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INFO-DIR-SECTION Compression
START-INFO-DIR-ENTRY
* Lziprecover: (lziprecover).   Data recovery tool for the lzip format
END-INFO-DIR-ENTRY


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

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

This manual is for Lziprecover (version 1.23, 21 January 2022).

* Menu:

* Introduction::            Purpose and features of lziprecover
* Invoking lziprecover::    Command line interface
* Data safety::             Protecting data from accidental loss
* Repairing one byte::      Fixing bit flips and similar errors
* Merging files::           Fixing several damaged copies
* Reproducing one sector::  Fixing a missing (zeroed) sector
* Tarlz::                   Options supporting the tar.lz format
* File names::              Names of the files produced by lziprecover
* File format::             Detailed format of the compressed file
* Trailing data::           Extra data appended to the file
* Examples::                A small tutorial with examples
* Unzcrash::                Testing the robustness of decompressors
* Problems::                Reporting bugs
* Concept index::           Index of concepts


   Copyright (C) 2009-2022 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). Lziprecover is able to repair slightly damaged
files (up to one single-byte error per member), produce a correct file by
merging the good parts of two or more damaged copies, reproduce a missing
(zeroed) sector using a reference file, extract data from damaged files,
decompress files, and test integrity of files.

   Lziprecover can remove the damaged members from multimember files, for
example multimember tar.lz archives.

   Lziprecover provides random access to the data in multimember files; it
only decompresses the members containing the desired data.

   Lziprecover facilitates the management of metadata stored as trailing
data in lzip files.

   Lziprecover is not a replacement for regular backups, but a last line of
defense for the case where the backups are also damaged.

   The lzip file format is designed for data sharing and long-term
archiving, taking into account both data integrity and decoder availability:

   * The lzip format provides very safe integrity checking and some data
     recovery means. The program lziprecover can repair bit flip errors
     (one of the most common forms of data corruption) in lzip files, and
     provides data recovery capabilities, including error-checked merging
     of damaged copies of a file. *Note Data safety::.

   * The lzip format is as simple as possible (but not simpler). The lzip
     manual provides the source code of a simple decompressor along with a
     detailed explanation of how it works, so that with the only help of the
     lzip manual it would be possible for a digital archaeologist to extract
     the data from a lzip file long after quantum computers eventually
     render LZMA obsolete.

   * Additionally the lzip reference implementation is copylefted, which
     guarantees that it will remain free forever.

   A nice feature of the lzip format is that a corrupt byte is easier to
repair the nearer it is from the beginning of the file. Therefore, with the
help of lziprecover, losing an entire archive just because of a corrupt
byte near the beginning is a thing of the past.

   Compression may be good for long-term archiving. For compressible data,
multiple compressed copies may provide redundancy in a more useful form and
may have a better chance of surviving intact than one uncompressed copy
using the same amount of storage space. This is specially true if the format
provides recovery capabilities like those of lziprecover, which is able to
find and combine the good parts of several damaged copies.

   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.

   If the cause of file corruption is a damaged medium, the combination
GNU ddrescue + lziprecover is the recommended option for recovering data
from damaged lzip files. *Note ddrescue-example::, and *note
ddrescue-example2::, for examples.

   If a file is too damaged for lziprecover to repair it, all the
recoverable data in all members of the file can be extracted with the
following command (the resulting file may contain errors and some garbage
data may be produced at the end of each damaged member):

     lziprecover -cd -i file.lz > file

   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 kept unchanged.

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

   LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never
have been compressed. Decompressed is used to refer to data which have
undergone the process of decompression.


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

2 Invoking lziprecover
**********************

The format for running lziprecover is:

     lziprecover [OPTIONS] [FILES]

When decompressing or testing, a hyphen '-' used as a FILE argument means
standard input. It can be mixed with other FILES and is read just once, the
first time it appears in the command line. If no file names are specified,
lziprecover decompresses from standard input to standard output.

   lziprecover supports the following options: *Note Argument syntax:
(arg_parser)Argument syntax.

'-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. This version number should be included in all bug reports.

'-a'
'--trailing-error'
     Exit with error status 2 if any remaining input is detected after
     decompressing the last member. Such remaining input is usually trailing
     garbage that can be safely ignored. *Note concat-example::.

'-A'
'--alone-to-lz'
     Convert lzma-alone files to lzip format without recompressing, just
     adding a lzip header and trailer. The conversion minimizes the
     dictionary size of the resulting file (and therefore the amount of
     memory required to decompress it). Only streamed files with default
     LZMA properties can be converted; non-streamed lzma-alone files lack
     the "End Of Stream" marker required in lzip files.

     The name of the converted lzip file is derived from that of the
     original lzma-alone file as follows:

     filename.lzma   becomes   filename.lz
     filename.tlz    becomes   filename.tar.lz
     anyothername    becomes   anyothername.lz

'-c'
'--stdout'
     Write decompressed data to standard output; keep input files
     unchanged. This option (or '-o') is needed when reading from a named
     pipe (fifo) or from a device. Use it also to recover as much of the
     decompressed data as possible when decompressing a corrupt file. '-c'
     overrides '-o'. '-c' has no effect when merging, removing members,
     repairing, reproducing, splitting, testing or listing.

'-d'
'--decompress'
     Decompress the files specified. If a file does not exist, can't be
     opened, or the destination file already exists and '--force' has not
     been specified, lziprecover continues decompressing the rest of the
     files and exits with error status 1. If a file fails to decompress, or
     is a terminal, lziprecover exits immediately with error status 2
     without decompressing the rest of the files. A terminal is considered
     an uncompressed file, and therefore invalid.

'-D RANGE'
'--range-decompress=RANGE'
     Decompress only a range of bytes starting at decompressed byte position
     BEGIN and up to byte position END - 1. Byte positions start at 0. This
     option provides random access to the data in multimember files; it
     only decompresses the members containing the desired data. 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.

     Four formats of RANGE are recognized, 'BEGIN', 'BEGIN-END',
     'BEGIN,SIZE', and ',SIZE'. If only BEGIN is specified, END is taken as
     the end of the file. If only SIZE is specified, BEGIN is taken as the
     beginning of the file. The bytes produced are sent to standard output
     unless the option '--output' is used.

