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Normally, tar will request data in full block increments from
an archive storage device. If the device cannot return a full block,
tar will report an error. However, some devices do not always
return full blocks, or do not require the last block of an archive to
be padded out to the next block boundary. To keep reading until you
obtain a full block, or to accept an incomplete block if it contains
an end-of-archive marker, specify the `--read-full-blocks' (`-B') option
in conjunction with the `--extract' (`-x') or `--list' (`-t') operations.
FIXME: xref Listing Contents.
The `--read-full-blocks' (`-B') option is turned on by default when
tar reads an archive from standard input, or from a remote
machine. This is because on BSD Unix systems, attempting to read a
pipe returns however much happens to be in the pipe, even if it is
less than was requested. If this option were not enabled, tar
would fail as soon as it read an incomplete block from the pipe.
If you're not sure of the blocking factor of an archive, you can read the archive by specifying `--read-full-blocks' (`-B') and `--block-size=512-size' (`-b 512-size'), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive.
FIXME: xref Blocking Factor.
Normally tar stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
`--ignore-zeros' (`-i') allows tar to completely read an archive
which contains a block of zeros before the end (i.e. a damaged
archive, or one which was created by cat-ing several archives
together).
The `--ignore-zeros' (`-i') option is turned off by default because many
versions of tar write garbage after the end-of-archive entry,
since that part of the media is never supposed to be read. GNU
tar does not write after the end of an archive, but seeks to
maintain compatablity among archiving utilities.
If you are using a machine with a small amount of memory, and you
need to process large list of file names, you can reduce the amount
of space tar needs to process the list. To do so, specify the
`--same-order' (`-s') option and provide an ordered list of file names.
This option tells tar that the name arguments provided
on the command line, or read from a file using the `--files-from=file-of-names' (`-T file-of-names')
option, are listed in the same order as the files in the archive.
You can create a file containing an ordered list of files in the archive by storing the output produced by `tar --list --file=archive-name'.
FIXME: xref Listing Contents, for information on the `--list' (`-t') operation.
This option is probably never needed on modern computer systems.
FIXME: we don't need/want --preserve to exist any more
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