Add incremented number to AD attribute based on last value - powershell

Question

I'm trying to use PowerShell to search the AD attribute otherTelephone, and if the value is null then add the next number in the sequence.
For instance, if I add a new user called John, I'd like PowerShell to check AD and see that the last otherTelephone number used was 999 so it'll auto add 1000 for John.
So far I've managed to export a list of users in AD along with their respective otherTelephone numbers:
Get-ADUser -Filter * -Properties otherTelephone |
select name, #{L='otherTelephone'; E={$_.otherTelephone[0]}}, |
Export-Csv c:\aduser.csv -NoTypeInformation
but I'm not sure how to proceed from there.

Answer 1

As #Paul suggested in the comments to your question: read the phone numbers from AD, get the highest number, and increment it.
$nextPhoneNumber = Get-ADUser -Filter * -Properties otherTelephone |
? { $_.otherTelephone } |
select -Expand otherTelephone |
% { [int]$_ } | sort -Desc | select -First 1
$nextPhoneNumber++
Explanation:
? { $_.otherTelephone }: Ensure that only objects with a non-empty attribute are processed.
select -Expand otherTelephone: Expand the attribute so that you get a stream of phone numbers. This also takes care of multiple numbers assigned to a user (the attribute is multi-valued).
% { [int]$_ }: Convert each telephone number from a string to an integer, so they can be sorted numerically.
sort -Desc: Sort the list in descending order (first number is highest).
select -First 1: Select the first number from the list and discard the rest.
$nextPhoneNumber++: increment the number by 1.
If you need to do this for all existing users that don't already have the otherTelephone attribute populated, you could do something like this:
$users = Get-ADUser -Filter * -Properties otherTelephone
$nextPhoneNumber = $users | ? { $_.otherTelephone } |
select -Expand otherTelephone |
% { [int]$_ } | sort -Desc | select -First 1
$users | ? { -not $_.otherTelephone } | % {
$nextPhoneNumber++
Set-ADUser -Identity $_.DistinguishedName -Replace #{
'otherTelephone' = $nextPhoneNumber
}
}