Bak file

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 they are 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.

Other naming schemes are also in widespread use: file~, file.orig, file.old, and so on.

Database Applications like FoxPro and SQL Server use .bak files to back up their databases and other applications, like XML shell, create .bak files in their autosave process.[1] They do not get automatically deleted, so they need to be manually deleted after the process using it is stopped.