Bak file
（重定向自.bak）
In computing, ".bak" is a filename extension commonly used to signify a backup copy of a file.
When a program is about to overwrite an existing file (for example, when the user saves the document he or she is working on), the program may first make a copy of the existing file, with .bak appended to the filename. This common .bak naming scheme makes it possible to retrieve the original contents of the file. In a similar manner, a user may also manually make a copy of the file before the change and append .bak to the filename.