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A backup bit is a single unit of binary data that shows a value of either one or zero. Some processes use the backup bit to determine whether a file has been backed up or modified.
A backup bit is also known as an archive bit.
Essentially, the single backup bit stored in the system is a piece of metadata that allows programs to flag whether the file has been backed up. However, on some conventional operating systems, outside features like timestamps track the modification or backup of files. In these cases, a backup bit or archive bit may not be relevant.
Where backup bits or archive bits are used, they are sometimes prone to error. There has to be a chain of command that shows whether a backup bit has been subsequently manipulated by a third-party program – for instance, if one backup utility flags the backup bit, and another utility flags it again, it goes back to the zero position, indicating it has not been backed up, and that can cause confusion.