powershell to copy data from AD attribute

Yes, of course. Do you want to change only those with no extensionAttribute1, or do you need to overwrite on each run?

Get-ADUser -Filter * -Property homePhone | ? { $_.homePhone } | Set-ADUser -Replace @{extensionAttribute1 = $_.homePhone}

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would do the latter, but only if there is a homePhone set. This also means that you cannot remove the property by just emptying homePhone.

Better to filter directly at the source (that is, the DC), instead of retrieving all user accounts and then filtering in PS:

Get-ADUser -Filter "homePhone -like '*'" -Property homePhone | ForEach-Object {Set-ADUser -Identity $_.SamAccountName -Replace @{'extensionAttribute1' = $_.homePhone}}

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I was assuming most of the  accounts are to be changed anyway, so it should not matter, but in general oBdA is correct. With exception of the unnecessary ForEach, that is.
It would even get better if we were able to check for differences in the filter, but we can only compare against values, not properties ...

Not unnecessary, since the pipeline variable is referenced.
Try your script, it won't work:

PS C:\PS> Get-ADUser -Filter * -Property homePhone | ? { $_.homePhone } | Set-ADUser -Replace @{extensionAttribute1 = $_.homePhone}
Set-ADUser : Cannot validate argument on parameter 'Replace'. The argument is null or an element of the argument collection contains a null value.

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And for the sake of completeness, this one would clear extensionAttribute1 for all users were homePhone is empty, but extensionAttribute1 isn't:

Get-ADUser -Filter "(extensionAttribute1 -like '*') -and -not (homePhone -like '*')" | Set-ADUser -Clear extensionAttribute1

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@#~*!!!

Hi,

Thanks for the quick responses

 Get-ADUser -Filter * -Property homePhone | ? { $_.homePhone } | Set-ADUser -Replace @{extensionAttribute4 = $_.homePhone}

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did return an error
Set-ADUser : Cannot validate argument on parameter 'Replace'. The argument is null or an element of the argument collection contains a null value.
At line:1 char:85
+ ... mePhone } | Set-ADUser -Replace @{extensionAttribute4 = $_.homePhone}
+                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Set-ADUser], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.ActiveDirectory.Management.Commands.SetADUser

Get-ADUser -Filter "(extensionAttribute1 -like '*') -and -not (homePhone -like '*')" | Set-ADUser -Clear extensionAttribute1

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executed as normal but the homePhone number did not copy to extensionAttribute1 ?

Regards,