PowerShell output is crossing between functions

Question

I am writing a PowerShell script in version 5.1 on Windows 10 that gets certain pieces of information about a local system ( and eventually its subnets ) and outputs them in

Answer 1

tl; dr:

Force synchronous output to the console by using a formatting cmdlet explicitly:

getUsersAndGroups | Out-Host
getRunningProcesses | Out-Host

^{Note: You can alternatively use one of the Format-* cmdlets, which also forces synchronous output; e.g., getUsersAndGroups | Format-Table.}

Note that this is primarily a display problem, and you do not need this workaround for capturing output in a file or passing it on through the pipeline.

However, sending to Out-Host means that the commands' output can no longer be captured or redirected; see this answer for a - suboptimal - workaround.

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It's helpful to demonstrate the problem with an MCVE (Minimal, Complete, and Verifiable Example):

Write-Host "-- before"
[pscustomobject] @{ one = 1; two = 2; three = 3 }
Write-Host "-- after"

In PSv5+, this yields:

-- before

-- after
one two three
--- --- -----
  1   2     3

What happened?

• The Write-Host calls produced output synchronously.

  • It is worth noting that Write-Host bypasses the normal success output stream and (in effect) writes directly to the console - mostly, even though there are legitimate uses, Write-Host should be avoided.

• The implicit output - from not capturing the output from statement [pscustomobject] @{ one = 1; two = 2; three = 3 } - was unexpectedly not synchronous:

  • A blank line was initially produced.
  • All actual output followed the final Write-Host call.

This helpful answer explains why that happens; in short:

• Implicit output is formatted based on the type of objects being output; in the case at hand, Format-Table is implicitly used.

• In Psv5+, implicitly applied Format-Table now waits up to 300 msecs. in order to determine suitable column widths.

  • Note, however, that this only applies to output objects for whose type table-formatting instructions are not predefined; if they are, they determine the column widths ahead of time, and no waiting occurs.

  • To test whether a given type with full name <FullTypeName> has table-formatting data associated with it, you can use the following command:

    # Outputs $true, if <FullTypeName> has predefined table-formatting data.
    Get-FormatData <FullTypeName> -PowerShellVersion $PSVersionTable.PSVersion |
      Where-Object { 
        $_.FormatViewDefinition.Control.ForEach('GetType') -contains [System.Management.Automation.TableControl] 
      }
    

• Unfortunately, that means that subsequent commands execute inside that time window and may produce unrelated output (via pipeline-bypassing output commands such as Write-Host) or prompt for user input before Format-Table output starts.

The problematic behavior is discussed in this GitHub issue; while there's still hope for a solution, there has been no activity in a long time.

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Note: This answer originally incorrectly "blamed" the PSv5+ 300 msec. delay for potentially surprising standard output formatting behavior (namely that the first object sent to a pipeline determines the display format for all objects in the pipeline, if table formatting is applied - see this answer).