'-e'
'--reproduce'
     Try to recover a missing (zeroed) sector in FILE using a reference
     file and the same version of lzip that created 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 member containing the
     zeroed sector could be repaired, 2 otherwise. Note that
     'FILE_fixed.lz' may still contain errors in the members following the
     one repaired. *Note Reproducing one sector::, for a complete
     description of the reproduce mode.

'--lzip-level=DIGIT|a|m[LENGTH]'
     Try only the given compression level or match length limit when
     reproducing a zeroed sector. '--lzip-level=a' tries all the
     compression levels (0 to 9), while '--lzip-level=m' tries all the
     match length limits (5 to 273).

'--lzip-name=NAME'
     Set the name of the lzip executable used by '--reproduce'. If
     '--lzip-name' is not specified, 'lzip' is used.

'--reference-file=FILE'
     Set the reference file used by '--reproduce'. It must contain the
     uncompressed data corresponding to the missing compressed data of the
     zeroed sector, plus some context data before and after them.

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

'-i'
'--ignore-errors'
     Make '--decompress', '--test', and '--range-decompress' ignore format
     and data errors and continue decompressing the remaining members in
     the file; keep input files unchanged. For example, the commands
     'lziprecover -cd -i file.lz > file' or
     'lziprecover -D0 -i file.lz > file' decompress all the recoverable
     data in all members of 'file.lz' without having to split it first. The
     '-cd -i' method resyncs to the next member header after each error,
     and is immune to some format errors that make '-D0 -i' fail. The range
     decompressed may be smaller than the range requested, because of the
     errors. The exit status is set to 0 unless other errors are found (I/O
     errors, for example).

     Make '--list', '--dump', '--remove', and '--strip' ignore format
     errors. The sizes of the members with errors (specially the last) may
     be wrong.

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

'-l'
'--list'
     Print the uncompressed size, compressed size, and percentage saved of
     the files specified. Trailing data are ignored. The values produced
     are correct even for multimember files. If more than one file is
     given, a final line containing the cumulative sizes is printed. With
     '-v', the dictionary size, the number of members in the file, and the
     amount of trailing data (if any) are also printed. With '-vv', the
     positions and sizes of each member in multimember files are also
     printed. With '-i', format errors are ignored, and with '-ivv', gaps
     between members are shown. The member numbers shown coincide with the
     file numbers produced by '--split'.

     If any file is damaged, does not exist, can't be opened, or is not
     regular, the final exit status will be > 0. '-lq' can be used to verify
     quickly (without decompressing) the structural integrity of the files
     specified. (Use '--test' to verify the data integrity). '-alq'
     additionally verifies that none of the files specified contain
     trailing data.

'-m'
'--merge'
     Try to produce a correct file by merging the good parts of two or more
     damaged copies. If successful, a repaired copy is written to the file
     'FILE_fixed.lz'. The exit status is 0 if a correct file could be
     produced, 2 otherwise. *Note Merging files::, for a complete
     description of the merge mode.

'-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, or converting lzma-alone files, and '-c' has not been
     also specified, write the decompressed or converted output to FILE;
     keep input files unchanged. This option (or '-c') is needed when
     reading from a named pipe (fifo) or from a device. '-o -' is
     equivalent to '-c'. '-o' has no effect when testing or listing.

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

'-R'
'--repair'
     Try to repair a FILE with small errors (up to one single-byte error
     per member). 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. *Note Repairing one byte::,
     for a complete description of the repair mode.

'-s'
'--split'
     Search for members in FILE and write each member in its own file. Gaps
     between members are detected and each gap is saved in its own file.
     Trailing data (if any) are saved alone in the last 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. Gaps may contain garbage or may be
     members with corrupt headers or trailers. If other lziprecover
     functions fail to work on a multimember FILE because of damage in
     headers or trailers, try to split FILE and then work on each member
     individually.

     The names of the files produced are in the form 'rec01FILE',
     'rec02FILE', etc, and are designed so that the use of wildcards in
     subsequent processing, for example,
     'lziprecover -cd rec*FILE > 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 files specified, 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 files. If a file
     fails the test, does not exist, can't be opened, or is a terminal,
     lziprecover continues checking the rest of the files. A final
     diagnostic is shown at verbosity level 1 or higher if any file fails
     the test when testing multiple files.

'-v'
'--verbose'
     Verbose mode.
     When decompressing or testing, further -v's (up to 4) increase the
     verbosity level, showing status, compression ratio, dictionary size,
     trailer contents (CRC, data size, member size), and up to 6 bytes of
     trailing data (if any) both in hexadecimal and as a string of printable
     ASCII characters.
     Two or more '-v' options show the progress of decompression.
     In other modes, increasing verbosity levels show final status, progress
     of operations, and extra information (for example, the failed areas).

'--loose-trailing'
     When decompressing, testing, or listing, allow trailing data whose
     first bytes are so similar to the magic bytes of a lzip header that
     they can be confused with a corrupt header. Use this option if a file
     triggers a "corrupt header" error and the cause is not indeed a
     corrupt header.

'--dump=[MEMBER_LIST][:damaged][:tdata]'
     Dump the members listed, the damaged members (if any), or the trailing
     data (if any) of one or more regular multimember files to standard
     output, or to a file if the option '--output' is used. If more than
     one file is given, the elements dumped from all files are concatenated.
     If a file does not exist, can't be opened, or is not regular,
     lziprecover continues processing the rest of the files. If the dump
     fails in one file, lziprecover exits immediately without processing the
     rest of the files. Only '--dump=tdata' can write to a terminal.

