An archive bit is a single bit within a file that indicates whether the file has been backed up or not. A value of 0 indicates the current version of the file has been backed up, while a 1 indicates it has not. Various backup utilities use the archive bit to determine which files to back up and which files to skip.
The archive bit is a file attribute supported by several operating systems, including Windows and OS/2. It is typically not visible to the user but is part of the file's metadata. Setting the archive bit modifies the file itself, so the feature cannot be used on operating systems such as macOS and Unix. These operating systems use file timestamps to track the last modified dates of files and compare them with the last backup time to determine what files need to be backed up.
While the archive bit provides an easy way to label files for backup, it is not always accurate. For example, if more than one backup program is running on a computer, the archive bit may be turned off by one program when it has not been backed up by another one. For this reason, many modern programs use their own backup system, such as a journal of backed up files. Advanced backup systems may also support incremental backups, which back up multiple versions of a file over time.
NOTE: Since archive bits are used exclusively for backup purposes, they are sometimes called "backup bits."