A relatively new PowerShell user came into #PowerShell on IRC.FreeNode.net this week to ask a question about scripts and the pipeline, and the conversation went so well, that I thought I’d share it with you all in case it helps clear things up for you. We’ll call him “user” since he left before I could get his permission to paste this, and I’ve cleaned up, reordered and in a couple of cases added lines to make the conversation seem a little more linear than the chat really was.

user: Can you explain the distinction between $_ and a param in a script context?
Jaykul: I sure can. And I think I can do it simply … take an example script|function … actually, we’ll just use a scriptblock
Jaykul: it can have pipeline blocks, or not (the pipeline blocks are the BEGIN{...} PROCESS{...} END{...})
user: yeah, I’m with you
Jaykul: so if you pass a value via an argument, it’s present in all three blocks, and has the same value the whole time (unless you change it in your script)
user: really
Jaykul: But when you put that script in a pipeline, your BEGIN {} block is called first, and then for EACH item in the pipeline, powershell sets $_ and calls your PROCESS block, and finally, your END{} block is called after all pipeline items have been processed
Jaykul: Here, check this out (run it so you can see) (more…)

I wrote a post last week about how to write functions for use in the PowerShell pipeline and I’ve been using the template I wrote in that post as the basis for several of my other scripts … and I’ve been gradually fleshing it out, and improving it, so I thought I’d drop it here with all of it’s inline comments. Hopefully that will be better than what you would get if I just trying to explain it in a blog post .

Incidentally, this script is also on PowerShellCentral scripts Repository where I will probably post any future modifications … (more…)