pipes and foreach loops

Question

Recently, I've been playing with PowerShell, and I've noticed some weird behavior when using pipes and foreach loops that I couldn't understand.

This simple code works:

$x = foreach ($i in gci){$i.length}
$x | measure -max

Makes sense.

But this code won't:

foreach ($i in gci){$i.length} | measure -max

And I'm getting the following error:

An empty pipe element is not allowed.
At line:1 char:33
+ foreach ($i in gci){$i.length} | <<<<  measure -max
+ CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : EmptyPipeElement

What is the difference between those two methods, and why does the second one fails?

Answer 1

The foreach statement doesn't use the pipeline architecture, so its output cannot be passed to a pipeline directly (i.e. item by item). To be able to pass output from a foreach loop to a pipeline you must run the loop in a subexpression:

$(foreach ($item in Get-ChildItem) { $item.Length }) | ...

or collect it in a variable first:

$len = foreach ($item in Get-ChildItem) { ... }
$len | ...

If you want to process data in a pipeline use the ForEach-Object cmdlet instead:

Get-ChildItem | ForEach-Object { $_.Length } | ...

For further explanation of the differences between foreach statement and ForEach-Object cmdlet see the Scripting Guy blog and the chapter on loops from Master-PowerShell.

Answer 2

You need to evaluate the foreach before piping the resulting Object like you did in the first test:

$(foreach ($i in gci){$i.length}) | measure -max

Alternatively, use the % shorthand to which will evaluate it before piping it as well:

gci | % { $_.Length } | measure -max