     The argument to '--dump' is a colon-separated list of the following
     element specifiers; a member list (1,3-6), a reverse member list
     (r1,3-6), and the strings "damaged" and "tdata" (which may be shortened
     to 'd' and 't' respectively). A member list selects the members (or
     gaps) listed, whose numbers coincide with those shown by '--list'. A
     reverse member list selects the members listed counting from the last
     member in the file (r1). Negated versions of both kinds of lists exist
     (^1,3-6:r^1,3-6) which selects all the members except those in the
     list. The strings "damaged" and "tdata" select the damaged members and
     the trailing data respectively. If the same member is selected more
     than once, for example by '1:r1' in a single-member file, it is dumped
     just once. See the following examples:

     '--dump' argument      Elements dumped
     ---------------------------------------------------------------------
     '1,3-6'                members 1, 3, 4, 5, 6
     'r1-3'                 last 3 members in file
     '^13,15'               all but 13th and 15th members in file
     'r^1'                  all but last member in file
     'damaged'              all damaged members in file
     'tdata'                trailing data
     '1-5:r1:tdata'         members 1 to 5, last member, trailing data
     'damaged:tdata'        damaged members, trailing data
     '3,12:damaged:tdata'   members 3, 12, damaged members, trailing data

'--remove=[MEMBER_LIST][:damaged][:tdata]'
     Remove the members listed, the damaged members (if any), or the
     trailing data (if any) from regular multimember files in place. The
     date of each file is preserved if possible. If all members in a file
     are selected to be removed, the file is left unchanged and the exit
     status is set to 2. If a file does not exist, can't be opened, is not
     regular, or is left unchanged, lziprecover continues processing the
     rest of the files. In case of I/O error, lziprecover exits immediately
     without processing the rest of the files. See '--dump' above for a
     description of the argument.

     This option may be dangerous even if only the trailing data is being
     removed because the file may be corrupt or the trailing data may
     contain a forbidden combination of characters. *Note Trailing data::.
     It is advisable to make a backup before attempting the removal. At
     least verify that 'lzip -cd file.lz | wc -c' and the uncompressed size
     shown by 'lzip -l file.lz' match before attempting the removal of
     trailing data.

'--strip=[MEMBER_LIST][:damaged][:tdata]'
     Copy one or more regular multimember files to standard output (or to a
     file if the option '--output' is used), stripping the members listed,
     the damaged members (if any), or the trailing data (if any) from each
     file. If all members in a file are selected to be stripped, the
     trailing data (if any) are also stripped even if 'tdata' is not
     specified. If more than one file is given, the files are concatenated.
     In this case the trailing data are also stripped from all but the last
     file even if 'tdata' is not specified. If a file does not exist, can't
     be opened, or is not regular, lziprecover continues processing the
     rest of the files. If a file fails to copy, lziprecover exits
     immediately without processing the rest of the files. See '--dump'
     above for a description of the argument.


   Lziprecover also supports the following debug options (for experts):

'-E RANGE[,SECTOR_SIZE]'
'--debug-reproduce=RANGE[,SECTOR_SIZE]'
     Load the compressed FILE into memory, set all bytes in the positions
     specified by RANGE to 0, and try to reproduce a correct compressed
     file. *Note --reproduce::. *Note range-format::, for a description of
     RANGE. If a SECTOR_SIZE is specified, set each sector to 0 in sequence
     and try to reproduce the file, printing to standard output final
     statistics of the number of sectors reproduced successfully. Exit with
     nonzero status only in case of fatal error.

'-M'
'--md5sum'
     Print to standard output the MD5 digests of the input FILES one per
     line in the same format produced by the 'md5sum' tool. Lziprecover
     uses MD5 digests to verify the result of some operations. This option
     allows the verification of lziprecover's implementation of the MD5
     algorithm.

'-S[VALUE]'
'--nrep-stats[=VALUE]'
     Compare the frequency of sequences of N repeated bytes of a given
     VALUE in the compressed LZMA streams of the input FILES with the
     frequency expected for random data (1 / 2^(8N)). If VALUE is not
     specified, print the frequency of repeated sequences of all possible
     byte values. Print cumulative data for all files followed by the name
     of the first file with the longest sequence.

'-U 1|BSIZE'
'--unzcrash=1|BSIZE'
     With argument '1', test 1-bit errors in the LZMA stream of the
     compressed input FILE like the command
     'unzcrash -b1 -p7 -s-20 'lzip -t' FILE' but in memory, and therefore
     much faster. *Note Unzcrash::. This option tests all the members
     independently in a multimember file, skipping headers and trailers. If
     a decompression succeeds, the decompressed output is compared with the
     decompressed output of the original FILE using MD5 digests. FILE must
     not contain errors and must decompress correctly for the comparisons to
     work.

     With argument 'B', test zeroed sectors (blocks of bytes) in the LZMA
     stream of the compressed input FILE like the command
     'unzcrash --block=SIZE -d1 -p7 -s-(SIZE+20) 'lzip -t' FILE' but in
     memory, and therefore much faster. Testing and comparisons work just
     like with the argument '1' explained above.

     By default '--unzcrash' only prints the interesting cases; CRC
     mismatches, size mismatches, unsupported marker codes, unexpected EOFs,
     apparently successful decompressions, and decoder errors detected
     50_000 or more bytes beyond the byte (or the start of the block) being
     tested. At verbosity level 1 (-v) it also prints decoder errors
     detected 10_000 or more bytes beyond the byte being tested. At
     verbosity level 2 (-vv) it prints all cases for 1-bit errors or the
     decoder errors detected beyond the end of the block for zeroed blocks.

'-W POSITION,VALUE'
'--debug-decompress=POSITION,VALUE'
     Load the compressed FILE into memory, set the byte at POSITION to
     VALUE, and decompress the modified compressed data to standard output.
     If the damaged member is decompressed fully (just fails with a CRC
     mismatch), the members following it are also decompressed.

'-X[POSITION,VALUE]'
'--show-packets[=POSITION,VALUE]'
     Load the compressed FILE into memory, optionally set the byte at
     POSITION to VALUE, decompress the modified compressed data (discarding
     the output), and print to standard output descriptions of the LZMA
     packets being decoded.

'-Y RANGE'
'--debug-delay=RANGE'
     Load the compressed FILE into memory and then repeatedly decompress
     it, increasing 256 times each byte of the subset of the compressed data
     positions specified by RANGE, so as to test all possible one-byte
     errors. For each decompression error find the error detection delay and
     print to standard output the maximum delay. The error detection delay
     is the difference between the position of the error and the position
     where the decoder realized that the data contains an error. *Note
     range-format::, for a description of RANGE.

'-Z POSITION,VALUE'
'--debug-repair=POSITION,VALUE'
     Load the compressed FILE into memory, set the byte at POSITION to
     VALUE, and then try to repair the error. *Note --repair::.


   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 (e.g., bug) which caused
lziprecover to panic.


File: lziprecover.info,  Node: Data safety,  Next: Repairing one byte,  Prev: Invoking lziprecover,  Up: Top

3 Protecting data from accidental loss
**************************************

It is a fact of life that sometimes data will become corrupt. Software has
errors. Hardware may misbehave or fail. RAM may be struck by a cosmic ray.
This is why a safe enough integrity checking is needed in compressed
formats, and the reason why a data recovery tool is sometimes needed.

