Capture program stdout and stderr to separate variables

Question

Is it possible to redirect stdout from an external program to a variable and stderr from external programs to another variable in one run?

For example:

$global:ERRORS = @();
$global:PROGERR = @();

function test() {
    # Can we redirect errors to $PROGERR here, leaving stdout for $OUTPUT?
    $OUTPUT = (& myprogram.exe 'argv[0]', 'argv[1]');

    if ( $OUTPUT | select-string -Pattern "foo" ) {
        # do stuff
    } else {
        $global:ERRORS += "test(): oh noes! 'foo' missing!";
    }
}

test;
if ( @($global:ERRORS).length -gt 0 ) {
    Write-Host "Script specific error occurred";
    foreach ( $err in $global:ERRORS ) {
        $host.ui.WriteErrorLine("err: $err");
    }
} else {
    Write-Host "Script ran fine!";
}

if ( @($global:PROGERR).length -gt 0 ) {
    # do stuff
} else {
    Write-Host "External program ran fine!";
}

A dull example however I am wondering if that is possible?

Answer 1

One option is to combine the output of stdout and stderr into a single stream, then filter.

Data from stdout will be strings, while stderr produces System.Management.Automation.ErrorRecord objects.

$allOutput = & myprogram.exe 2>&1
$stderr = $allOutput | ?{ $_ -is [System.Management.Automation.ErrorRecord] }
$stdout = $allOutput | ?{ $_ -isnot [System.Management.Automation.ErrorRecord] }

Answer 2

The easiest way to do this is to use a file for the stderr output, e.g.:

$output = & myprogram.exe 'argv[0]', 'argv[1]' 2>stderr.txt
$err = get-content stderr.txt
if ($LastExitCode -ne 0) { ... handle error ... }

I would also use $LastExitCode to check for errors from native console EXE files.

Answer 3

You should be using Start-Process with -RedirectStandardError -RedirectStandardOutput options. This other post has a great example of how to do this (sampled from that post below):

$pinfo = New-Object System.Diagnostics.ProcessStartInfo
$pinfo.FileName = "ping.exe"
$pinfo.RedirectStandardError = $true
$pinfo.RedirectStandardOutput = $true
$pinfo.UseShellExecute = $false
$pinfo.Arguments = "localhost"
$p = New-Object System.Diagnostics.Process
$p.StartInfo = $pinfo
$p.Start() | Out-Null
$p.WaitForExit()
$stdout = $p.StandardOutput.ReadToEnd()
$stderr = $p.StandardError.ReadToEnd()
Write-Host "stdout: $stdout"
Write-Host "stderr: $stderr"
Write-Host "exit code: " + $p.ExitCode

Answer 4

This is also an alternative that I have used to redirect stdout and stderr of a command line while still showing the output during PowerShell execution:

$command = "myexecutable.exe my command line params"

Invoke-Expression $command -OutVariable output -ErrorVariable errors
Write-Host "STDOUT"
Write-Host $output
Write-Host "STDERR"
Write-Host $errors

It is just another possibility to supplement what was already given.

Keep in mind this may not always work depending upon how the script is invoked. I have had problems with -OutVariable and -ErrorVariable when invoked from a standard command line rather than a PowerShell command line like this:

PowerShell -File ".\FileName.ps1"

An alternative that seems to work under most circumstances is this:

$stdOutAndError = Invoke-Expression "$command 2>&1"

Unfortunately, you will lose output to the command line during execution of the script and would have to Write-Host $stdOutAndError after the command returns to make it "a part of the record" (like a part of a Jenkins batch file run). And unfortunately it doesn't separate stdout and stderr.

Answer 5

Copied from my answer on how to capture both output and verbose information in different variables.

---

Using Where-Object(The alias is symbol ?) is an obvious method, but it's a bit too cumbersome. It needs a lot of code.

In this way, it will not only take longer time, but also increase the probability of error.

In fact, there is a more concise method that separate different streams to different variable in PowerShell(it came to me by accident).

