An archive containing all the files in the file system is called a "full backup" or "full dump". It is more efficient to do a full dump only occasionally. To back up files between full dumps, you can a incremental dump. A "level one" dump archives all the files that have changed since the last full dump.

A typical dump strategy would be to perform a full dump once a week, and a level one dump once a day. This means some versions of files will in fact be archived more than once, but this dump strategy makes it possible to restore a file system to within one day of accuracy by only extracting two archives—the last weekly (full) dump and the last daily (level one) dump.

GNU `tar' comes with scripts you can use to do full and level-one dumps. The name of the restore script is `restore'. The names of the level one and full backup scripts are, respectively, `level-1' and `level-0'. The `level-0' script also exists under the name `weekly', and the `level-1' under the name `daily'

Before you use these scripts, you need to edit the file `backup-specs', which specifies parameters used by the backup scripts and by the restore script.

`tar' will normally automatically cross file system boundaries in order to archive files which are part of a directory tree. You can change this behavior by running `tar' and specifying `—one-file-system' (`-l').

`--one-file-system'
`-l'
    Prevents `tar' from crossing file system boundaries when
    archiving.

If a file in a directory is not on the same filesystem as the directory itself, then `tar' will not archive that file. If the file is a directory itself, `tar' will not archive anything beneath it; in other words, `tar' will not cross mount points. The mount point is is archived, but nothing under it.

This option only affects files that are archived because they are in a directory that is being archived; `tar' will still archive files explicitly named on the command line or through `—files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES'), regardless of where they reside.

To create an archive that is larger than will fit on a single unit of the media, use the `—multi-volume' (`-M') option in conjunction with the `—create' (`-c') option (*note create::). A "multi-volume" archive can be manipulated like any other archive (provided the `—multi-volume' (`-M') option is specified), but is stored on more than one tape or disk.

You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use `—list' (`-t'), without `—multi-volume' (`-M') specified. To extract an archive member from one volume (assuming it is described that volume), use `—extract' (`—get', `-x'), again without `—multi-volume' (`-M').

If an archive member is split across volumes (ie. its entry begins on one volume of the media and ends on another), you need to specify `—multi-volume' (`-M') to extract it successfully. In this case, you should load the volume where the archive member starts, and use `tar —extract —multi-volume' — `tar' will prompt for later volumes as it needs them.

Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive.

Often you might want to write a large archive, one larger than will fit on the actual tape you are using.

Use `—multi-volume' (`-M') on the command line, and then `tar' will, when it reaches the end of the tape, prompt for another tape, and continue the archive. You can also use the `—tape-length=1024-SIZE' (`-L 1024-SIZE') option to specify the volume size.

`-M'
`--multi-volume'
     Create/list/extract multi-volume archive.

This option causes `tar' to write a "multi-volume" archive--one
that may be larger than will fit on the medium used to hold it.

`-L NUM'
`--tape-length=NUM'
     Change tape after writing NUM x 1024 bytes.

This option might be useful when your tape drivers do not properly
detect end of physical tapes.  By being slightly conservative on
the maximum tape length, you might avoid the problem entirely.

Each tape will have an independent archive, and can be read without needing the other. (As an exception to this, the file that `tar' was archiving when it ran out of tape will usually be split between the two archives; in this case you need to extract from the first archive, using `—multi-volume' (`-M'), and then put in the second tape when prompted, so `tar' can restore both halves of the file.)

The volume number used by `tar' in its tape-change prompt can be changed; if you give the `—volno-file=FILE-OF-NUMBER' option, then FILE-OF-NUMBER should be an unexisting file to be created, or else, a file already containing a decimal number. That number will be used as the volume number of the first volume written. When `tar' is finished, it will rewrite the file with the now-current volume number.

GNU `tar' multi-volume archives do not use a truly portable format. You need GNU `tar' at both end to process them properly.

When prompting for a new tape, `tar' accepts any of the following responses:

`y'
    Request `tar' to begin writing the next volume.

   (You should only type `y' after you have changed the tape; otherwise
`tar' will write over the volume it just finished.)

If you want more elaborate behavior than this, give `tar' the `—info-script=SCRIPT-NAME' (`—new-volume-script=SCRIPT-NAME', `-F SCRIPT-NAME') option. The file SCRIPT-NAME is expected to be a program (or shell script) to be run instead of the normal prompting procedure. When the program finishes, `tar' will immediately begin writing the next volume.

`-F FILE'
`--info-script=FILE'
`--new-volume-script=FILE'
    Execute `file' at end of each tape.  If `file' exits with nonzero
    status, exit.  This implies `--multi-volume' (`-M').

if you give `tar' multiple `—file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') options, then the specified files will be used, in sequence, as the successive volumes of the archive. Only when the first one in the sequence needs to be used again will `tar' prompt for a tape change (or run the info script).

When `tar' writes an archive to tape, it creates a single tape file. If multiple archives are written to the same tape, one after the other, they each get written as separate tape files.

When extracting, it is necessary to position the tape at the right place before running `tar'. To do this, use the `mt' command. For more information on the `mt' command and on the organization of tapes into a sequence of tape files, see *Note mt::.

To give the archive a name which will be recorded in it, use the `—label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option. This will write a special block identifying VOLUME-LABEL as the name of the archive to the front of the archive which will be displayed when the archive is listed with `—list' (`-t'). If you are creating a multi-volume archive with `—multi-volume' (`-M'), then the volume label will have `Volume NNN' appended to the name you give, where NNN is the number of the volume of the archive. (If you use the `—label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option when reading an archive, it checks to make sure the label on the tape matches the one you give.

People seem to often do:

--label="SOME-PREFIX `date +SOME-FORMAT`"

or such, for pushing a common date in all volumes or an archive set.

The `—gzip' (`—gunzip', `—ungzip', `-z') option does not work with the `—multi-volume' (`-M') option, or with the `—update' (`-u'), `—append' (`-r'), `—concatenate' (`—catenate', `-A'), or `—delete' operations.

`—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.

`—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.