Management.Automation.ErrorCategory]::InvalidData), # the PS error category
      $null # the target object (what object the error relates to)
  ))

}

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