i found a workaround (not clean but working…):
i inserted tags (tar start and tar end)in my log file and i used /bin/sed to extract tar logs from it.
extract tar logs from backup.log to backuped_files.lst
sed -n '/^tar start/,/^tar end/p' backup.log > backuped_files.lst
remove tar logs from backup.log
sed '/^tar start/,/^tar end/d' backup.log > backup.log