How to convert infinity while loop to a pipeline statement

Question

I'm trying to use PowerShell pipeline for some recurring tasks and checks, like perform certain checks X times or skip forward after the response in pipeline will have different state.

The simplest script I can write to do such checks is something like this:

do {
    $result=Update-ACMEIdentifier dns1 -ChallengeType dns-01
    if($result.Status -ne 'pending')
    { 
        "Hurray"
        break 
    }

    "Still pending"
    Start-Sleep -s 3
} while ($true)

The question is - how can I write this script as a single pipeline. It looks like the only thing I need is infinity pipeline to start with:

  1..Infinity |
    %{ Start-Sleep -Seconds 1 | Out-Null; $_ } |
    %{Update-ACMEIdentifier dns1 -ChallengeType dns-01 } |
    Select -ExpandProperty Status | ?{$_ -eq 'pending'} |
    #some code here to stop infinity producer or stop the pipeline

So is there any simple one-liner, which allows me to put infinity object producer on one side of the pipeline?

Good example of such object may be a tick generator that generates current timestamp into pipeline every 13 seconds

Answer 1

@PetSerAl gave the crucial pointer in a comment on the question: A script block containing an infinite loop, invoked with the call operator (&), creates an infinite source of objects that can be sent through a pipeline:

& { while ($true) { ... } }

A later pipeline segment can then stop the pipeline on demand.

Note:

• As of PS v5, only Select-Object is capable of directly stopping a pipeline.

  • An imperfect generic pipeline-stopping function can be found in this answer of mine.

• Using break to stop the pipeline is tricky, because it doesn't just stop the pipeline, but breaks out of any enclosing loop - safe use requires wrapping the pipeline in a dummy loop.

• Alternatively, a Boolean variable can be used to terminate the infinite producer.

Here are examples demonstrating each approach:

---

A working example with Select-Object -First:

& { while ($true) { Get-Date; Start-Sleep 1 } } | Select-Object -First 5

This executes Get-Date every second indefinitely, but is stopped by Select-Object after 5 iterations.

An equivalent example with break and a dummy loop:

do { 
   & { while ($true) { Get-Date; Start-Sleep 1 } } |
     % { $i = 0 } { $_; if (++$i -eq 5) { break } }  # `break` stops the pipeline and
                                                     # breaks out of the dummy loop
} while ($false)

An equivalent example with a Boolean variable that terminates the infinite producer:

& { while (-not $done) { Get-Date; Start-Sleep 1 } } |
  % { $done = $false; $i = 0 } { $_; if (++$i -eq 5) { $done = $true } }

Note how even though $done is only initialized in the 2nd pipeline segment - namely in the ForEach-Object (%) cmdlet's (implicit) -Begin block - that initialization still happens before the 1st pipeline segment - the infinite producer - starts executing.^{Thanks again, @PetSerAl.}

Answer 2

Not sure why you'd want to use a pipeline over a loop in this scenario, but it is possible by using a bit of C#; e.g.

$Source = @"
using System.Collections.Generic;
public static class Counter
{
    public static bool Running = false;
    public static IEnumerable<long> Run()
    {
        Running = true;
        while(Running)
        {
            for (long l = 0; l <= long.MaxValue; l++) 
            {
                yield return l;
                if (!Running) {
                    break;
                }
            }
        }
    }
}
"@

Add-Type -TypeDefinition $Source -Language CSharp

[Counter]::Run() | %{
    start-sleep -seconds 1
    $_
} | %{
    "Hello $_"
    if ($_ -eq 12) {
        [Counter]::Running = $false;
    }
}

NB: Because the numbers are generated in parallel with the pipeline execution it's possible that the generator may create a backlog of numbers before it's stopped. In my testing that didn't happen; but I believe that scenario is possible.

You'll also notice that I've stuck a for loop inside the while loop; that's to ensure that the values produced are valid; i.e. so I don't overrun the max value for the data type.

---

Update

Per @PetSerAl's comment above, here's an adapted version in pure PowerShell:

$run=$true; &{for($i=0;$run;$i++){$i}} | %{ #infinite loop outputting to pipeline demo
    "hello $_";
        if($_ -eq 10){"stop";$run=$false <# stopping condition demo #>}
}