How to capture error output only in a variable in

Question

I want to store the stderr output of a PowerShell command in a variable. I don't want to store it in a file and I don't want standard output included, just the error output.

This redirects to a file named error.txt:

& $command $params 2> error.txt

This redirects both stderr and stdout to the $output variable:

$output = & $command $params 2>&1

But I want to store only the error output in a variable (the same as the content of the error.txt file above), without writing anything to file. How do I do that?

Answer 1

You can call the command a slightly different way and use the -ErrorVariable parameter in PowerShell:

Invoke-Expression "$command $params" -ErrorVariable badoutput

$badoutput will now contain the contents of the error string.

Answer 2

To add to arco444, the specific exception can be obtained by using $badoutput[0].Exception.

-ErrorAction and -ErrorVariable has more information on to effectively use the ErrorAction and ErrorVariable parameters built into PowerShell.

Here is the easiest way to show this working:

PS> Stop-Process 13,23
Stop-Process : Cannot find a process with the process identifier 13.
At line:1 char:13
+ Stop-Process  <<<< 13,23
Stop-Process : Cannot find a process with the process identifier 23.
At line:1 char:13
+ Stop-Process  <<<< 13,23

PS> Stop-Process 13,23 -ErrorAction Stop  # Only 1 error
Stop-Process : Command execution stopped because the shell variable “ErrorA
ctionPreference” is set to Stop: Cannot find a process with the process ide
ntifier 13.
At line:1 char:13
+ Stop-Process  <<<< 13,23 -ErrorAction Stop  # Only 1 error

PS> Stop-Process 13,23 -ErrorAction silentlycontinue  # No errors

PS> Stop-Process 13,23 -ErrorAction inquire  # ASK
Confirm
Cannot find a process with the process identifier 13.
[Y] Yes  [A] Yes to All  [H] Halt Command  [S] Suspend  [?] Help
(default is “Y”):h
Stop-Process : Command execution stopped because the user selected the Halt
 option.
At line:1 char:13
+ Stop-Process  <<<< 13,23 -ErrorAction inquire  # ASK
PS>
PS>
PS> Stop-Process 13,23 -ErrorVariable a -ErrorAction SilentlyContinue

PS> $a[0]
Stop-Process : Cannot find a process with the process identifier 13.
At line:1 char:13
+ Stop-Process  <<<< 13,23 -ErrorVariable a -ErrorAction SilentlyContinue

PS> $a[0] |fl * -Force

Exception             : Microsoft.PowerShell.Commands.ProcessCommandExcepti
                        on: Cannot find a process with the process identifi
                        er 13.
TargetObject          : 13
CategoryInfo          : ObjectNotFound: (13:Int32) [Stop-Process], ProcessC
                        ommandException
FullyQualifiedErrorId : NoProcessFoundForGivenId,Microsoft.PowerShell.Comma
                        nds.StopProcessCommand
ErrorDetails          :
InvocationInfo        : System.Management.Automation.InvocationInfo

PS> $a |ft TargetObject -force -auto
TargetObject
————
          13
          23

Now one thing that is not obvious to people is that you can specify a “+” in front of the variable name for ErrorVariable and we will ADD the errors to that variable.

PS> $err=@()
PS> stop-process 13 -ea silentlycontinue -ErrorVariable err
PS> $err.count
1

PS> stop-process 23 -ea silentlycontinue -ErrorVariable +err
PS> $err.count
2
PS> $err
Stop-Process : Cannot find a process with the process identifier 13.
At line:1 char:13
+ stop-process  <<<< 13 -ea silentlycontinue -ErrorVariable err
Stop-Process : Cannot find a process with the process identifier 23.
At line:1 char:13
+ stop-process  <<<< 23 -ea silentlycontinue -ErrorVariable +err

Lastly, you don’t need to type out –ErrorAction or –ErrorVariable, we have defined parameter aliases for these so you can just type –EA and -EV.