Powershell again. Pipes, and 'This is not an output'

My experiences with PowerShell continue and every so often realize that I never really understood something I thought I’d “got” some while back. I’ve spent most of my time trying not to use the Write- cmdlets to send output to the screen. After all, if I want to write “Hello, World” to the screen I can have a function

function hw {
    'Hello, World'
}

No need for a Print, Write, Echo or any other command: in PowerShell, an output with nowhere else to go ends up on the screen.
If I want formatted output, Format-List and Format-Table do a great job, especially since I “discovered” calculated fields. Since I always used to get my queries in Access to do loads of work, shoving the work onto Format-Table seems natural enough, even if the syntax is little weird. A calculated field is written as a hash table:
@{label="Text", Expression={some code} }. Of course, “Some code” can do anything you want and what it returns goes in the table. So, I had a bright idea over the weekend. I can have a menu using Format-Table. Here the code:

function Choose-Process {
  $global:Counter=-1
  $proc=Get-Process
  Format-Table -inputobject $Proc @{ Label = "ID"; Expression={($global:counter++) }} , processName
  $proc[(Read-Host "Which one ?")]
}

So I set a counter, get a list of processes, and then format a table; in formatting the table I output and increment the counter. Then I ask the user to input a number to choose one and that gives me the offset into the array of processes of the one to return. When I run it, here’s what I get:

    PS C:\Users\Jamesone\> choose-proc
    ID ProcessName
    -- -----------
    0 audiodg
    1 CcmExec
    Which one ?: 1

    Handles NPM(K) PM(K) WS(K) VM(M) CPU(s)   Id ProcessName
    ------- ------ ----- ----- ----- ------   -- -----------
        744     29 13128 10560    92        2204 CcmExec

Of course, I want to save the result, so I type in $proc=choose-proc and I get this:
    Which one ?:
What the … ? Where did my menu go ? And the answer is: the menu was OUTPUT. Where did I tell PowerShell that Format-Table wasn’t to send to standard output but $proc[offset] was? I didn’t: so both ended up being stored in the result.

Anyone who’s explored PowerShell’s providers will have found there is one for variables. Now I get it
$varName= MyFunction is equivalent to MyFunction > Variable:varName.
If it goes to standard output it goes into the variable. This behaviour can work for or against you depending on the situation, and here I need to pipe Format-Table into Out-Host to force output to go the console and not to standard Output

I guess I was still thinking in Basic, where the last line of your function usually returns the result. PowerShell’s Outputs are different, they go down the “Standard Output” pipe unless told otherwise and standard output ends up on the screen if the “end of pipe” is left Open; just like you’d expect from a Shell. And since I was quoting my university lecturers a little while ago here’s something which was written on the board in our first programming class.

It is practically impossible to teach good programming style to students that have had prior exposure to BASIC; as potential programmers they are mentally mutilated beyond hope of regeneration.
( Edsger Dijkstra )

This post originally appeared on my TechNet blog in 2007

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