path: root/doc/lziprecover.texinfo

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename lziprecover.info
@settitle Lziprecover Manual
@finalout
@c %**end of header

@set UPDATED 12 November 2011
@set VERSION 1.13-rc1

@dircategory Data Compression
@direntry
* Lziprecover: (lziprecover).   Data recovery tool for lzipped files
@end direntry


@ifnothtml
@titlepage
@title Lziprecover
@subtitle Data recovery tool for lzipped files
@subtitle for Lziprecover version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

@page
@vskip 0pt plus 1filll
@end titlepage

@contents
@end ifnothtml

@node Top
@top

This manual is for Lziprecover (version @value{VERSION}, @value{UPDATED}).

@menu
* Introduction::          Purpose and features of lziprecover
* Invoking Lziprecover::  Command line interface
* File Format::           Detailed format of the compressed file
* Examples::              A small tutorial with examples
* Problems::              Reporting bugs
* Concept Index::         Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2009, 2010, 2011 Antonio Diaz Diaz.

This manual is free documentation: you have unlimited permission
to copy, distribute and modify it.


@node Introduction
@chapter Introduction
@cindex introduction

Lziprecover is a data recovery tool and decompressor for files in the
lzip compressed data format (.lz) able to repair slightly damaged files,
recover badly damaged files from two or more copies, extract undamaged
members from multi-member files, decompress files and test integrity of
files.

Lziprecover is able to recover or decompress files produced by any of
the compressors in the lzip family; lzip, plzip, minilzip/lzlib, clzip
and pdlzip. This recovery capability contributes to make the lzip format
one of the best options for long-term data archiving.

When recovering data, lziprecover takes as arguments the names of the
damaged files and writes zero or more recovered files depending on the
operation selected and whether the recovery succeeded or not. The
damaged files themselves are never modified.

When decompressing or testing file integrity, lziprecover behaves like
lzip or lunzip.

If the files are too damaged for lziprecover to repair them, data from
damaged members can be partially recovered writing it to stdout as shown
in the following example (the resulting file may contain some garbage
data at the end):

@example
lziprecover -cd rec00001file.lz > rec00001file
@end example

If the cause of file corruption is damaged media, the combination
@w{GNU ddrescue + lziprecover} is the best option for recovering data
from multiple damaged copies. @xref{ddrescue-example}, for an example.

Return values: 0 for a normal exit, 1 for environmental problems (file
not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused lziprecover to panic.


@node Invoking Lziprecover
@chapter Invoking Lziprecover
@cindex invoking lziprecover

The format for running lziprecover is:

@example
lziprecover [@var{options}] [@var{files}]
@end example

Lziprecover supports the following options:

@table @samp
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of lziprecover on the standard output and exit.

@item -c
@itemx --stdout
Decompress to standard output. Needed when reading from a named pipe
(fifo) or from a device. Use it to recover as much of the uncompressed
data as possible when decompressing a corrupt file.

@item -d
@itemx --decompress
Decompress.

@item -f
@itemx --force
Force overwrite of output files.

@item -k
@itemx --keep
Keep (don't delete) input files during decompression.

@item -m
@itemx --merge
Try to produce a correct file merging the good parts of two or more
damaged copies. The copies must be single-member files. The merge will
fail if the copies have too many damaged areas or if the same byte is
damaged in all copies. If successful, a repaired copy is written to the
file @samp{@var{file}_fixed.lz}. The exit status is 0 if the file could
be repaired, 2 otherwise.

To give you an idea of its possibilities, when merging two copies each
of them with one damaged area affecting 1 percent of the copy, the
probability of obtaining a correct file is about 98 percent. With three
such copies the probability rises to 99.97 percent. For large files with
small errors, the probability approaches 100 percent even with only two
copies.

@item -o @var{file}
@itemx --output=@var{file}
Place the output into @samp{@var{file}} instead of into
@samp{@var{file}_fixed.lz}. If splitting, the names of the files
produced are in the form @samp{rec00001@var{file}},
@samp{rec00002@var{file}}, etc. If decompressing from standard input and
@samp{--stdout} has not been specified, use @samp{@var{file}} as the
name of the decompressed file.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -R
@itemx --repair
Try to repair a small error, affecting only one byte, in a single-member
@var{file}. If successful, a repaired copy is written to the file
@samp{@var{file}_fixed.lz}. @samp{@var{file}} is not modified at all.
The exit status is 0 if the file could be repaired, 2 otherwise.

@item -s
@itemx --split
Search for members in @samp{@var{file}} and write each member in its own
@samp{.lz} file. You can then use @samp{lziprecover -t} to test the
integrity of the resulting files, decompress those which are undamaged,
and try to repair or partially decompress those which are damaged.

The names of the files produced are in the form
@samp{rec00001@var{file}.lz}, @samp{rec00002@var{file}.lz}, etc, and are
designed so that the use of wildcards in subsequent processing, for
example, @w{@samp{lziprecover -cd rec*@var{file}.lz > recovered_data}},
processes the files in the correct order.

@item -t
@itemx --test
Check integrity of the specified file(s), but don't decompress them.
This really performs a trial decompression and throws away the result.
Use it together with @samp{-v} to see information about the file.

@item -v
@itemx --verbose
Verbose mode.@*
When decompressing or testing, further -v's (up to 4) increase the
verbosity level, showing status, dictionary size, compression ratio,
trailer contents (CRC, data size, member size), and up to 6 bytes of
trailing garbage (if any).