   There are 3 main types of data corruption that may cause data loss:
single-byte errors, multibyte errors (generally affecting a whole sector in
a block device), and total device failure.

   Lziprecover protects natively against single-byte errors as long as file
integrity is checked frequently enough that a second single-byte error does
not develop in the same member before the first one is repaired. *Note
Repairing one byte::.

   Lziprecover also protects against multibyte errors if at least one backup
copy of the file is made (*note Merging files::), or if the error is a
zeroed sector and the uncompressed data corresponding to the zeroed sector
are available (*note Reproducing one sector::). If you can choose between
merging and reproducing, try merging first because it is usually faster,
easier to use, and has a high probability of success.

   Lziprecover can't help in case of device failure. The only remedy for
total device failure is storing backup copies in separate media.

   The extraordinary safety of the lzip format allows lziprecover to exploit
the redundance that occurrs naturally when making compressed backups.
Lziprecover can recover data that would not be recoverable from files
compressed in other formats. Let's see two examples of how much better is
lzip compared with gzip and bzip2 with respect to data safety:

* Menu:

* Merging with a backup::   Recovering a file using a damaged backup
* Reproducing a mailbox::   Recovering new messages using an old backup


File: lziprecover.info,  Node: Merging with a backup,  Next: Reproducing a mailbox,  Up: Data safety

3.1 Recovering a file using a damaged backup
============================================

Let's suppose that you made a compressed backup of your valuable scientific
data and stored two copies on separate media. Years later you notice that
both copies are corrupt.

   If you compressed the data with gzip and both copies suffer any damage in
the data stream, even if it is just one altered bit, the original data can
only be recovered by an expert, if at all.

   If you used bzip2, and if the file is large enough to contain more than
one compressed data block (usually larger than 900 kB uncompressed), and if
no block is damaged in both files, then the data can be manually recovered
by splitting the files with bzip2recover, verifying every block, and then
copying the right blocks in the right order into another file.

   But if you used lzip, the data can be automatically recovered with
'lziprecover --merge' as long as the damaged areas don't overlap.

   Note that each error in a bzip2 file makes a whole block unusable, but
each error in a lzip file only affects the damaged bytes, making it
possible to recover a file with thousands of errors.


File: lziprecover.info,  Node: Reproducing a mailbox,  Prev: Merging with a backup,  Up: Data safety

3.2 Recovering new messages using an old backup
===============================================

Let's suppose that you make periodic backups of your email messages stored
in one or more mailboxes. (A mailbox is a file containing a possibly large
number of email messages). New messages are appended to the end of each
mailbox, therefore the initial part of two consecutive backups is identical
unless some messages have been changed or deleted in the meantime. The new
messages added to each backup are usually a small part of the whole mailbox.

+========================================================+
|  Older backup containing some messages                 |
+========================================================+
+========================================================+================+
|  Newer backup containing the messages above plus some  |  new messages  |
+========================================================+================+

   One day you discover that your mailbox has disappeared because you
deleted it inadvertently or because of a bug in your email reader. Not only
that. You need to recover a recent message, but the last backup you made of
the mailbox (the newer backup above) has lost the data corresponding to a
whole sector because of an I/O error in the part containing the old
messages.

   If you compressed the mailbox with gzip, usually none of the new messages
can be recovered even if they are intact because all the data beyond the
missing sector can't be decoded.

   If you used bzip2, and if the newer backup is large enough that the new
messages are in a different compressed data block than the one damaged
(usually larger than 900 kB uncompressed), then you can recover the new
messages manually with bzip2recover. If the backups are identical except for
the new messages appended, you may even recover the whole newer backup by
combining the good blocks from both backups.

   But if you used lzip, the whole newer backup can be automatically
recovered with 'lziprecover --reproduce' as long as the missing bytes can be
recovered from the older backup, even if other messages in the common part
have been changed or deleted. Mailboxes seem to be specially easy to
reproduce. The probability of reproducing a mailbox (*note
performance-of-reproduce::) is almost as high as that of merging two
identical backups (*note performance-of-merge::).


File: lziprecover.info,  Node: Repairing one byte,  Next: Merging files,  Prev: Data safety,  Up: Top

4 Repairing one byte
********************

Lziprecover can repair perfectly most files with small errors (up to one
single-byte error per member), without the need of any extra redundance at
all. If the reparation is successful, the repaired file will be identical
bit for bit to the original. This makes lzip files resistant to bit flip,
one of the most common forms of data corruption.

   The file is repaired in memory. Therefore, enough virtual memory
(RAM + swap) to contain the largest damaged member is required.

   The error may be located anywhere in the file except in the first 5
bytes of each member header or in the 'Member size' field of the trailer
(last 8 bytes of each member). If the error is in the header it can be
easily repaired with a text editor like GNU Moe (*note File format::). If
the error is in the member size, it is enough to ignore the message about
'bad member size' when decompressing.

   Bit flip happens when one bit in the file is changed from 0 to 1 or vice
versa. It may be caused by bad RAM or even by natural radiation. I have
seen a case of bit flip in a file stored on an USB flash drive.

   One byte may seem small, but most file corruptions not produced by
transmission errors or I/O errors just affect one byte, or even one bit, of
the file. Also, unlike magnetic media, where errors usually affect a whole
sector, solid-state storage devices tend to produce single-byte errors,
making of lzip the perfect format for data stored on such devices.

   Repairing a file can take some time. Small files or files with the error
located near the beginning can be repaired in a few seconds. But repairing
a large file compressed with a large dictionary size and with the error
located far from the beginning, may take hours.

   On the other hand, errors located near the beginning of the file cause
much more loss of data than errors located near the end. So lziprecover
repairs more efficiently the worst errors.


File: lziprecover.info,  Node: Merging files,  Next: Reproducing one sector,  Prev: Repairing one byte,  Up: Top

5 Merging files
***************

If you have several copies of a file but all of them are too damaged to
repair them (*note Repairing one byte::), lziprecover can try to produce a
correct file by merging the good parts of the damaged copies.

   The merge may succeed even if some copies of the file have all the
headers and trailers damaged, as long as there is at least one copy of
every header and trailer intact, even if they are in different copies of
the file.

