PowerShell Capture Write-Host output

Note: As for why you should never use Write-Host to output data, see this answer.

In PSv5+:

$result = Test-Cluster 6>&1

Since version 5, Write-Host writes to the newly introduced information stream, whose number is 6.

6>&1 redirects that stream to the success output stream (number 1), so that it too can be captured in $result.

Caveat: The related Out-Host cmdlet does not write to the information stream; its output cannot be captured – see this answer for the differences between Write-Host and Out-Host.

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In PSv4-:

There is no way to capture Write-Host output in-session.

The only workaround is to launch another instance of Powershell with the target command specified as a string.
Caveats:

• Such an invocation is slow,
• prevents passing of arguments with their original data type
• invariably only returns string data (lines of text)
• returns output from all output streams, including error output
  • for a list of all output streams, see Get-Help about_Redirection

$result = powershell -noprofile -command 'Test-Cluster'

^{Note that using a script block to pass the command (-command { Test-Cluster }) would not work, because PowerShell then uses serialization and deserialization to emulate the in-session behavior.}

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Optional reading: output streams in PowerShell and how to redirect them:

Get-Help about_Redirection discusses a list of all output streams, which can be targeted by their numbers; since PSv5, these are:

1 ... success output stream (implicit output and Write-Output output)
2 ... error output stream (Write-Error and unhandled errors)
3 ... warnings (Write-Warning)
4 ... verbose output (Write-Verbose)
5 ... debug output (Write-Debug)
6 ... (v5+) Write-Information and Write-Host output

Note that some streams are silent by default and require opt-in to produce output, either via a preference variable (e.g., $VerbosePreference) or a common parameter (e.g., -Verbose)

• {n}> allows redirecting the number {n} stream; if {n} is omitted, 1 is implied:

  • to a file (e.g., 3> c:/tmp/warnings.txt
  • to “nowhere”, i.e suppressing the output (e.g., 3> $null)
  • to the success output stream (e.g., 3>&1); note: only stream 1 can be targeted this way.

• *> targets all output streams.

Note: Unlike in POSIX-like shells (e.g., bash), the order of multiple redirection expression does not matter.

Therefore, the following POSIX-like shell idiom – which redirects error output to the success stream and silences only the original success output – does NOT work:

... 2>&1 1>$null # !! NO output in PowerShell

To achieve this in PowerShell, you mustn’t redirect 1 and instead filter the objects in the success by their stream of origin.

Case in point: In the end, the OP wanted the following: capture only warning output, without the regular (success) output:

Test-Cluster 3>&1 | Where-Object { $_ -is [System.Management.Automation.WarningRecord] }

Objects that came from the warning stream have type [System.Management.Automation.WarningRecord], which is what enables the filtering above.