This file is about using GNU tar to write backups.

Stores special files (c and b) just fine.

The string:string format of destination archives is only used for
invoking "rmt" on the remote host, so it's not useful for my 
typical goals.

Useful options
Can abbreviate at least -- style params as long as still distinctive.
E.e. can use "--one-f" instead of "--one-file-system".
There is no option to extract without subdirectories, corresponding to unzip -j.

    -F     vol-change-script (elsewhere says to use --new-volume-script)
    --exclude=PATTERN   (pattern uses shell wildcards except * matches /
            BUT THESE ARE APPLIED TO EACH DIR LEVEL!)

    (Can't use these with -M!:)
    # According to
    # https://community.centminmod.com/threads/compression-comparison-benchmarks-zstd-vs-brotli-vs-pigz-vs-bzip2-vs-xz-etc.12764/
    # xz has best compression ratio and zstd has fastest compress/decompress.
    -j     filter through bzip2  (write *.bz2)
    -Z     filter through compress  (write *.Z?)
    -z     filter through gzip  (write *.gz)
    -J     filter through xz  (write *.xz)
    --zstd filter through zstd (write *.zst)
    -C     like unzip's -d.  Local extract-to directory.

    --one-file-system  Does honor btrfs subvolumes too
    -L 5   tape length is 5 kB.  Unit is kB.
           (If use like $Date: text $ and $Source: text$, can
           see it with "ident file.tar").
    -M     multi-volume archive.  (Can't use with compression!)
           (N.b., if using stdout, will re-open stdout upon EOF,
           even if -M is not set).
    -V name  volume label
    -f archive.tar.*
    --numeric-owner
    -C localdir

    -p    preserve permissions (on extract only).  No umasking.
    --occurrence   Only extracts or lists the first occurrence of the
                   specified file(s), then quits.  NOT WORKING!!
    -P Preserve leading slashes.  Definitely use for both input and output.


WORKS:

    tar -czf - -V '$Date: 1-2-30 $ $Source: my backup. $' /tmp/wesnoth.old |
    /tmp/ddblocker.bash

See the tar file keywords (like $keyword: ...$), by running
"gunzip -c file.tgz | ident | head".  The tar file keywords will be before
keywords for any contained files.  ("what" or similar SCCS command is just
like RCS ident).

--wildcards

Empirically, it looks like -mount is default?

Multi-Volume creation:
...-ML <KB_EACH> -f first/vol.tar ...
    Answer "Prepare volume..." prompt answers:
        n second/vol.tar
        n third/vol.tar
        etc.
Multi-Volume extraction:
...-Mf first/vol.tar ...
    Answer "Prepare volume..." prompt answers:
        n second/vol.tar
        n third/vol.tar
        etc.
The "Prepare volume..." ! command isn't used for stdin.
Therefore, must stage compressed files if you want compression + multi-volume.