Lziprecover Manual

This manual is for Lziprecover (version 1.14-rc3, 23 April 2013).

• 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

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

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. See 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.

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).

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)

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

+---+
|   | <-- the vertical bars might be missing
+---+

represents one byte; a box like this:

+==============+
|              |
+==============+

represents a variable number of bytes.

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.

`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.

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

     lziprecover -tv file.lz

     lziprecover -D 10KiB file.lz

     lziprecover -D 10000-15000 file.lz

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

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

     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

     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

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'.

Concept Index

• bugs: Problems
• examples: Examples
• file format: File Format
• getting help: Problems
• introduction: Introduction
• invoking lziprecover: Invoking Lziprecover