PowerShell Capture Write-Host output

Question

I am having to run a Microsoft cmdlet, and the important bit of information is written to console using a Write-Host line within the cmdlet.

It is NOT returned, so I cannot do $result = Commandlet ... A different value is returned that is not of use to me, what I actually need is printed to console within the commandlet is there anyway I can 'sniff' or 'scrape' the console to get the information I want?

$result = Test-Cluser

Test-Cluster will print stuff like: 'HadUnselectedTests', 'ClusterConditionallyApproved', etc. But the value it returns in the path to the .htm report file. And the .htm report file does not contain one of those status codes unfortunately so I cannot just parse the .htm file for it either.

Any suggestions?

mklement0 mklement0 · Accepted Answer · 2018-06-19T21:52:53

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.

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.}

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.

PowerShell Capture Write-Host output

3 Answers

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