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(tar.info)incremental and listed-incremental

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The Incremental Options
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   `--incremental' (`-G') is used in conjunction with `--create' (`-c'),
`--extract' (`--get', `-x') or `--list' (`-t') when backing up and
restoring file systems.  An archive cannot be extracted or listed with
the `--incremental' (`-G') option specified unless it was created with
the option specified.  This option should only be used by a script, not
by the user, and is usually disregarded in favor of
`--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE'), which is
described below.

   `--incremental' (`-G') in conjunction with `--create' (`-c') causes
`tar' to write, at the beginning of the archive, an entry for each of
the directories that will be archived.  The entry for a directory
includes a list of all the files in the directory at the time the
archive was created and a flag for each file indicating whether or not
the file is going to be put in the archive.

   Note that this option causes `tar' to create a non-standard archive
that may not be readable by non-GNU versions of the `tar' program.

   `--incremental' (`-G') in conjunction with `--extract' (`--get',
`-x') causes `tar' to read the lists of directory contents previously
stored in the archive, _delete_ files in the file system that did not
exist in their directories when the archive was created, and then
extract the files in the archive.

   This behavior is convenient when restoring a damaged file system from
a succession of incremental backups: it restores the entire state of
the file system to that which obtained when the backup was made.  If
`--incremental' (`-G') isn't specified, the file system will probably
fill up with files that shouldn't exist any more.

   `--incremental' (`-G') in conjunction with `--list' (`-t') causes
`tar' to print, for each directory in the archive, the list of files in
that directory at the time the archive was created.  This information
is put out in a format that is not easy for humans to read, but which
is unambiguous for a program: each file name is preceded by either a
`Y' if the file is present in the archive, an `N' if the file is not
included in the archive, or a `D' if the file is a directory (and is
included in the archive).  Each file name is terminated by a null
character.  The last file is followed by an additional null and a
newline to indicate the end of the data.

   `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') acts like
`--incremental' (`-G'), but when used in conjunction with `--create'
(`-c') will also cause `tar' to use the file SNAPSHOT-FILE, which
contains information about the state of the file system at the time of
the last backup, to decide which files to include in the archive being
created.  That file will then be updated by `tar'.  If the file FILE
does not exist when this option is specified, `tar' will create it, and
include all appropriate files in the archive.

   The file FILE, which is archive independent, contains the date it
was last modified and a list of devices, inode numbers and directory
names.  `tar' will archive files with newer mod dates or inode change
times, and directories with an unchanged inode number and device but a
changed directory name.  The file is updated after the files to be
archived are determined, but before the new archive is actually created.

   Incremental dumps depend crucially on time stamps, so the results are
unreliable if you modify a file's time stamps during dumping (e.g.
with the `--atime-preserve' option), or if you set the clock backwards.

   Despite it should be obvious that a device has a non-volatile value,
NFS devices have non-dependable values when an automounter gets in the
picture.  This led to a great deal of spurious redumping in incremental
dumps, so it is somewhat useless to compare two NFS devices numbers
over time.  So `tar' now considers all NFS devices as being equal when
it comes to comparing directories; this is fairly gross, but there does
not seem to be a better way to go.