[powershell] Capturing standard out and error with Start-Process

Is there a bug in PowerShell's Start-Process command when accessing the StandardError and StandardOutput properties?

If I run the following I get no output:

$process = Start-Process -FilePath ping -ArgumentList localhost -NoNewWindow -PassThru -Wait
$process.StandardOutput
$process.StandardError

But if I redirect the output to a file I get the expected result:

$process = Start-Process -FilePath ping -ArgumentList localhost -NoNewWindow -PassThru -Wait -RedirectStandardOutput stdout.txt -RedirectStandardError stderr.txt

Here is my version of function that is returning standard System.Diagnostics.Process with 3 new properties

Function Execute-Command ($commandTitle, $commandPath, $commandArguments)
{
    Try {
        $pinfo = New-Object System.Diagnostics.ProcessStartInfo
        $pinfo.FileName = $commandPath
        $pinfo.RedirectStandardError = $true
        $pinfo.RedirectStandardOutput = $true
        $pinfo.UseShellExecute = $false
        $pinfo.WindowStyle = 'Hidden'
        $pinfo.CreateNoWindow = $True
        $pinfo.Arguments = $commandArguments
        $p = New-Object System.Diagnostics.Process
        $p.StartInfo = $pinfo
        $p.Start() | Out-Null
        $stdout = $p.StandardOutput.ReadToEnd()
        $stderr = $p.StandardError.ReadToEnd()
        $p.WaitForExit()
        $p | Add-Member "commandTitle" $commandTitle
        $p | Add-Member "stdout" $stdout
        $p | Add-Member "stderr" $stderr
    }
    Catch {
    }
    $p
}

I really had troubles with those examples from Andy Arismendi and from LPG. You should always use:

$stdout = $p.StandardOutput.ReadToEnd()

before calling

$p.WaitForExit()

A full example is:

$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
$stdout = $p.StandardOutput.ReadToEnd()
$stderr = $p.StandardError.ReadToEnd()
$p.WaitForExit()
Write-Host "stdout: $stdout"
Write-Host "stderr: $stderr"
Write-Host "exit code: " + $p.ExitCode

IMPORTANT:

We have been using the function as provided above by LPG.

However, this contains a bug you might encounter when you start a process that generates a lot of output. Due to this you might end up with a deadlock when using this function. Instead use the adapted version below:

Function Execute-Command ($commandTitle, $commandPath, $commandArguments)
{
  Try {
    $pinfo = New-Object System.Diagnostics.ProcessStartInfo
    $pinfo.FileName = $commandPath
    $pinfo.RedirectStandardError = $true
    $pinfo.RedirectStandardOutput = $true
    $pinfo.UseShellExecute = $false
    $pinfo.Arguments = $commandArguments
    $p = New-Object System.Diagnostics.Process
    $p.StartInfo = $pinfo
    $p.Start() | Out-Null
    [pscustomobject]@{
        commandTitle = $commandTitle
        stdout = $p.StandardOutput.ReadToEnd()
        stderr = $p.StandardError.ReadToEnd()
        ExitCode = $p.ExitCode
    }
    $p.WaitForExit()
  }
  Catch {
     exit
  }
}

Further information on this issue can be found at MSDN:

A deadlock condition can result if the parent process calls p.WaitForExit before p.StandardError.ReadToEnd and the child process writes enough text to fill the redirected stream. The parent process would wait indefinitely for the child process to exit. The child process would wait indefinitely for the parent to read from the full StandardError stream.

I also had this issue and ended up using Andy's code to create a function to clean things up when multiple commands need to be run.

It'll return stderr, stdout, and exit codes as objects. One thing to note: the function won't accept .\ in the path; full paths must be used.

Function Execute-Command ($commandTitle, $commandPath, $commandArguments)
{
    $pinfo = New-Object System.Diagnostics.ProcessStartInfo
    $pinfo.FileName = $commandPath
    $pinfo.RedirectStandardError = $true
    $pinfo.RedirectStandardOutput = $true
    $pinfo.UseShellExecute = $false
    $pinfo.Arguments = $commandArguments
    $p = New-Object System.Diagnostics.Process
    $p.StartInfo = $pinfo
    $p.Start() | Out-Null
    $p.WaitForExit()
    [pscustomobject]@{
        commandTitle = $commandTitle
        stdout = $p.StandardOutput.ReadToEnd()
        stderr = $p.StandardError.ReadToEnd()
        ExitCode = $p.ExitCode
    }
}

Here's how to use it:

$DisableACMonitorTimeOut = Execute-Command -commandTitle "Disable Monitor Timeout" -commandPath "C:\Windows\System32\powercfg.exe" -commandArguments " -x monitor-timeout-ac 0"

Here's a kludgy way to get the output from another powershell process:

start-process -wait -nonewwindow powershell 'ps | Export-Clixml out.xml'; import-clixml out.xml

In the code given in the question, I think that reading the ExitCode property of the initiation variable should work.

$process = Start-Process -FilePath ping -ArgumentList localhost -NoNewWindow -PassThru -Wait
$process.ExitCode

Note that (as in your example) you need to add the -PassThru and -Wait parameters (this caught me out for a while).

To get both stdout and stderr, I use:

Function GetProgramOutput([string]$exe, [string]$arguments)
{
    $process = New-Object -TypeName System.Diagnostics.Process
    $process.StartInfo.FileName = $exe
    $process.StartInfo.Arguments = $arguments

    $process.StartInfo.UseShellExecute = $false
    $process.StartInfo.RedirectStandardOutput = $true
    $process.StartInfo.RedirectStandardError = $true
    $process.Start()

    $output = $process.StandardOutput.ReadToEnd()   
    $err = $process.StandardError.ReadToEnd()

    $process.WaitForExit()

    $output
    $err
}

$exe = "cmd"
$arguments = '/c echo hello 1>&2'   #this writes 'hello' to stderr

$runResult = (GetProgramOutput $exe $arguments)
$stdout = $runResult[-2]
$stderr = $runResult[-1]

[System.Console]::WriteLine("Standard out: " + $stdout)
[System.Console]::WriteLine("Standard error: " + $stderr)