CODEWITHSUNDEEP
powershellerror-handling
Andrew J. Brehm
Andrew J. Brehm

Reputation: 4758

Powershell: How can I stop errors from being displayed in a script?

When my PowerShell script tries, for example, to create a SQL Server object for a server that doesn't exist ("bla" in my case), PowerShell displays lots of PowerShell errors in red.

Since my script checks the value of $? after such calls, and displays and logs errors, I'd rather not have the several lines of PowerShell errors displayed as well.

How can I deactivate those being displayed for my script?

Upvotes: 147

Views: 426557

Answers (9)

jpmc26
jpmc26

Reputation: 29866

You're way off track here. Silencing errors is almost never a good idea, and manually checking $? explicitly after every single command is enormously cumbersome and easy to forget to do (error prone). Don't set yourself up to easily make a mistake. If you're getting lots and lots of red, that means your script kept going when it should have stopped instead. It can no longer do useful work if most of its commands are failing. Continuing a program when it and the system are in an unknown state will have unknown consequences; you could easily leave the system in a corrupt state.

The correct solution is to stop the algorithm on the first error. This principle is called "fail fast," and PowerShell has a built in mechanism to enable that behavior. It is a setting called the error preference, and setting it to the highest level will make your script (and the child scopes if they don't override it) behave this way:

$ErrorActionPreference = 'Stop'

This will produce a nice, big error message for your consumption and prevent the following commands from executing the first time something goes wrong, without having to check $? every single time you run a command. This makes the code vastly simpler and more reliable. I put it at the top of every single script I ever write, and you almost certainly should as well.

In the rare cases where you can be absolutely certain that allowing the script to continue makes sense, you can use one of two mechanisms:

  • catch: This is the better and more flexible mechanism. You can wrap a try/catch block around multiple commands, allowing the first error to stop the sequence and jump into the handler where you can log it and then otherwise recover from it or rethrow it to bubble the error up even further. You can also limit the catch to specific errors, meaning that it will only be invoked in specific situations you anticipated rather than any error. (For example, failing to create a file because it already exists warrants a different response than a security failure.)
  • The common -ErrorAction parameter: This parameter changes the error handling for one single function call, but you cannot limit it to specific types of errors. You should only use this if you can be certain that the script can continue on any error, not just the ones you can anticipate.

In your case, you probably want one big try/catch block around your entire program. Then your process will stop on the first error and the catch block can log it before exiting. This will remove a lot of duplicate code from your program in addition to cleaning up your log file and terminal output and making your program less likely to cause problems.

Do note that this doesn't handle the case when external executables fail (exit code nonzero, conventionally), so you do still need to check $LASTEXITCODE if you invoke any. Despite this limitation, the setting still saves a lot of code and effort.

Additional reliability

You might also want to consider using strict mode:

Set-StrictMode -Version Latest

This prevents PowerShell from silently proceeding when you use a non-existent variable and in other weird situations. (See the -Version parameter for details about what it restricts.)

Combining these two settings makes PowerShell much more of fail-fast language, which makes programming in it vastly easier.

Upvotes: 8

Patrice Thimothee
Patrice Thimothee

Reputation: 977

To extend on Mikkel's answer.

If you still want to capture the error, you can use "-ErrorAction stop" combined with a try - catch.

"-ErrorAction silentlycontinue" will ignore the error.

For instance:

try
{
    New-Item -Path "/somepath" -Name "somename" -ErrorAction Stop | Out-Null
}
catch
{
    echo "You must run this command in an elevated mode."
}

NOTE: There is no "silentlyStop" action, and I believe Mickel's answer refers to the "stop" action. It is likely a typo. The idea of using a try-catch combined with the "stop" action is to be able to not just dismiss eventual errors but to show something in case of errors.

Upvotes: 3

skeetastax
skeetastax

Reputation: 1658

You can also append 2>$null to your command.

Example:

$rec = Resolve-DnsName $fqdn -Server $dns 2>$null

Upvotes: 30

Berry
Berry

Reputation: 77

In some cases you can pipe after the command a Out-Null

command | Out-Null

Upvotes: 2

Jayant Rajwani
Jayant Rajwani

Reputation: 833

Add -ErrorAction SilentlyContinue to your script and you'll be good to go.

Upvotes: 6

JNK
JNK

Reputation: 65147

You have a couple of options. The easiest involve using the ErrorAction settings.

-Erroraction is a universal parameter for all cmdlets. If there are special commands you want to ignore you can use -erroraction 'silentlycontinue' which will basically ignore all error messages generated by that command. You can also use the Ignore value (in PowerShell 3+):

Unlike SilentlyContinue, Ignore does not add the error message to the $Error automatic variable.

If you want to ignore all errors in a script, you can use the system variable $ErrorActionPreference and do the same thing: $ErrorActionPreference= 'silentlycontinue'

See about_CommonParameters for more info about -ErrorAction. See about_preference_variables for more info about $ErrorActionPreference.

Upvotes: 206

Mikkel Vejlby
Mikkel Vejlby

Reputation: 31

If you want the powershell errormessage for a cmdlet suppressed, but still want to catch the error, use "-erroraction 'silentlyStop'"

Upvotes: -1

D-W
D-W

Reputation: 89

I had a similar problem when trying to resolve host names using [system.net.dns]. If the IP wasn't resolved .Net threw a terminating error. To prevent the terminating error and still retain control of the output, I created a function using TRAP.

E.G.

Function Get-IP 
{PARAM   ([string]$HostName="")
PROCESS {TRAP 
             {"" ;continue} 
             [system.net.dns]::gethostaddresses($HostName)
        }
}

Upvotes: 8

JPBlanc
JPBlanc

Reputation: 72610

Windows PowerShell provides two mechanisms for reporting errors: one mechanism for terminating errors and another mechanism for non-terminating errors.

Internal CmdLets code can call a ThrowTerminatingError method when an error occurs that does not or should not allow the cmdlet to continue to process its input objects. The script writter can them use exception to catch these error.

EX :

try
{
  Your database code
}
catch
{
  Error reporting/logging
}

Internal CmdLets code can call a WriteError method to report non-terminating errors when the cmdlet can continue processing the input objects. The script writer can then use -ErrorAction option to hide the messages, or use the $ErrorActionPreference to setup the entire script behaviour.

Upvotes: 24

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