   The merge will fail if the damaged areas overlap (at least one byte is
damaged in all copies), or are adjacent and the boundary can't be
determined, or if the copies have too many damaged areas.

   All the copies to be merged must have the same size. If any of them is
larger or smaller than it should, either because it has been truncated or
because it got some garbage data appended at the end, it can be brought to
the correct size with the following command before merging it with the
other copies:

     ddrescue -s<correct_size> -x<correct_size> file.lz correct_size_file.lz

   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 (a few
MB) with small errors (one sector damaged per copy), the probability
approaches 100 percent even with only two copies. (Supposing that the
errors are randomly located inside each copy).

   Some types of solid-state device (NAND flash, for example) can produce
bursts of scattered single-bit errors. Lziprecover is able to merge files
with thousands of such scattered errors by grouping the errors into
clusters and then merging the files as if each cluster were a single error.

   Here is a real case of successful merging. Two copies of the file
'icecat-3.5.3-x86.tar.lz' (compressed size 9 MB) became corrupt while
stored on the same NAND flash device. One of the copies had 76 single-bit
errors scattered in an area of 1020 bytes, and the other had 3028 such
errors in an area of 31729 bytes. Lziprecover produced a correct file,
identical to the original, in just 5 seconds:

     lziprecover -vvm a/icecat-3.5.3-x86.tar.lz b/icecat-3.5.3-x86.tar.lz
     Merging member 1 of 1  (2552 errors)
       2552 errors have been grouped in 16 clusters.
       Trying variation 2 of 2, block 2
     Input files merged successfully.

   Note that the number of errors reported by lziprecover (2552) is lower
than the number of corrupt bytes (3104) because contiguous corrupt bytes
are counted as a single multibyte error.


Example 1: 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 -d -r1 -b2048 /dev/cdrom cdimage1 mapfile1
     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 -d -r1 -b2048 /dev/cdrom cdimage2 mapfile2
     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 backup.tar.lz rescued1.tar.lz rescued2.tar.lz
       Input files merged successfully.
     lziprecover -tv backup.tar.lz
       backup.tar.lz: ok


Example 2: 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. The
correct file produced is saved in 'big_db_00001.lz'.

     lziprecover -m -v -o big_db_00001.lz big_db1_00001.lz big_db2_00001.lz
       Input files merged successfully.
     lziprecover -tv big_db_00001.lz
       big_db_00001.lz: ok


File: lziprecover.info,  Node: Reproducing one sector,  Next: Tarlz,  Prev: Merging files,  Up: Top

6 Reproducing one sector
************************

Lziprecover can recover a zeroed sector in a lzip file by concatenating the
decompressed contents of the file up to the beginning of the zeroed sector
and the uncompressed data corresponding to the zeroed sector, and then
feeding the concatenated data to the same version of lzip that created the
file. For this to work, a reference file is required containing the
uncompressed data corresponding to the missing compressed data of the zeroed
sector, plus some context data before and after them. It is possible to
recover a large file using just a few KB of reference data.

   The difficult part is finding a suitable reference file. It must contain
the exact data required (possibly mixed with other data). Containing similar
data is not enough.

   A zeroed sector may be caused by the incomplete recovery of a damaged
storage device (with I/O errors) using, for example, ddrescue. The
reproduction can't be done if the zeroed sector overlaps with the first 15
bytes of a member, or if the zeroed sector is smaller than 8 bytes.

   The file is reproduced in memory. Therefore, enough virtual memory
(RAM + swap) to contain the damaged member is required.

   To understand how it works, take any lzipped file, say 'foo.lz',
decompress it (keeping the original), and try to reproduce an artificially
zeroed sector in it by running the following commands:

     lzip -kd foo.lz
     lziprecover -vv --debug-reproduce=65536,512 --reference-file=foo foo.lz

which should produce an output like the following:

     Reproducing:    foo.lz
     Reference file: foo
     Testing sectors of size 512 at file positions 65536 to 66047
       (master mpos = 65536, dpos = 296892)
     foo: Match found at offset 296892
     Reproduction succeeded at pos 65536

            1 sectors tested
            1 reproductions returned with zero status
              all comparisons passed

   Using 'foo' as reference file guarantees that any zeroed sector in
'foo.lz' can be reproduced because both files contain the same data. In
real use, the reference file needs to contain the data corresponding to the
zeroed sector, but the rest of the data (if any) may differ between both
files. The reference data may be obtained from the partial decompression of
the damaged file itself if it contains repeated data. For example if the
damaged file is a compressed tarball containing several partially modified
versions of the same file.

   The offset reported by lziprecover is the position in the reference file
of the first byte that could not be decompressed. This is the first byte
that will be compressed to reproduce the zeroed sector.

   The reproduce mode tries to reproduce the missing compressed data
originally present in the zeroed sector. It is based on the perfect
reproducibility of lzip files (lzip produces identical compressed output
from identical input). Therefore, the same version of lzip that created the
file to be reproduced should be used to reproduce the zeroed sector. Near
versions may also work because the output of lzip changes infrequently. If
reproducing a tar.lz archive created with tarlz, the version of lzip,
clzip, or minilzip corresponding to the version of the lzlib library used
by tarlz to create the archive should be used.

   When recovering a tar.lz archive and using as reference a file from the
filesystem, if the zeroed sector encodes (part of) a tar header, the archive
can't be reproduced. Therefore, the less overhead (smaller headers) a tar
archive has, the more probable is that the zeroed sector does not include a
header, and that the archive can be reproduced. The tarlz format has minimum
overhead. It uses basic ustar headers, and only adds extended pax headers
when they are required.

