Archive bit
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An archive bit is a file attribute present in many computer file systems, notably FAT, FAT32, and NTFS. The purpose of an archive bit is to track incremental changes to files for the purpose of backup, also called archiving.
As the archive bit is a binary bit, it is either 1 or 0, or in this case more frequently called set (1) and clear (0). The operating system sets the archive bit any time a file is created, moved, renamed, or otherwise modified in any way. The archive bit therefore represents one of two states: "changed" and "not changed" since the last backup.
Archive bits are not affected by simply reading a file. When a file is copied, the original file's archive bit is unaffected, however the copy's archive bit will be set at the time the copy is made.
In NTFS, changing the permissions of a file also re-sets the file's archive bit. However, changing other file attributes (such as hidden, system) usually does not.
Backup software optionally clears the archive bit when making a backup copy of the file. File compression utilities such as PKZIP and WinRAR, as well as the built-in XCOPY command, can do this as well. Because this behavior is optional, it only happens when specifically requested by the user with a command-line switch or with a configuration option.
Backup software offers the option to only back up files with the archive bit set, which due to the operating system's behavior, equates to all the files that have been changed in some way since the last backup when the bits were cleared. When doing so, files that have not been changed since the most recent backup are skipped. A backup made in this manner is called an incremental backup. A typical incremental backup creates a copy of all files with the archive bit set, and then clears the bit on those files as the backup is made.
Backup software also provides the option to do a full backup while clearing archive bits - that is, to back up all files irrespective of the archive bit, but to clear the archive bit of all files in the backup for the future. This allows for the creation of an initial full backup that will be supplemented by incremental backups in the future.
The operating system never clears the archive bit on its own, unless explicitly told to do so by the user. In MS-DOS as well as nearly all versions of Windows, the archive bit can be seen or changed with the attrib command-line utility, or by viewing the properties of a given file with the Windows shell.
When a file with a clear archive bit is moved from one place on a file system to another, the archive bit reverts to being set.
Typically, on a computer system that has never been backed up in a manner that clears archive bits, a full 100% of the files on that computer will have their archive bits set, as there are few applications that make use of archive bits without the user's request.
As the archive bit is a file attribute and not part of the file itself, the contents of the file are unrelated to the status of the archive bit and remain unchanged even if the setting of the archive bit is changed.
