[SOLVED] Why does Write-Host not work when run in a powershell job?

Why does Write-Host not work when run in a powershell job?

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

Solution

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.

The following example receives the job output synchronously, i.e. it blocks execution until the job finishes (-Wait) and then removes it (-AutoRemoveJob); see the bottom section for an asynchronous (polling, non-blocking) approach.

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

Caveat re use of Write-Host in jobs:

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+); e.g.:
```
# 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
```

Waiting for a job to complete in a non-blocking fashion, passing job output through as it becomes available:

# Start a simple job that writes a "." to the host once a second,
# for 5 seconds
$job = Start-Job $job -ScriptBlock { 
         1..5| ForEach-Object { Write-Host -NoNewLine .; Start-Sleep 1 } 
       } 

"Waiting for job $($job.Id) to terminate while passing its output through..."

do {
  $job | Receive-Job  # See if job output is available (non-blocking) and pass it through
  Start-Sleep 1       # Do other things or sleep a little.
} while (($job | Get-Job).State -in 'NotStarted', 'Running')

"`nJob terminated with state '$($job.State)'."

$job | Remove-Job     # Clean up.

Note: In this simple case, the expected termination states are Completed (either no or only non-terminating errors occurred) or Failed (a script-terminating error was generated with throw (and not caught inside the job)).
- See the [System.Management.Automation.JobState] enumeration for the complete list of possible states.
The job object returned by Start-Job - rather than a self-chosen name via the -Name parameter - is used to interact with the job. This eliminates the ambiguity of possibly multiple jobs being present with a given -Name, all of which would be targeted.