6.1 Performance of '--reproduce'
================================

Reproduce mode is specially useful when recovering a corrupt backup (or a
corrupt source tarball) that is part of a series. Usually only a small
fraction of the data changes from one backup to the next or from one version
of a source tarball to the next. This makes sometimes possible to reproduce
a given corrupted version using reference data from a near version. The
following two tables show the fraction of reproducible sectors (reproducible
sectors divided by total sectors in archive) for some archives, using sector
sizes of 512 and 4096 bytes. 'mailbox-aug.tar.lz' is a backup of some of my
mailboxes. 'backup-feb.tar.lz' and 'backup-apr.tar.lz' are real backups of
my own working directory:

Reference file   File                Reproducible (512)
---------------------------------------------------------
backup-feb.tar   backup-apr.tar.lz   3273 / 4342 = 75.38%
backup-apr.tar   backup-feb.tar.lz   3259 / 4161 = 78.32%
gawk-5.0.0.tar   gawk-5.0.1.tar.lz   4369 / 5844 = 74.76%
gawk-5.0.1.tar   gawk-5.0.0.tar.lz   4379 / 5603 = 78.15%
gmp-6.1.1.tar    gmp-6.1.2.tar.lz    2454 / 3787 = 64.8%
gmp-6.1.2.tar    gmp-6.1.1.tar.lz    2461 / 3782 = 65.07%

Reference file    File                 Reproducible (4096)
-----------------------------------------------------------
mailbox-mar.tar   mailbox-aug.tar.lz   4036 / 4252 = 94.92%
backup-feb.tar    backup-apr.tar.lz    264 / 542 = 48.71%
backup-apr.tar    backup-feb.tar.lz    264 / 520 = 50.77%
gawk-5.0.0.tar    gawk-5.0.1.tar.lz    327 / 730 = 44.79%
gawk-5.0.1.tar    gawk-5.0.0.tar.lz    326 / 700 = 46.57%
gmp-6.1.1.tar     gmp-6.1.2.tar.lz     175 / 473 = 37%
gmp-6.1.2.tar     gmp-6.1.1.tar.lz     181 / 472 = 38.35%

   Note that the "performance of reproduce" is a probability, not a partial
recovery. The data is either recovered fully (with the probability X shown
in the last column of the tables above) or not recovered at all (with
probability 1 - X).

   Example 1: Recover a damaged source tarball with a zeroed sector of 512
bytes at file position 1019904, using as reference another source tarball
for a different version of the software.

     lziprecover -vv -e --reference-file=gmp-6.1.1.tar gmp-6.1.2.tar.lz
     Reproducing bad area in member 1 of 1
       (begin = 1019904, size = 512, value = 0x00)
       (master mpos = 1019904, dpos = 6292134)
     warning: gmp-6.1.1.tar: Partial match found at offset 6277798, len 8716.
     Reference data may be mixed with other data.
     Trying level -9
       Reproducing position 1015808
     Member reproduced successfully.
     Copy of input file reproduced successfully.


Example 2: Recover a damaged backup with a zeroed sector of 4096 bytes at
file position 1019904, using as reference a previous backup. The damaged
backup comes from a damaged partition copied with ddrescue.

     ddrescue -b4096 -r10 /dev/sdc1 hdimage mapfile
     mount -o loop,ro hdimage /mnt/hdimage
     cp /mnt/hdimage/backup.tar.lz backup.tar.lz
     umount /mnt/hdimage
     lzip -t backup.tar.lz
       backup.tar.lz: Decoder error at pos 1020530
     lziprecover -vv -e --reference-file=old_backup.tar backup.tar.lz
     Reproducing bad area in member 1 of 1
       (begin = 1019904, size = 4096, value = 0x00)
       (master mpos = 1019903, dpos = 5857954)
     warning: old_backup.tar: Partial match found at offset 5743778, len 9546.
     Reference data may be mixed with other data.
     Trying level -9
       Reproducing position 1015808
     Member reproduced successfully.
     Copy of input file reproduced successfully.


Example 3: Recover a damaged backup with a zeroed sector of 4096 bytes at
file position 1019904, using as reference a file from the filesystem. (If
the zeroed sector encodes (part of) a tar header, the tarball can't be
reproduced).

     # List the contents of the backup tarball to locate the damaged member.
     tarlz -n0 -tvf backup.tar.lz
       [...]
       example.txt
     tarlz: Skipping to next header.
     tarlz: backup.tar.lz: Archive ends unexpectedly.
     # Find in the filesystem the last file listed and use it as reference.
     lziprecover -vv -e --reference-file=/somedir/example.txt backup.tar.lz
     Reproducing bad area in member 1 of 1
       (begin = 1019904, size = 4096, value = 0x00)
       (master mpos = 1019903, dpos = 5857954)
     /somedir/example.txt: Match found at offset 9378
     Trying level -9
       Reproducing position 1015808
     Member reproduced successfully.
     Copy of input file reproduced successfully.

   If 'backup.tar.lz' is a multimember file with more than one member
damaged and lziprecover shows the message 'One member reproduced. Copy of
input file still contains errors.', the procedure shown in the example
above can be repeated until all the members have been reproduced.

   'tarlz --keep-damaged -n0 -xf backup.tar.lz example.txt' produces a
partial copy of the reference file 'example.txt' that may help locate a
complete copy in the filesystem or in another backup, even if 'example.txt'
has been renamed.


File: lziprecover.info,  Node: Tarlz,  Next: File names,  Prev: Reproducing one sector,  Up: Top

7 Options supporting the tar.lz format
**************************************

Tarlz is a massively parallel (multi-threaded) combined implementation of
the tar archiver and the lzip compressor.

   Tarlz creates tar archives using a simplified and safer variant of the
POSIX pax format compressed in lzip format, keeping the alignment between
tar members and lzip members. The resulting multimember tar.lz archive is
fully backward compatible with standard tar tools like GNU tar, which treat
it like any other tar.lz archive. *Note tarlz manual: (tarlz)Top, and *note
lzip manual: (lzip)Top.

   Multimember tar.lz archives have some safety advantages over solidly
compressed tar.lz archives. For example, in case of corruption, tarlz can
extract all the undamaged members from the tar.lz archive, skipping over the
damaged members, just like the standard (uncompressed) tar. Keeping the
alignment between tar members and lzip members minimizes the amount of data
lost in case of corruption. In this chapter we'll explain the ways in which
lziprecover can recover and process multimember tar.lz archives.


