Skipping files with a given prefix when renaming - powershell

Question

I have the following script for renaming a bunch of files in a directory, adding the name of the directory to the start of them:
$s = "Y:\Teknisk Arkiv\Likeretter 111-Å1\E2_Kretsskjema\02_Likeretter styring\3AJJ000302-222"
Get-ChildItem -Path $s -Exclude $_.Directory.Name* | rename-item -NewName { $_.Directory.Name + '_' + $_.Name }
Before running the script the files in the folder looks something like this
after like this
As you can see it does more or less what I want, except that -exclude $_.DirectoryName* doesn't prevent files which already have the foldername as a prefix from being renamed. What am I doing wrong here?

Answer 1

$_ in a pipeline is only defined inside a script block used in a non-initial pipeline segment, where it refers to the input object at hand, so in your Get-ChildItem command it is effectively undefined.
Even if $_.Directory.Name did have a value, $_.Directory.Name* wouldn't work as expected, because it would be passed as 2 arguments (you'd have to use "$($_.Directory.Name)*" or ($_.Directory.Name + '*').
You instead want to extract the directory name from the $s input path, which you can do with Split-Path -Leaf, and then append '*'.
In order for -Exclude to be effective, the input path must end in \*, because -Include and -Exclude filters - perhaps surprisingly - operate on the leaf component of the -Path argument, not on the child paths (unless -Recurse is also specified).
To put it all together:
Get-Item -Path $s\* -Exclude ((Split-Path -Leaf $s) + '*') |
Rename-Item -NewName { $_.Directory.Name + '_' + $_.Name }
I've switched to Get-Item, since \* is now being used to enumerate the children, but Get-ChildItem would work too.

Answer 2

The $_ is only valid when it is used on the right-side of a pipeline meaning when you have a collection of items and "pipe" them through the "$_" would represent the current item.
Since the directory name you want excluded is static you can just hardcode it and use as your exclude filter.
$s = "Y:\Teknisk Arkiv\Likeretter 111-Å1\E2_Kretsskjema\02_Likeretter styring\3AJJ000302-222"
$exclude_filter = "3AJJ000302-222*"
Get-ChildItem -Path $s -Exclude $exclude_filter | rename-item -NewName { $_.Directory.Name + '_' + $_.Name }
Also try to use "-whatif" with rename-item so you know what will happen before it happens.

Answer 3

$_ represents the currently processed item, what requires a ForEach-Object or a scriptblock inside a pipe, not present at the begin of your command.
Solution make the path a FileInfoObject and use -Exclude
$s = Get-Item "Y:\Teknisk Arkiv\Likeretter 111-Å1\E2_Kretsskjema\02_Likeretter styring\3AJJ000302-222"
Get-ChildItem -Path $s -Exclude "$($s.Name)*"|Rename-Item -NewName {$_.Directory.Name+'_'+$_.Name}
solution use a Where-Object to filter files already starting with the directory name
Get-ChildItem -Path $s | Where-Object {$_.Directory.Name -notlike "$($_.Name)*"} |
Rename-Item -NewName { $_.Directory.Name + '_' + $_.Name }
Solution use the RegEx based -replace operator to prepend the directory name and use a negative lookahead assertion to exclude files which already have it.
Get-ChildItem -Path $s |
Rename-Item -NewName {$x=$_.Directory.Name;$_.Name -replace "^(?!$x)",$x}