Which is why I suggested making the third copy, which could be "rolled back" in case of a problem. It's similar to DB transactions with redo logs. They don't have to worry about the file permissions problems, though. 

 eval {
 copy file to temp_file
 copy file to backup_file
 modify temp_file
 cat temp_file > file
 verify file is OK
 remove temp_file and backup_file
 };
 if ( $@ ) {
   remove temp_file
   cat backup_file > file
   remove backup_file
 }

[download]
Worst case scenario the cat of the backup_file to temp_file fails and you're left with a corrupt "file" and the extra "backup_file" which would have to be moved back manually.

In reply to Re^3: Writing to a file atomically without renaming by Transient
in thread Writing to a file atomically without renaming by nomis80

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