Fun with the PowerShell Pipeline

I've been having some fun with the PowerShell Pipeline. This started with a post to a PowerShell question over on Spiceworks (http://community.spiceworks.com/topic/737915-powershell-question-help). The OP wanted to stop the spooler, remove any temp files then restart. Simple enough – but the first response suggested using pipes like this:

Stop-Service -Name Spooler |
Remove-Item C:\Windows\System32\spool\PRINTERS\* |
Start-Service -Name Spooler

I looked at this for a long while wondering what the heck the pipeline would be doing here. More importantly, I could not believe it would work – surely the output of Stop-Service would be null thus the remove-item would fail, etc. Then it tried it and much to my surprise – it actually works!

Then I began to wonder two things – WHY does it work at all – and what is the performance impact. To understand why requires a good understanding of how the pipeline works. In this case, Jason Shirk has a great explanation of the underpinnings over on Stack Overflow: http://stackoverflow.com/questions/22343187/why-is-an-empty-powershell-pipeline-not-the-same-as-null. I highly recommend reading it to help you better understand what the pipeline is doing.

Ok – so I know WHY it works, but what is the performance impact if any? To answer this question, I first constructed a little script.

Function Measureit {$m1 = (Measure-Command -Expression {
      Stop-Service -Name Spooler |
      Remove-Item C:\Windows\System32\spool\PRINTERS\* |
      Start-Service -Name Spooler
}).milliseconds

$m2 = (Measure-Command -Expression {
      Stop-Service -Name Spooler
      Remove-Item C:\Windows\System32\spool\PRINTERS\*
      Start-Service -Name Spooler
}).Milliseconds

"{0,-10}{1}" -f $m1, $m2
}

1..20 | %{measureit}

Now please read the script and guess which is going to be faster – with or without pipelines? The results absolutely astounded me:

15        307
2         273
2         270
254       280
2         277
10        267
2         271
2         269
2         276
2         268
2         268
2         277
2         272
2         268
2         268
2         268
10        267
2         269
2         267
2         271

I know why this works, but I sure as heck have NO idea how adding the pipeline, in this care can offer up 100-fold improvement in the speed of execution in most cases. Hmmm.