# First, declare a method that outputs both streams at the same time.
function thisFunc {
    [cmdletbinding()]
    param()
    Write-Output 'Output'
    Write-Verbose 'Verbose'
}
# The separation is done in a single statement.Our goal has been achieved.
$VerboseStream = (thisFunc -Verbose | Tee-Object -Variable 'String' | Out-Null) 4>&1

Then we verify the contents of these two variables

$VerboseStream.getType().FullName
$String.getType().FullName

The following information should appear on the console:

PS> System.Management.Automation.VerboseRecord
System.String

'4>&1' means to redirect the verboseStream to the success stream, which can then be saved to a variable, of course you can change this number to any number between 2 and 5.

Answer 6

In case you want to get any from a PowerShell script and to pass a function name followed by any arguments you can use dot sourcing to call the function name and its parameters.

Then using part of James answer to get the $output or the $errors.

The .ps1 file is called W:\Path With Spaces\Get-Something.ps1 with a function inside named Get-It and a parameter FilePath.

Both the paths are wrapped in quotes to prevent spaces in the paths breaking the command.

$command = '. "C:\Path Spaces\Get-Something.ps1"; Get-It -FilePath "W:\Apps\settings.json"'

Invoke-Expression $command -OutVariable output -ErrorVariable errors | Out-Null

# This will get its output.
$output

# This will output the errors.
$errors

Answer 7

Separately, preserving formatting

cls
function GetAnsVal {
    param([Parameter(Mandatory=$true, ValueFromPipeline=$true)][System.Object[]][AllowEmptyString()]$Output,
          [Parameter(Mandatory=$false, ValueFromPipeline=$true)][System.String]$firstEncNew="UTF-8",
          [Parameter(Mandatory=$false, ValueFromPipeline=$true)][System.String]$secondEncNew="CP866"
    )
    function ConvertTo-Encoding ([string]$From, [string]$To){#"UTF-8" "CP866" "ASCII" "windows-1251"
        Begin{
            $encFrom = [System.Text.Encoding]::GetEncoding($from)
            $encTo = [System.Text.Encoding]::GetEncoding($to)
        }
        Process{
            $Text=($_).ToString()
            $bytes = $encTo.GetBytes($Text)
            $bytes = [System.Text.Encoding]::Convert($encFrom, $encTo, $bytes)
            $encTo.GetString($bytes)
        }
    }
    $all = New-Object System.Collections.Generic.List[System.Object];
    $exception = New-Object System.Collections.Generic.List[System.Object];
    $stderr = New-Object System.Collections.Generic.List[System.Object];
    $stdout = New-Object System.Collections.Generic.List[System.Object]
    $i = 0;$Output | % {
        if ($_ -ne $null){
            if ($_.GetType().FullName -ne 'System.Management.Automation.ErrorRecord'){
                if ($_.Exception.message -ne $null){$Temp=$_.Exception.message | ConvertTo-Encoding $firstEncNew $secondEncNew;$all.Add($Temp);$exception.Add($Temp)}
                elseif ($_ -ne $null){$Temp=$_ | ConvertTo-Encoding $firstEncNew $secondEncNew;$all.Add($Temp);$stdout.Add($Temp)}
            } else {
                #if (MyNonTerminatingError.Exception is AccessDeniedException)
                $Temp=$_.Exception.message | ConvertTo-Encoding $firstEncNew $secondEncNew;
                $all.Add($Temp);$stderr.Add($Temp)
            }   
         }
    $i++
    }
    [hashtable]$return = @{}
    $return.Meta0=$all;$return.Meta1=$exception;$return.Meta2=$stderr;$return.Meta3=$stdout;
    return $return
}
Add-Type -AssemblyName System.Windows.Forms;
& C:\Windows\System32\curl.exe 'api.ipify.org/?format=plain' 2>&1 | set-variable Output;
$r = & GetAnsVal $Output
$Meta2=""
foreach ($el in $r.Meta2){
    $Meta2+=$el
}
$Meta2=($Meta2 -split "[`r`n]") -join "`n"
$Meta2=($Meta2 -split "[`n]{2,}") -join "`n"
[Console]::Write("stderr:`n");
[Console]::Write($Meta2);
[Console]::Write("`n");
$Meta3=""
foreach ($el in $r.Meta3){
    $Meta3+=$el
}
$Meta3=($Meta3 -split "[`r`n]") -join "`n"
$Meta3=($Meta3 -split "[`n]{2,}") -join "`n"
[Console]::Write("stdout:`n");
[Console]::Write($Meta3);
[Console]::Write("`n");