powershell cmdlet how to pipe information or error to write-eventlog

Question

i'm trying to output to eventlog to the correct Entry Type (Information,Warning,Error) based on the stream that is coming out of my cmdlet, something like this:

function myfunction {
   Param(
   [switch]$stream1,
   [switch]$stream2
)
   if ($stream1) {write-output 'stream 1 msg'}
   if ($stream2) {write-error 'stream 2 msg'}
}

$eventlogparams = @{'logname'='application';'source'='myapp';'eventid'='1'}

myfunction -stream1 -stream2 `
  1> write-eventlog @eventlogparams -entrytype information -message $_ `
  2> write-eventlog @eventlogparams -entrytype error -message $_

does anyone have an idea of how to accomplish this?

Answer 1

You can merge the error stream and others into the success stream and distinguish between the origin streams by the data type of each pipeline object:

myfunction -channel1 -channel2 *>&1 | ForEach-Object { 
  $entryType = switch ($_.GetType().FullName) {
        'System.Management.Automation.ErrorRecord' { 'error'; break }
        'System.Management.Automation.WarningRecord' { 'warning'; break }
        default { 'information'}
    }
  write-eventlog @eventlogparams -entrytype $entryType -message $_
}

Redirection *>&1 sends the output from all (*) streams to (&) the success stream (1), so that all output, irrespective of what stream it came from, is sent through the pipeline.

The above only deals with errors and warnings specifically, and reports everything else, including the success output, as information, but it's easy to extend the approach - see bottom.

See about_Redirections for an overview of all 6 output streams available in PowerShell (as of v6).

---

To illustrate the technique with a simpler example:

& { Write-Output good; Write-Error bad; Write-Warning problematic } *>&1 | ForEach-Object {
  $entryType = switch ($_.GetType().FullName) {
        'System.Management.Automation.ErrorRecord' { 'error'; break }
        'System.Management.Automation.WarningRecord' { 'warning'; break }
        default { 'information'}
    }
  '[{0}] {1}' -f $entryType, $_
}

The above yields:

[information] good
[error] bad
[warning] problematic

---

The list of data types output by the various streams:

Stream           Type
------           ----
#1 (Success)     (whatever input type is provided).
#2 (Error)       [System.Management.Automation.ErrorRecord]
#3 (Warning)     [System.Management.Automation.WarningRecord]
#4 (Verbose)     [System.Management.Automation.VerboseRecord]
#5 (Debug)       [System.Management.Automation.DebugRecord]
#6 (Information) [System.Management.Automation.InformationRecord]

The following code was used to produce the above list (except for the first data row):

& {
    $VerbosePreference = $DebugPreference = $InformationPreference = 'Continue'
    $ndx = 2
    "Write-Error", "Write-Warning", "Write-Verbose", "Write-Debug", "Write-Information" | % {
        & $_ ($_ -split '-')[-1] *>&1
        ++$ndx
    } | Select-Object @{n='Stream'; e={"#$ndx ($_)"} }, @{n='Type'; e={"[$($_.GetType().FullName)]"} }
}