@end table


@node File Format
@chapter File Format
@cindex file format

In the diagram below, a box like this:
@verbatim
+---+
|   | <-- the vertical bars might be missing
+---+
@end verbatim

represents one byte; a box like this:
@verbatim
+==============+
|              |
+==============+
@end verbatim

represents a variable number of bytes.

@sp 1
A lzip file consists of a series of "members" (compressed data sets).
The members simply appear one after another in the file, with no
additional information before, between, or after them.

Each member has the following structure:
@verbatim
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | Lzma stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@end verbatim

All multibyte values are stored in little endian order.

@table @samp
@item ID string
A four byte string, identifying the lzip format, with the value "LZIP".

@item VN (version number, 1 byte)
Just in case something needs to be modified in the future. Valid values
are 0 and 1. Version 0 files are deprecated. They can contain only one
member and lack the @samp{Member size} field.

@item DS (coded dictionary size, 1 byte)
Bits 4-0 contain the base 2 logarithm of the base dictionary size.@*
Bits 7-5 contain the number of "wedges" to substract from the base
dictionary size to obtain the dictionary size. The size of a wedge is
(base dictionary size / 16).@*
Valid values for dictionary size range from 4KiB to 512MiB.

@item Lzma stream
The lzma stream, finished by an end of stream marker. Uses default values
for encoder properties.

@item CRC32 (4 bytes)
CRC of the uncompressed original data.

@item Data size (8 bytes)
Size of the uncompressed original data.

@item Member size (8 bytes)
Total size of the member, including header and trailer. This facilitates
safe recovery of undamaged members from multimember files.

@end table


@node Examples
@chapter A small tutorial with examples
@cindex examples

Example 1: Restore a regular file from its compressed version
@samp{file.lz}. If the operation is successful, @samp{file.lz} is
removed.

@example
lziprecover -d file.lz
@end example

@sp 1
@noindent
Example 2: Verify the integrity of the compressed file @samp{file.lz}
and show status.

@example
lziprecover -tv file.lz
@end example

@sp 1
@noindent
Example 3: Decompress @samp{file.lz} partially until 10KiB of
decompressed data are produced.

@example
lziprecover -cd file.lz | dd bs=1024 count=10
@end example

@sp 1
@noindent
Example 4: Decompress @samp{file.lz} partially from decompressed byte
10000 to decompressed byte 15000 (5000 bytes are produced).

@example
lziprecover -cd file.lz | dd bs=1000 skip=10 count=5
@end example

@sp 1
@noindent
Example 5: Repair a one-byte corruption in the single-member file
@samp{file.lz}. (Indented lines are abridged error messages from
lziprecover).

@example
lziprecover -v -R file.lz
  Copy of input file repaired successfully.
mv file_fixed.lz file.lz
@end example

@sp 1
@noindent
Example 6: Split the multi-member file @samp{file.lz} and write each
member in its own @samp{recXXXXXfile.lz} file. Then use
@w{@samp{lziprecover -t}} to test the integrity of the resulting files.

@example
lziprecover -s file.lz
lziprecover -tv rec*file.lz
@end example

@sp 1
@anchor{ddrescue-example}
@noindent
Example 7: Recover a compressed backup from two copies on CD-ROM (see
the GNU ddrescue manual for details about ddrescue)

@example
ddrescue -b2048 /dev/cdrom cdimage1 logfile1
mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz
umount /mnt/cdimage
  (insert second copy in the CD drive)
ddrescue -b2048 /dev/cdrom cdimage2 logfile2
mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz
umount /mnt/cdimage
lziprecover -m -v -o rescued.tar.lz rescued1.tar.lz rescued2.tar.lz
@end example

@sp 1
@noindent
Example 8: Recover the first volume of those created with the command
@w{@code{lzip -b 32MiB -S 650MB big_db}} from two copies,
@samp{big_db1_00001.lz} and @samp{big_db2_00001.lz}, with member 00007
damaged in the first copy, member 00018 damaged in the second copy, and
member 00012 damaged in both copies. Two correct copies are produced and
compared.

@example
lziprecover -s big_db1_00001.lz
lziprecover -s big_db2_00001.lz
lziprecover -t rec*big_db1_00001.lz
  rec00007big_db1_00001.lz: crc mismatch
  rec00012big_db1_00001.lz: crc mismatch
lziprecover -t rec*big_db2_00001.lz
  rec00012big_db2_00001.lz: crc mismatch
  rec00018big_db2_00001.lz: crc mismatch
lziprecover -m -v rec00012big_db1_00001.lz rec00012big_db2_00001.lz
  Input files merged successfully
cp rec00007big_db2_00001.lz rec00007big_db1_00001.lz
cp rec00012big_db1_00001_fixed.lz rec00012big_db1_00001.lz
cp rec00012big_db1_00001_fixed.lz rec00012big_db2_00001.lz
cp rec00018big_db1_00001.lz rec00018big_db2_00001.lz
cat rec*big_db1_00001.lz > big_db3_00001.lz
cat rec*big_db2_00001.lz > big_db4_00001.lz
zcmp big_db3_00001.lz big_db4_00001.lz
@end example


@node Problems
@chapter Reporting Bugs
@cindex bugs
@cindex getting help

There are probably bugs in lziprecover. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

If you find a bug in lziprecover, please send electronic mail to
@email{lzip-bug@@nongnu.org}. Include the version number, which you can
find by running @w{@samp{lziprecover --version}}.


@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye