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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 option (see create). A multi-volume archive can be manipulated like any other archive (provided the --multi-volume option is specified), but is stored on more than one tape or disk.
When you specify --multi-volume, tar does not report an error when it comes to the end of an archive volume (when reading), or the end of the media (when writing). Instead, it prompts you to load a new storage volume. If the archive is on a magnetic tape, you should change tapes when you see the prompt; if the archive is on a floppy disk, you should change disks; etc.
$ tar --create --multi-volume --file=/dev/tape files
The method tar uses to detect end of tape is not perfect, and fails on some operating systems or on some devices. If tar cannot detect the end of the tape itself, you can use --tape-length option to inform it about the capacity of the tape:
$ tar --create --tape-length=41943040 --file=/dev/tape files
When GNU tar comes to the end of a storage media, it asks you to change the volume. The built-in prompt for POSIX locale is1:
Prepare volume #n for `archive' and hit return:
where n is the ordinal number of the volume to be created and archive is archive file or device name.
When prompting for a new tape, tar accepts any of the following responses:
(You should only type ‘y’ after you have changed the tape; otherwise tar will write over the volume it just finished.)
The volume number used by tar in its tape-changing prompt can be changed; if you give the --volno-file=file-of-number option, then file-of-number should be an non-existing 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 more elaborate behavior than this, you can write a special new volume script, that will be responsible for changing the volume, and instruct tar to use it instead of its normal prompting procedure:
The script-name is executed without any command line arguments. It inherits tar's shell environment. Additional data is passed to it via the following environment variables:
The volume 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.
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). For example, suppose 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
The second method is to use the ‘n’ response to the tape-change prompt.
Finally, the most flexible approach is to use a volume script, that writes new archive name to the file descriptor #3. For example, the following volume script will create a series of archive files, named archive-vol, where archive is the name of the archive being created (as given by --file option) and vol is the ordinal number of the archive being created:
#! /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
The same script cant be used while listing, comparing or extracting from the created archive. For example:
# Create a multi-volume archive: $ tar -c -L1024 -f archive.tar -F new-volume . # Extract from the created archive: $ tar -x -f archive.tar -F new-volume .
Notice, that the first command had to use -L option, since otherwise GNU tar will end up writing everything to file archive.tar.
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, without --multi-volume specified. To extract an archive member from one volume (assuming it is described that volume), use --extract, again without --multi-volume.
If an archive member is split across volumes (i.e., its entry begins on one volume of the media and ends on another), you need to specify --multi-volume 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. See extracting archives, for more information about extracting archives.
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.
If a multi-volume archive was labeled using --label=archive-label (see label) when it was created, tar will not automatically label volumes which are added later. To label subsequent volumes, specify --label=archive-label again in conjunction with the --append, --update or --concatenate operation.
Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using GNU tar. If you absolutely have to process such archives using a third-party tar implementation, read Split Recovery.