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INFO-DIR-SECTION Data Compression
START-INFO-DIR-ENTRY
* Lziprecover: (lziprecover).   Data recovery tool for lzipped files
END-INFO-DIR-ENTRY


File: lziprecover.info,  Node: Top,  Next: Introduction,  Up: (dir)

Lziprecover Manual
******************

This manual is for Lziprecover (version 1.14, 31 May 2013).

* 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


   Copyright (C) 2009, 2010, 2011, 2012, 2013 Antonio Diaz Diaz.

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


File: lziprecover.info,  Node: Introduction,  Next: Invoking Lziprecover,  Prev: Top,  Up: Top

1 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.

   Lziprecover is able to efficiently extract a range of bytes from a
multi-member file, because it only decompresses the members containing
the desired data.

   Lziprecover can print correct total file sizes and ratios even for
multi-member files.

   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):

     lziprecover -cd rec01file.lz > rec01file

   If the cause of file corruption is damaged media, the combination
GNU ddrescue + lziprecover is the best option for recovering data from
multiple damaged copies. *Note ddrescue-example::, for an example.


File: lziprecover.info,  Node: Invoking Lziprecover,  Next: File Format,  Prev: Introduction,  Up: Top

2 Invoking Lziprecover
**********************

The format for running lziprecover is:

     lziprecover [OPTIONS] [FILES]

   Lziprecover supports the following options:

`-h'
`--help'
     Print an informative help message describing the options and exit.

`-V'
`--version'
     Print the version number of lziprecover on the standard output and
     exit.

`-c'
`--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.

`-d'
`--decompress'
     Decompress.

`-D RANGE'
`--range-decompress=RANGE'
     Decompress only a range of bytes starting at decompressed byte
     position `BEGIN' and up to byte position `END - 1'.  Three formats
     of RANGE are recognized, `BEGIN', `BEGIN-END', and `BEGIN,SIZE'.
     If only BEGIN is specified, END is taken as the end of the file.
     The produced bytes are sent to standard output unless the
     `--output' option is used. In order to guarantee the correctness
     of the data produced, all members containing any part of the
     desired data are decompressed and their integrity is verified.
     This operation is more efficient in multi-member files because it
     only decompresses the members containing the desired data.

`-f'
`--force'
     Force overwrite of output files.

`-i'
`--ignore-errors'
     Make `--range-decompress' ignore data errors and continue
     decompressing the remaining members in the file. For example,
     `lziprecover -i -D0 file.lz > file' decompresses all the
     recoverable data in all members of `file.lz' without having to
     split it first.

`-k'
`--keep'
     Keep (don't delete) input files during decompression.

`-l'
`--list'
     Print total file sizes and ratios. The values produced are correct
     even for multi-member files. Use it together with `-v' to see
     information about the members in the file.

`-m'
`--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 `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.

`-o FILE'
`--output=FILE'
     Place the output into `FILE' instead of into `FILE_fixed.lz'. If
     splitting, the names of the files produced are in the form
     `rec01FILE', `rec02FILE', etc. If decompressing from standard
     input and `--stdout' has not been specified, use `FILE' as the
     name of the decompressed file.

`-q'
`--quiet'
     Quiet operation. Suppress all messages.

`-R'
`--repair'
     Try to repair a small error, affecting only one byte, in a
     single-member FILE. If successful, a repaired copy is written to
     the file `FILE_fixed.lz'. `FILE' is not modified at all.  The exit
     status is 0 if the file could be repaired, 2 otherwise.

`-s'
`--split'
     Search for members in `FILE' and write each member in its own
     `.lz' file. You can then use `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 `rec01FILE.lz',
     `rec02FILE.lz', etc, and are designed so that the use of wildcards
     in subsequent processing, for example,
     `lziprecover -cd rec*FILE.lz > recovered_data', processes the
     files in the correct order. The number of digits used in the names
     varies depending on the number of members in `FILE'.

`-t'
`--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 `-v' to see information about
     the file.

`-v'
`--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).



   Numbers given as arguments to options may be followed by a multiplier
and an optional `B' for "byte".

   Table of SI and binary prefixes (unit multipliers):

Prefix   Value                     |   Prefix   Value
k        kilobyte  (10^3 = 1000)   |   Ki       kibibyte (2^10 = 1024)
M        megabyte  (10^6)          |   Mi       mebibyte (2^20)
G        gigabyte  (10^9)          |   Gi       gibibyte (2^30)
T        terabyte  (10^12)         |   Ti       tebibyte (2^40)
P        petabyte  (10^15)         |   Pi       pebibyte (2^50)
E        exabyte   (10^18)         |   Ei       exbibyte (2^60)
Z        zettabyte (10^21)         |   Zi       zebibyte (2^70)
Y        yottabyte (10^24)         |   Yi       yobibyte (2^80)


   Exit status: 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.


