GNU tar & incremental backups
Bill Bogstad
bill at green.bph.jhu.edu
Sat Jan 7 01:55:26 AEST 1989
A number of people have been questioning whether or not
GNU tar can really be used for incremental file system backups.
Below I provide the GNU documentation for their "-G" option
which seems to fit the bill. They also have -M (multi-volume) and
-V (volume header) options.
For creation
`-G'
This option should only be used when creating an incremental backup of
a filesystem. When the `-G' option is used, `tar' writes at
the beginning of the archive an entry for each of the directories that
will be operated on. The entry for a directory includes a list of all
the files in the directory at the time the dump was done, and a flag
for each file indicating whether the file is going to be put in the
archive. This information is used when doing a complete incremental
restore.
For extraction
`-G'
The `-G' option means the archive is an incremental backup.
Its meaning depends on the command that it modifies.
If the `-G' option is used with `-t', `tar' will list,
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 filename is preceded by either a
`Y' if the file is present in the archive, or an `N' if the
file is a directory, or is not included in the archive. Each
filename 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.
If the `-G' option is used with `-x', then when the entry
for a directory is found, all files that currently exist in that directory
but are not listed in the archive *are deleted from the directory*.
This behavior is convenient when you are 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 you don't use `-G', the file system will probably fill up
with files that shouldn't exist any more.
Bill Bogstad
bogstad at crabcake.cs.jhu.edu
More information about the Comp.unix.wizards
mailing list