Powershell file rename syntax

Question

I'm trying to rename all the files in a given directory so that they have the sequential format 001a.jpg, 001b.jpg, 002a.jpg etc.

I have tried:

$c=1
get-childitem * | foreach {
if($c/2=[int]($c/2)){
rename-item -NewName {($c-1) + 'b.jpg'} else rename-item - NewName {$c + 'a.jpg'}}$c++} - whatif

But I am getting "the assignment operator is not valid".

The = in the third line is obviously not supposed to be an assignment operator. I just want to check whether the counter is on an even number. I found -ne for not equal to but PS didn't seem to like the -.

I am not sure about the syntax of the new names but the parser doesn't get that far ATM.

Grateful for any pointers.

Edit: I've just realised the numbers would skip but I can fix that once I understand the problem with what I already have.

Leave spaces around comparison operators -eq, -ne etc, otherwise the dash is seen as a minus to the equasion $c/2 — Theo
– Theo, Commented Nov 14, 2020 at 20:34
P.S. to test if a number is odd, it might be quicker to do if ($c % 2) or if ($c -band 1) — Theo
– Theo, Commented Nov 14, 2020 at 20:46
Thanks a lot. I think the comparison operator is OK now but there seems to a problem with the foreach. I am getting "ForEach-Object : Cannot bind parameter 'RemainingScripts'. Cannot convert the "-" value of type "System.String" to type "System.Management.Automation.ScriptBlock"." — rchivers
– rchivers, Commented Nov 14, 2020 at 20:49
Because of your edit "I can fix that once I understand the problem with what I already have" I only addressed the comparison problem you had. Petty you didn't ask to review/repair all of your code.. — Theo
– Theo, Commented Nov 14, 2020 at 21:19

Theo · Accepted Answer · 2020-11-14 20:51:53Z

3

As commented, if you do not leave spaces around the -eq or -ne operators, PowerShell will interpret the dash as belonging to the math, so

if($c/2-ne[int]($c/2))

Will fail.

Also, to test if a number is odd, it is probably quicker to do if ($c % 2) or if ($c -band 1).

Use -not to test for even numbers.

answered Nov 14, 2020 at 20:51

Theo

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Comments

marsze · Accepted Answer · 2020-11-14 21:01:33Z

Several things you need to know (some are just recommendations):

Every if-else branch requires curly brackets
If you do not input the items to Rename-Item via the pipeline, you have to specify the path
If you specify the path for Rename-Item, you cannot use a script block for -NewName
You cannot use + if the left operand is int and the right is string, because it would be interpreted as addition and requires two numbers.
You cannot use the -WhatIf switch on the whole construct, but only individual commands
It will suffice to use the command only once, and use the if-else only to set the new name
You can simplify your condition using the modulus operator % (as explained in other answers and comments)
You can use the -Begin parameter for initialization
The * for Get-ChildItem is not necessary

Here is the updated code:

Get-ChildItem | foreach -Begin { $c = 1 } -Process {
    if ($c % 2) {
        $newName = "${c}a.jpg"
    }
    else {
        $newName = "$($c-1)b.jpg"
    }
    Rename-Item -LiteralPath $_.FullName -NewName $newName -WhatIf
    $c++
}

Here is an even shorter version, that pipes directly into Rename-Item

$c = 0
Get-ChildItem |
  Rename-Item -NewName {if (++$script:c % 2) {"${c}a.jpg"} else {"$($c-1)b.jpg"}} -WhatIf

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