.bak File — Backup File

BAK is a widely used file extension for backup copies of files across various applications and contexts. The most significant usage is in Microsoft SQL Server, where .bak files contain complete database backups created by the BACKUP DATABASE command, including data files, transaction logs, and metadata needed to restore the database to its exact state at backup time. SQL Server BAK files support full backups, differential backups, and transaction log backups. Beyond SQL Server, many applications automatically create .bak files before modifying originals — text editors, configuration tools, and database utilities often rename the original file to .bak before writing the updated version. AutoCAD, Notepad++, and many other applications follow this convention. The BAK extension communicates backup purpose clearly, making it easy to identify these files during cleanup operations. SQL Server BAK files can be restored using RESTORE DATABASE, examined with RESTORE HEADERONLY/FILELISTONLY, and compressed for storage efficiency. In enterprise environments, BAK files are critical components of disaster recovery strategies, typically stored on separate storage systems and replicated to offsite locations according to retention policies.