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BAK is the most generic file extension in computing — it simply means "this is a backup copy of something." There's no standard format, no specification, no magic bytes. A .bak file could be a database backup (SQL Server), a renamed Word document, an AutoCAD drawing, a configuration file, or any other file that software (or a human) duplicated before making changes.
The convention is simple: before modifying `important.cfg`, copy it to `important.cfg.bak`. If the changes break something, you can restore the original. Many applications automate this — AutoCAD creates .bak files alongside every .dwg save, SQL Server's BACKUP command produces .bak files, and countless scripts and configuration management tools use the extension for their backup copies.
Since .bak files have no inherent format, the only way to determine what's inside is to examine the content. Open it in a text editor — if it's readable, it's probably a config file, script, or data file. If it's binary, check the magic bytes or try renaming it to the original extension. The file is exactly what it was before someone appended .bak to the name.
* Rename to original extension and open with appropriate application * For text-based backups