path: root/doc/lziprecover.texinfo

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename lziprecover.info
@documentencoding ISO-8859-15
@settitle Lziprecover Manual
@finalout
@c %**end of header

@set UPDATED 23 April 2013
@set VERSION 1.14-rc3

@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, 2012, 2013 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.

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):

@example
lziprecover -cd rec01file.lz > rec01file
@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 -D @var{range}
@itemx --range-decompress=@var{range}
Decompress only a range of bytes starting at decompressed byte position
@samp{@var{begin}} and up to byte position @w{@samp{@var{end} - 1}}.
Three formats of @var{range} are recognized, @samp{@var{begin}},
@samp{@var{begin}-@var{end}}, and @samp{@var{begin},@var{size}}. If only
@var{begin} is specified, @var{end} is taken as the end of the file. The
produced bytes are sent to standard output unless the @samp{--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.

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

@item -i
@itemx --ignore-errors
Make @samp{--range-decompress} ignore data errors and continue
decompressing the remaining members in the file. For example,
@w{@samp{lziprecover -i -D0 file.lz > file}} decompresses all the
recoverable data in all members of @samp{file.lz} without having to
split it first.

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

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


@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{rec01@var{file}}, @samp{rec02@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{rec01@var{file}.lz}, @samp{rec02@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. The number of digits used in
the names varies depending on the number of members in @samp{@var{file}}.

@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

@sp 1
Numbers given as arguments to options may be followed by a multiplier
and an optional @samp{B} for "byte".

Table of SI and binary prefixes (unit multipliers):

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


@node File Format
@chapter File Format
@cindex 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

@sp 1
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. 1 for now.

@item 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.

@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 field acts
as a distributed index, allows the verification of stream integrity, and
facilitates safe recovery of undamaged members from multi-member 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 -D 10KiB file.lz
@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 -D 10000-15000 file.lz
@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 with
error-checked merging of copies
@ifnothtml
(@xref{Top,GNU ddrescue manual,,ddrescue},
@end ifnothtml
@ifhtml
(See the
@uref{http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html,,ddrescue manual}
@end ifhtml
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 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.

@example
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
@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