7.1 Recovering damaged multimember tar.lz archives
==================================================

If you have several copies of the damaged archive, try merging them first
because merging has a high probability of success. *Note Merging files::. If
the command below prints something like 'Input files merged successfully.'
you are done and 'archive.tar.lz' now contains the recovered archive:

     lziprecover -m -v -o archive.tar.lz a/archive.tar.lz b/archive.tar.lz

   If you only have one copy of the damaged archive with a zeroed block of
data caused by an I/O error, you may try to reproduce the archive. *Note
Reproducing one sector::. If the command below prints something like
'Copy of input file reproduced successfully.' you are done and
'archive_fixed.tar.lz' now contains the recovered archive:

     lziprecover -vv -e --reference-file=old_archive.tar archive.tar.lz

   If you only have one copy of the damaged archive, you may try to repair
the archive, but this has a lower probability of success. *Note Repairing
one byte::. If the command below prints something like
'Copy of input file repaired successfully.' you are done and
'archive_fixed.tar.lz' now contains the recovered archive:

     lziprecover -v -R archive.tar.lz

   If all the above fails, and the archive was created with tarlz, you may
save the damaged members for later and then copy the good members to another
archive. If the two commands below succeed, 'bad_members.tar.lz' will
contain all the damaged members and 'archive_cleaned.tar.lz' will contain a
good archive with the damaged members removed:

     lziprecover -v --dump=damaged -o bad_members.tar.lz archive.tar.lz
     lziprecover -v --strip=damaged -o archive_cleaned.tar.lz archive.tar.lz

   You can then use 'tarlz --keep-damaged' to recover as much data as
possible from each damaged member in 'bad_members.tar.lz':

     mkdir tmp
     cd tmp
     tarlz --keep-damaged -xvf ../bad_members.tar.lz


7.2 Processing multimember tar.lz archives
==========================================

Lziprecover is able to copy a list of members from a file to another. For
example the command
'lziprecover --dump=1-10:r1:tdata archive.tar.lz > subarch.tar.lz' creates
a subset archive containing the first ten members, the end-of-file blocks,
and the trailing data (if any) of 'archive.tar.lz'. The 'r1' part selects
the last member, which in an appendable tar.lz archive contains the
end-of-file blocks.


File: lziprecover.info,  Node: File names,  Next: File format,  Prev: Tarlz,  Up: Top

8 Names of the files produced by lziprecover
********************************************

The name of the fixed file produced by '--merge' and '--repair' is made by
appending the string '_fixed.lz' to the original file name. If the original
file name ends with one of the extensions '.tar.lz', '.lz', or '.tlz', the
string '_fixed' is inserted before the extension.


File: lziprecover.info,  Node: File format,  Next: Trailing data,  Prev: File names,  Up: Top

9 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 independent "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 can
encode in compressed form up to 16 EiB - 1 byte of uncompressed data. The
size of a multimember file is unlimited.

   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 (the "magic" bytes)'
     A four byte string, identifying the lzip format, with the value "LZIP"
     (0x4C, 0x5A, 0x49, 0x50).

'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)'
     The dictionary size is calculated by taking a power of 2 (the base
     size) and subtracting from it a fraction between 0/16 and 7/16 of the
     base size.
     Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).
     Bits 7-5 contain the numerator of the fraction (0 to 7) to subtract
     from the base size to obtain the dictionary size.
     Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB
     Valid values for dictionary size range from 4 KiB to 512 MiB.

'LZMA stream'
     The LZMA stream, finished by an "End Of Stream" marker. Uses default
     values for encoder properties. *Note Stream format: (lzip)Stream
     format, for a complete description.

'CRC32 (4 bytes)'
     Cyclic Redundancy Check (CRC) of the original uncompressed data.

'Data size (8 bytes)'
     Size of the original uncompressed 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 the safe recovery of undamaged members from
     multimember files. Member size should be limited to 2 PiB to prevent
     the data size field from overflowing.



File: lziprecover.info,  Node: Trailing data,  Next: Examples,  Prev: File format,  Up: Top

10 Extra data appended to the file
**********************************

Sometimes extra data are found appended to a lzip file after the last
member. Such trailing data may be:

   * Padding added to make the file size a multiple of some block size, for
     example when writing to a tape. It is safe to append any amount of
     padding zero bytes to a lzip file.

   * Useful data added by the user; a cryptographically secure hash, a
     description of file contents, etc. It is safe to append any amount of
     text to a lzip file as long as none of the first four bytes of the text
     match the corresponding byte in the string "LZIP", and the text does
     not contain any zero bytes (null characters). Nonzero bytes and zero
     bytes can't be safely mixed in trailing data.

   * Garbage added by some not totally successful copy operation.

   * Malicious data added to the file in order to make its total size and
     hash value (for a chosen hash) coincide with those of another file.

   * In rare cases, trailing data could be the corrupt header of another
     member. In multimember or concatenated files the probability of
     corruption happening in the magic bytes is 5 times smaller than the
     probability of getting a false positive caused by the corruption of the
     integrity information itself. Therefore it can be considered to be
     below the noise level. Additionally, the test used by lziprecover to
     discriminate trailing data from a corrupt header has a Hamming
     distance (HD) of 3, and the 3 bit flips must happen in different magic
     bytes for the test to fail. In any case, the option '--trailing-error'
     guarantees that any corrupt header will be detected.

   Trailing data are in no way part of the lzip file format, but tools
reading lzip files are expected to behave as correctly and usefully as
possible in the presence of trailing data.

   Trailing data can be safely ignored in most cases. In some cases, like
that of user-added data, they are expected to be ignored. In those cases
where a file containing trailing data must be rejected, the option
'--trailing-error' can be used. *Note --trailing-error::.

   Lziprecover facilitates the management of metadata stored as trailing
data in lzip files. See the following examples:

Example 1: Add a comment or description to a compressed file.

     # First append the comment as trailing data to a lzip file
     echo 'This file contains this and that' >> file.lz
     # This command prints the comment to standard output
     lziprecover --dump=tdata file.lz
     # This command outputs file.lz without the comment
     lziprecover --strip=tdata file.lz > stripped_file.lz
     # This command removes the comment from file.lz
     lziprecover --remove=tdata file.lz


Example 2: Add and verify a cryptographically secure hash. (This may be
convenient, but a separate copy of the hash must be kept in a safe place to
guarantee that both file and hash have not been maliciously replaced).

     sha256sum < file.lz >> file.lz
     lziprecover --strip=tdata file.lz | sha256sum -c \
       <(lziprecover --dump=tdata file.lz)


File: lziprecover.info,  Node: Examples,  Next: Unzcrash,  Prev: Trailing data,  Up: Top

11 A small tutorial with examples
*********************************

Example 1: Extract all the files from archive 'foo.tar.lz'.

       tar -xf foo.tar.lz
     or
       lziprecover -cd foo.tar.lz | tar -xf -


Example 2: Restore a regular file from its compressed version 'file.lz'. If
the operation is successful, 'file.lz' is removed.

     lziprecover -d file.lz


Example 3: Verify the integrity of the compressed file 'file.lz' and show
status.

     lziprecover -tv file.lz


Example 4: The right way of concatenating the decompressed output of two or
more compressed files. *Note Trailing data::.

