A bak file, also known as a backup file or backup, is a computer file that has the filename extension .bak or .bk, and is so named because it is a backup copy of another file. A variety of programs and applications make use of bak files, and some automatically create such files as part of their auto-save procedure or when overwriting original files, for example during a software update. A user can also add the .bak filename extension manually to create a bak file. Almost any kind of file can be saved as a bak file, including files from graphics programs, word processing programs, and database applications. A bak file usually has to be opened in the same application or program that the original content was created in, because it can contain data in almost any kind of format, and there is no special application available that can open all bak files.
The purpose of a bak file is to create a copy of an original file. This copy can then be accessed if the original is lost, corrupted, or altered in such a way that it is no longer usable or wanted. Saving backup copies of files before a software update makes it possible to revert to previous versions of files and data if there is a problem with the update.
Many computer programs including AutoCAD, XML Shell, and database applications like Foxpro and SQL server, make use of .bak files, for example when making backup copies of data for security purposes or when performing an auto-save. Backup files can also be created manually to store data that cannot be discarded, but is no longer actively used. Using the .bak filename extension for computer storage of such copies makes the files easier to organize and locate.
Backup files can sometimes create problems because they take up too much hard-drive space. This can happen especially if applications are creating .bak files automatically, without also deleting them regularly. These files can often be manually deleted without causing problems, but it is important to make sure that the files are not needed before doing so. The generic nature of bak files can make it difficult to know what type of data a specific bak file contains, or how to open it. The folder location and name of the file can sometimes provide clues, and one can also attempt to open the bak file in a text editor to help identify its content.