How to pipe output from foreach, in powershell

Question

I am trying to pipe the output from a foreach loop into a format command but it does not work. The reason I think is possible is because this works.

$op = foreach ($file in (Get-ChildItem -File)) {
    $file |
    Get-Member |
    Where-Object {$_.MemberType -eq "Method" -and $_.Definition -like "*system*" } |
    Select-Object -Property Name, MemberType
}

$op | Format-List

If I can assign the whole output to a variable, and pipe the variable into another command, why does the following NOT work?

(foreach ($file in (Get-ChildItem -File)) {
    $file |
    Get-Member |
    Where-Object {$_.MemberType -eq "Method" -and $_.Definition -like "*system*" } |
    Select-Object -Property Name, MemberType
}) | Format-List

Of course I tried without parents, but ultimately I think if anything, the parents make sense. It is like $file in (Get-ChildItem -File) where it evaluates the expression in the parents and uses the result as the actual object

Is there a way to make this work?

please note that the code is not supposed to achieve anything (else) than giving an example of the mechanics

Powershell distinguishes between Command parsing mode and Expression parsing mode - you've got both cases... — Peter Schneider
– Peter Schneider, Commented Nov 20, 2019 at 18:17
I don't know the exact reason or differences between $() and (). I've asked questions to this tag in the past, but no one was able to give me a concise answer. I just know wrapping things in a subexpression causes them to output differently. — Maximilian Burszley
– Maximilian Burszley, Commented Nov 20, 2019 at 18:24
@PeterSchneider Very nice. Your google-fu is stronger than mine — Maximilian Burszley
– Maximilian Burszley, Commented Nov 20, 2019 at 20:18

Maximilian Burszley · Accepted Answer · 2019-11-20 18:26:57Z

3

foreach does not have an output you can capture (besides the sugar you've found with variable assignment), but you can gather all the objects returned by wrapping it in a subexpression:

$(foreach ($file in Get-ChildItem -File) {
    # ...
}) | Format-List

This same pattern can be used for if and switch statements as well.

answered Nov 20, 2019 at 18:26

Maximilian Burszley

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Comments

js2010 · Accepted Answer · 2019-11-20 20:09:54Z

2

Here's another way to do it, without waiting for the whole foreach to finish. It's like defining a function on the fly:

& { foreach ($file in Get-ChildItem -File) {
      $file |
      Get-Member |
      Where-Object {$_.MemberType -eq "Method" -and $_.Definition -like "*system*" } |
      Select-Object -Property Name, MemberType
    } 
} | format-list

By the way, $( ) can go anywhere ( ) can go, but it can enclose multiple statements separated by newlines or semicolons.

Also, you can pipe it directly:

Get-ChildItem -File |
Get-Member |
Where-Object {$_.MemberType -eq "Method" -and $_.Definition -like "*system*" } |
Select-Object -Property Name, MemberType | 
Format-List

edited Nov 20, 2019 at 20:09

answered Nov 20, 2019 at 19:46

js2010

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4 Comments

Maximilian Burszley Over a year ago

Ah, nice use of IIFE. Forgot you could do that. It just has extra overhead of creating a new scope and managing invocation

The Fool Over a year ago

ok, nice! I knew this technique but a bit different. 1..3 | & {Process { $_*3 }} It makes sense now. I learned yesterday that a function will by default use the end block. That is why in my example here I need process explicitly but when you use it, the end block is correct and you return the result which is pipable. But why does it not need to wait for the foreach to finish?

js2010 Over a year ago

I don't know why. I just know that it does.

Maximilian Burszley Over a year ago

@TheFool pipeline and scriptblock black magic; it's not seen as a single command like a subexpression

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