How do I replace some text inside a text file with te buildId of a build/reease and save it back in the Azure DevOps ?

I would be a millionaire if I could ask a penny for each time that my colleagues ask me this question.

 

I would just run a PowerShell task with something like the below example:

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a=b+c

$oldValue = 'default-value'
$newValue = '$(Build.BuildId)'
$filePath='path-to-your-file/filename.txt'
 (Get-Content $filePath) -replace $oldValue , $newValue | Set-Content $filePath