The command groovy -pi -e "(line =~ 'a').replaceAll('b')" ok.txt fails with Caught: java.io.IOException: unable to rename ok.txt to C:\DOCUME~1\TOMWHI~1\LOC ALS~1\Temp\groovy_61377.tmp whereas groovy -pi.bak -e "(line =~ 'a').replaceAll('b')" ok.txt works fine. The problem is that after creating a temporary backup file using File.createTempFile() the code needs to delete it before renaming the original to the backup. The attached patch file fixes the problem.