Often you might want to write a large archive, one larger than will fit
on the actual tape you are using. In such a case, you can run multiple
tar commands, but this can be inconvenient, particularly if you
are using options like --exclude=pattern or dumping entire file systems.
Therefore, tar supports multiple tapes automatically.
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. 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, and then put in the second tape when
prompted, so tar can restore both halves of the file.)
GNU tar multi-volume archives do not use a truly portable format.
You need GNU tar at both ends to process them properly.
When prompting for a new tape, tar accepts any of the following
responses:
?
Request tar to explain possible responses
q
Request tar to exit immediately.
n file name
Request tar to write the next volume on the file file name.
!
Request tar to run a subshell. This option can be disabled
by giving --restrict command line option to tar.
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. It is executed without any command line
arguments. Additional data is passed to it via the following
environment variables:
TAR_VERSION
GNU tar version number.
TAR_ARCHIVE
The name of the archive tar is processing.
TAR_VOLUME
Ordinal number of the volume tar is about to start.
TAR_SUBCOMMAND
Short option describing the operation tar is executed.
See Operations, for a complete list of subcommand options.
TAR_FORMAT
Format of the archive being processed. See Formats, for a complete
list of archive format names.
The info script can instruct tar to use new archive name,
by writing in to file descriptor 3 (see below for an
example).
If the info script fails, tar exits; otherwise, it begins
writing the next volume.
The method tar uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices. You can use the
--tape-length=size (-L size) option if
tar can't detect the end of the tape itself. This option
selects --multi-volume (-M) automatically. The
size argument should then be the usable size of the tape in
units of 1024 bytes. But for many devices, and floppy disks in
particular, this option is never required for real, as far as we know.
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. (This does not change the volume number
written on a tape label, as per label, it only affects
the number used in the prompt.)
If you want tar to cycle through a series of files or tape
drives, there are three approaches to choose from. First of all, you
can give tar multiple --file options. In this case
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). Secondly, you can use the ‘n’ response to the
tape-change prompt, and, finally, you can use an info script, that
writes new archive name to file descriptor. The following example
illustrates this approach:
#! /bin/sh
echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
case $TAR_SUBCOMMAND in
-c) ;;
-d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1
;;
*) exit 1
esac
echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&3
Each volume of a multi-volume archive is an independent tar
archive, complete in itself. For example, you can list or extract any
volume alone; just don't specify --multi-volume
(-M). However, if one file in the archive is split across
volumes, the only way to extract it successfully is with a
multi-volume extract command --extract --multi-volume
(-xM) starting on or before the volume where the file begins.
For example, let's presume someone has two tape drives on a system
named /dev/tape0 and /dev/tape1. For having GNU tar
to switch to the second drive when it needs to write the
second tape, and then back to the first tape, etc., just do either of:
$ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 files
$ tar cMff /dev/tape0 /dev/tape1 files
