Looking at a Get-WebFile script over on PoshCode, http://poshcode.org/3226, I noticed this strange-to-me contraption:
$URL_Format_Error = [string]\"...\"
Wri
Addition to Andy Arismendi's answer:
Whether Write-Error terminates the process or not depends on the $ErrorActionPreference
setting.
For non-trivial scripts, $ErrorActionPreference = "Stop"
is a recommended setting to fail fast.
"PowerShell’s default behaviour with respect to errors, which is to continue on error ...feels very VB6 “On Error Resume Next”-ish"
(from http://codebetter.com/jameskovacs/2010/02/25/the-exec-problem/)
However, it makes Write-Error
calls terminating.
To use Write-Error as a non-terminating command regardless of other environment settings, you can use common parameter -ErrorAction
with value Continue
:
Write-Error "Error Message" -ErrorAction:Continue
The main difference between the Write-Error cmdlet and the throw keyword in PowerShell is that the former simply prints some text to the standard error stream (stderr), while the latter actually terminates the processing of the running command or function, which is then handled by PowerShell by sending out information about the error to the console.
You can observe the different behavior of the two in the examples you provided:
$URL_Format_Error = [string]"..."
Write-Error $URL_Format_Error
return
In this example the return
keyword has been added to explicitly stop the execution of the script after the error message has been sent out to the console. In the second example, on the other hand, the return
keyword is not necessary since termination is implicitly done by throw
:
$URL_Format_Error = New-Object System.FormatException "..."
Throw $URL_Format_Error
Important: There are 2 types of terminating errors, which the current help topics unfortunately conflate:
statement-terminating errors, as reported by cmdlets in certain non-recoverable situations and by expressions in which a .NET exception / a PS runtime error occurs; only the statement is terminated, and script execution continues by default.
script-terminating errors (more accurately: runspace-terminating), as either triggered by Throw
or by escalating one of the other error types via error-action preference-variable / parameter value Stop
.
Unless caught, they terminate the current runspace (thread); that is, they terminate not just the current script, but all its callers too, if applicable).
For a comprehensive overview of PowerShell's error handling, see this GitHub documentation issue.
The remainder of this post focuses on non-terminating vs. statement-terminating errors.
To complement the existing helpful answers with a focus on the core of the question: How do you choose whether to report a statement-terminating or non-terminating error?
Cmdlet Error Reporting contains helpful guidelines; let me attempt a pragmatic summary:
The general idea behind non-terminating errors is to allow "fault-tolerant" processing of large input sets: failure to process a subset of the input objects should not (by default) abort the - potentially long-running - process as a whole, allowing you to inspect the errors and reprocess only the failed objects later - as reported via the error records collected in automatic variable $Error
.
Report a NON-TERMINATING error, if your cmdlet / advanced function:
$PSCmdlet.WriteError()
to report a non-terminating error (Write-Error
, unfortunately, doesn't cause $?
to be set to $False
in the caller's scope - see this GitHub issue).$?
tells you whether the most recent command reported at least one non-terminating error.
$?
being $False
can either mean that any (nonempty) subset of input objects weren't properly processed, possibly the entire set.$ErrorActionPreference
and/or common cmdlet parameter -ErrorAction
can modify the behavior of non-terminating errors (only) in terms of error output behavior and whether non-terminating errors should be escalated to script-terminating ones.Report a STATEMENT-TERMINATING error in all other cases.
$PSCmdlet.ThrowTerminatingError()
in order to generate a statement-terminating error.Throw
keyword generates a script-terminating error that aborts the entire script (technically: the current thread).try/catch
handler or trap
statement may be used (which cannot be used with non-terminating errors), but note that even statement-terminating errors by default do not prevent the rest of the script from running. As with non-terminating errors, $?
reflects $False
if the previous statement triggered a statement-terminating error.Sadly, not all of PowerShell's own core cmdlets play by these rules:
While unlikely to fail, New-TemporaryFile (PSv5+) would report a non-terminating error if it failed, despite not accepting pipeline input and only producing one output object - this has been corrected as of at least PowerShell [Core] 7.0, however: see this GitHub issue.
Resume-Job 's help claims that passing an unsupported job type (such as a job created with Start-Job
, which is not supported, because Resume-Job
only applies to workflow jobs) causes a terminating error, but that's not true as of PSv5.1.
Write-Error
allows the consumer of the function to suppress the error message with -ErrorAction SilentlyContinue
(alternatively -ea 0
). While throw
requires a try{...} catch {..}
To use a try...catch with Write-Error
:
try {
SomeFunction -ErrorAction Stop
}
catch {
DoSomething
}
If your reading of the code is correct then you are correct. Terminating errors should use throw
, and if you're dealing with .NET types then it is helpful to also follow .NET exception conventions.
Write-Error
should be used if you want to inform the user of a non-critical error. By default all it does is print an error message in red text on the console. It does not stop a pipeline or a loop from continuing. Throw
on the other hand produces what is called a terminating error. If you use throw, the pipeline and/or current loop will be terminated. In fact all execution will be terminated unless you use a trap
or a try/catch
structure to handle the terminating error.
There is one thing to note, if you set $ErrorActionPreference
to "Stop"
and use Write-Error
it will produce a terminating error.
In the script you linked to we find this:
if ($url.Contains("http")) {
$request = [System.Net.HttpWebRequest]::Create($url)
}
else {
$URL_Format_Error = [string]"Connection protocol not specified. Recommended action: Try again using protocol (for example 'http://" + $url + "') instead. Function aborting..."
Write-Error $URL_Format_Error
return
}
It looks like the author of that function wanted to stop the execution of that function and display an error message on screen but did not want the entire script to stop executing. The script author could have used throw
however it would mean you would have to use a try/catch
when calling the function.
return
will exit the current scope which can be a function, script, or script block. This is best illustrated with code:
# A foreach loop.
foreach ( $i in (1..10) ) { Write-Host $i ; if ($i -eq 5) { return } }
# A for loop.
for ($i = 1; $i -le 10; $i++) { Write-Host $i ; if ($i -eq 5) { return } }
Output for both:
1
2
3
4
5
One gotcha here is using return
with ForEach-Object
. It will not break processing like one might expect.
More information:
$ErrorActionPreference
: about_Preference_Variablestry/catch
: about_Try_Catch_Finallytrap
: about_Trapthrow
: about_Throwreturn
: about_Return