为什么在 powershell 作业中运行时，Write-Host 不起作用？

Question

Sorry if I'm being a dumb powershell noob, but what's wrong with jobs apparently being unable to write to the terminal? And how can I fix that?

# test.ps1
function myjob {
    Write-Host "Hello, World!" # doesn't show
}
Start-Job -Name MyJob -ScriptBlock ${function:myjob}
Wait-Job MyJob
Remove-Job MyJob

Answer 1

It sounds like you're trying to use Write-Host to directly, synchronously write to the console (terminal) from a background job.

However, PowerShell jobs do not allow direct access to the caller's console . Any output - even to the PowerShell host (which in foreground use is the console, if run in one) is routed through PowerShell's system of output streams (see the conceptual about_Redirection help topic).

Therefore, you always need the Receive-Job cmdlet in order to receive output from a PowerShell job:

$null = Start-Job -Name MyJob -ScriptBlock { Write-Host "Hello, World!" } 
Receive-Job -Wait -AutoRemoveJob -Name  MyJob

Caveat :

• In foreground use, Write-Host output - even though primarily designed to go to the host (console) - can be redirected or captured via the information stream (whose number is 6 , available in PSv5+); eg:

   # OK - no output Write-Host 'silence me' 6>$null

• Write-Host output received via a (child-process-based) background job , however, can not be redirected or captured, as of PowerShell 7.2.1:

   #.! `silence me` still prints. Start-Job { Write-Host 'silence me' } | Receive-Job -Wait -AutoRemoveJob 6>$null

  • By contrast, it can be redirected/captured when using a (generally preferable) thread -based background job (as opposed to a child-process -based background job), via Start-ThreadJob :

     # OK - no output Start-ThreadJob { Write-Host 'silence me' } | Receive-Job -Wait -AutoRemoveJob 6>$null