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Using Powershell 7, this works perfectly:

dir foo* | Rename-Item -Path {$_.Name} -NewName{$_.Name -replace 'bar-(.*)', 'bar-$1'}

This fails:

dir foo* | Rename-Item {$_.Name} {$_.Name -replace 'bar-(.*)', 'bar-$1'}

With the following error:

Rename-Item: A positional parameter cannot be found that accepts argument '$_.Name'.

Based on the docs here -Path is the first position of Rename-Item. Also, the following command works: Rename-Item 'foo-one' 'foo-one-one'. Why does the above failure happen?

Choppy The Lumberjack
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    @AdminOfThings is almost right, `dir` is an alias to `Get-ChildItem` which outputs an object with a PSPath property. The `Rename-Item` function uses PSPath as an alias for the LiteralPath option, so you're actually using a different parameter set (which does not include -Path). – Dabombber Oct 29 '20 at 07:13
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    @Dabombber This makes the most sense since omitting {$_.Name} actually makes the command work. Feel free to post it as an answer. – Choppy The Lumberjack Oct 29 '20 at 07:29
  • @iRon When I do that it gives me an unexpected output showing me background jobs. Is that supposed to evaluate the into a ? – Choppy The Lumberjack Oct 29 '20 at 07:29

1 Answers1

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Although your example code does not actually rename any file (-replace 'bar-(.*)', 'bar-$1' will result in the exact same name.), the main problem here is that you add {$_.Name} directly after the Rename-Item cmdlet, where this is not needed, since you are already piping fileInfo objects through the pipeline, so now {$_.Name} is treated as if it were a parameter name.
Next, you forget to add the name for parameter -NewName.

By iterating through a path like foo* you do the same as -Path 'foo' -Recurse and there's a gotcha in that..

Especially when iterating through many files and passing them on through the pipeline, there is a chance that files that have already been renamed get picked up by the Get-ChildItem (alias dir) again and then will be renamed twice.

To overcome that, put the part to gather the files between brackets, so collecting all the FileInfo objects is complete and then pipe that to the next cmdlet like so:

(Get-ChildItem -Path 'foo' -File -Recurse) | 
 Rename-Item -NewName {$_.Name -replace 'bar-(.*)', 'baz-$1'}

for clarity, this does change part of the name from bar to baz

Theo
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