Brackets in the pipeline

It seems that brackets have some kind of inconsistent behavior when used in
the pipeline. For example if we try to append all filename names in a
directory and use pipeline from Get-ChildItem to Rename-Item round brackets
don't work. When I try the command below $_.Name just gets killed from the
expression with no error messages or anything:

dir |Rename-Item -NewName ($_.Name + "1") #This _renames_ to 1

On the other hand this works fine:

dir |Rename-Item -NewName {$_.Name + "1"} #This appends 1

However, if I use foreach instead of pipeline round brackets are OK again:

foreach ($f in dir) {Rename-Item $f -NewName ($f.Name + "1")} #This appends 1

Any insight?

--
Dmitry Sotnikov
http://dmitrysotnikov.wordpress.com

> dir |Rename-Item -NewName ($_.Name + "1") #This _renames_ to 1
$_.Name is evaluated once at the moment of invocation of Rename-Item (it is
invoked once), and $_ is not defined at this moment (or has no property
Name), so $_.Name is evaluated as $null and -NewName becomes '1' for any
object in the pipeline. It looks correct to me.
This case seems clear and correct, too: $f is defined for any call of
Rename-Item which is called as many times as @(dir).Count
First of all, this way is not documented: man Rename-Item -d: ... -newName
<string> Specifies the new name of the item ... (<string> only!)
But it works... That means Rename-Item invokes this script block for any
object in the pipeline which is presumably assigned to $_ variable.

--
Thanks,
Roman Kuzmin
PowerShellFar and FarNET: http://code.google.com/p/farnet/


"Dmitry Sotnikov" <DSotnikovREMOVETHIS@xxxxxx> wrote in message
news:8FB5ED87-944A-4E53-922D-8789DF9C68F1@xxxxxx

"Dmitry Sotnikov" <DSotnikovREMOVETHIS@xxxxxx> дÈëÏûÏ¢ÐÂÎÅ:8FB5ED87-944A-4E53-922D-8789DF9C68F1@xxxxxx This command is parsed under command mode. The ($_.Name + "1") is the same
as $null.name + "1" before the command starts. In other words, It just like
: dir |Rename-Item -NewName ("" + "1")
The {} means making it as an scriptblock, so this piece of code would be
evaluated every time when an object is piped into. As result, the $_ refers
the object which is piped by 'dir'.
The output of pipeline in foreach statement are collected firstly. So $f
refers to the entry in dir properly.
get-help about_parsing may help you.

Tao Ma (Edengundam)
Best wishes!