File: lziprecover.info,  Node: File Format,  Next: Examples,  Prev: Invoking Lziprecover,  Up: Top

3 File Format
*************

Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.
-- Antoine de Saint-Exupery


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

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

   represents a variable number of bytes.


   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:
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | Lzma stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All multibyte values are stored in little endian order.

`ID string'
     A four byte string, identifying the lzip format, with the value
     "LZIP".

`VN (version number, 1 byte)'
     Just in case something needs to be modified in the future. 1 for
     now.

`DS (coded dictionary size, 1 byte)'
     Lzip divides the distance between any two powers of 2 into 8
     equally spaced intervals, named "wedges". The dictionary size is
     calculated by taking a power of 2 (the base size) and substracting
     from it a number of wedges between 0 and 7. The size of a wedge is
     (base_size / 16).
     Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).
     Bits 7-5 contain the number of wedges (0 to 7) to substract from
     the base size to obtain the dictionary size.
     Example: 0xD3 = (2^19 - 6 * 2^15) = (512KiB - 6 * 32KiB) = 320KiB
     Valid values for dictionary size range from 4KiB to 512MiB.

`Lzma stream'
     The lzma stream, finished by an end of stream marker. Uses default
     values for encoder properties. See the lzip manual for a full
     description.

`CRC32 (4 bytes)'
     CRC of the uncompressed original data.

`Data size (8 bytes)'
     Size of the uncompressed original data.

`Member size (8 bytes)'
     Total size of the member, including header and trailer. This field
     acts as a distributed index, allows the verification of stream
     integrity, and facilitates safe recovery of undamaged members from
     multi-member files.



File: lziprecover.info,  Node: Examples,  Next: Problems,  Prev: File Format,  Up: Top

4 A small tutorial with examples
********************************

Example 1: Restore a regular file from its compressed version
`file.lz'. If the operation is successful, `file.lz' is removed.

     lziprecover -d file.lz


Example 2: Verify the integrity of the compressed file `file.lz' and
show status.

     lziprecover -tv file.lz


Example 3: Decompress `file.lz' partially until 10KiB of decompressed
data are produced.

     lziprecover -D 0,10KiB file.lz


Example 4: Decompress `file.lz' partially from decompressed byte 10000
to decompressed byte 15000 (5000 bytes are produced).

     lziprecover -D 10000-15000 file.lz


Example 5: Repair a one-byte corruption in the single-member file
`file.lz'. (Indented lines are abridged error messages from
lziprecover).

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


Example 6: Split the multi-member file `file.lz' and write each member
in its own `recXXXXXfile.lz' file. Then use `lziprecover -t' to test
the integrity of the resulting files.

     lziprecover -s file.lz
     lziprecover -tv rec*file.lz


Example 7: Recover a compressed backup from two copies on CD-ROM with
error-checked merging of copies (*Note GNU ddrescue manual:
(ddrescue)Top, for details about ddrescue).

     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


Example 8: Recover the first volume of those created with the command
`lzip -b 32MiB -S 650MB big_db' from two copies, `big_db1_00001.lz' and
`big_db2_00001.lz', with member 07 damaged in the first copy, member 18
damaged in the second copy, and member 12 damaged in both copies. Two
correct copies are produced and compared.

     lziprecover -s big_db1_00001.lz
     lziprecover -s big_db2_00001.lz
     lziprecover -t rec*big_db1_00001.lz
       rec07big_db1_00001.lz: crc mismatch
       rec12big_db1_00001.lz: crc mismatch
     lziprecover -t rec*big_db2_00001.lz
       rec12big_db2_00001.lz: crc mismatch
       rec18big_db2_00001.lz: crc mismatch
     lziprecover -m -v rec12big_db1_00001.lz rec12big_db2_00001.lz
       Input files merged successfully
     cp rec07big_db2_00001.lz rec07big_db1_00001.lz
     cp rec12big_db1_00001_fixed.lz rec12big_db1_00001.lz
     cp rec12big_db1_00001_fixed.lz rec12big_db2_00001.lz
     cp rec18big_db1_00001.lz rec18big_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


File: lziprecover.info,  Node: Problems,  Next: Concept Index,  Prev: Examples,  Up: Top

5 Reporting Bugs
****************

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
<lzip-bug@nongnu.org>. Include the version number, which you can find
by running `lziprecover --version'.


File: lziprecover.info,  Node: Concept Index,  Prev: Problems,  Up: Top

Concept Index
*************

[index]
* Menu:

* bugs:                                  Problems.              (line 6)
* examples:                              Examples.              (line 6)
* file format:                           File Format.           (line 6)
* getting help:                          Problems.              (line 6)
* introduction:                          Introduction.          (line 6)
* invoking lziprecover:                  Invoking Lziprecover.  (line 6)



Tag Table:
Node: Top231
Node: Introduction901
Node: Invoking Lziprecover2685
Node: File Format8447
Node: Examples10938
Ref: ddrescue-example12158
Node: Problems13938
Node: Concept Index14488

End Tag Table


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