     Don't do this
       cat file1.lz file2.lz file3.lz | lziprecover -d -
     Do this instead
       lziprecover -cd file1.lz file2.lz file3.lz
     You may also concatenate the compressed files like this
       lziprecover --strip=tdata file1.lz file2.lz file3.lz > file123.lz
     Or keeping the trailing data of the last file like this
       lziprecover --strip=damaged file1.lz file2.lz file3.lz > file123.lz


Example 5: Decompress 'file.lz' partially until 10 KiB of decompressed data
are produced.

     lziprecover -D 0,10KiB file.lz


Example 6: Decompress 'file.lz' partially from decompressed byte at offset
10000 to decompressed byte at offset 14999 (5000 bytes are produced).

     lziprecover -D 10000-15000 file.lz


Example 7: Repair small errors in the file 'file.lz'. (Indented lines are
abridged diagnostic messages from lziprecover).

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


Example 8: Split the multimember file 'file.lz' and write each member in
its own 'recXXXfile.lz' file. Then use 'lziprecover -t' to test the
integrity of the resulting files.

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


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

12 Testing the robustness of decompressors
******************************************

The lziprecover package also includes unzcrash, a program written to test
robustness to decompression of corrupted data, inspired by unzcrash.c from
Julian Seward's bzip2. Type 'make unzcrash' in the lziprecover source
directory to build it.

   By default, unzcrash reads the file specified and then repeatedly
decompresses it, increasing 256 times each byte of the compressed data, so
as to test all possible one-byte errors. Note that it may take years or even
centuries to test all possible one-byte errors in a large file (tens of MB).

   If the option '--block' is given, unzcrash reads the file specified and
then repeatedly decompresses it, setting all bytes in each successive block
to the value given, so as to test all possible full sector errors.

   If the option '--truncate' is given, unzcrash reads the file specified
and then repeatedly decompresses it, truncating the file to increasing
lengths, so as to test all possible truncation points.

   None of the three test modes described above should cause any invalid
memory accesses. If any of them does, please, report it as a bug to the
maintainers of the decompressor being tested.

   Unzcrash really executes as a subprocess the shell command specified in
the first non-option argument, and then writes the file specified in the
second non-option argument to the standard input of the subprocess,
modifying the corresponding byte each time. Therefore unzcrash can be used
to test any decompressor (not only lzip), or even other decoder programs
having a suitable command line syntax.

   If the decompressor returns with zero status, unzcrash compares the
output of the decompressor for the original and corrupt files. If the
outputs differ, it means that the decompressor returned a false negative;
it failed to recognize the corruption and produced garbage output. The only
exception is when a multimember file is truncated just after the last byte
of a member, producing a shorter but valid compressed file. Except in this
latter case, please, report any false negative as a bug.

   In order to compare the outputs, unzcrash needs a 'zcmp' program able to
understand the format being tested. For example the 'zcmp' provided by
zutils. If the 'zcmp' program used does not understand the format being
tested, all the comparisons will fail because the compressed files will be
compared without being decompressed first. Use '--zcmp=false' to disable
comparisons. *Note Zcmp: (zutils)Zcmp.

   The format for running unzcrash is:

     unzcrash [OPTIONS] 'lzip -t' FILE

The compressed FILE must not contain errors and the decompressor being
tested must decompress it correctly for the comparisons to work.

   unzcrash supports the following options:

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

'-V'
'--version'
     Print the version number of unzcrash on the standard output and exit.
     This version number should be included in all bug reports.

'-b RANGE'
'--bits=RANGE'
     Test N-bit errors only, instead of testing all the 255 wrong values for
     each byte. 'N-bit error' means any value differing from the original
     value in N bit positions, not a value differing from the original
     value in the bit position N.
     The number of N-bit errors per byte (N = 1 to 8) is:
     8 28 56 70 56 28 8 1

     Examples of RANGE   Tests errors of N-bits
     1                   1
     1,2,3               1, 2, 3
     2-4                 2, 3, 4
     1,3-5,8             1, 3, 4, 5, 8
     1-3,5-8             1, 2, 3, 5, 6, 7, 8

'-B[SIZE][,VALUE]'
'--block[=SIZE][,VALUE]'
     Test block errors of given SIZE, simulating a whole sector I/O error.
     SIZE defaults to 512 bytes. VALUE defaults to 0. By default, only
     contiguous, non-overlapping blocks are tested, but this may be changed
     with the option '--delta'.

'-d N'
'--delta=N'
     Test one byte, block, or truncation size every N bytes. If '--delta'
     is not specified, unzcrash tests all the bytes, non-overlapping
     blocks, or truncation sizes. Values of N smaller than the block size
     will result in overlapping blocks. (Which is convenient for testing
     because there are usually too few non-overlapping blocks in a file).

'-e POSITION,VALUE'
'--set-byte=POSITION,VALUE'
     Set byte at POSITION to VALUE in the internal buffer after reading and
     testing FILE but before the first test call to the decompressor. Byte
     positions start at 0. If VALUE is preceded by '+', it is added to the
     original value of the byte at POSITION. If VALUE is preceded by 'f'
     (flip), it is XORed with the original value of the byte at POSITION.
     This option can be used to run tests with a changed dictionary size,
     for example.

'-n'
'--no-verify'
     Skip initial verification of FILE and 'zcmp'. May speed up things a
     lot when testing many (or large) known good files.

'-p BYTES'
'--position=BYTES'
     First byte position to test in the file. Defaults to 0. Negative values
     are relative to the end of the file.

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

'-s BYTES'
'--size=BYTES'
     Number of byte positions to test. If not specified, the rest of the
     file is tested (from '--position' to end of file). Negative values are
     relative to the rest of the file.

'-t'
'--truncate'
     Test all possible truncation points in the range specified by
     '--position' and '--size'.

'-v'
'--verbose'
     Verbose mode.

'-z'
'--zcmp=<command>'
     Set zcmp command name and options. Defaults to 'zcmp'. Use
     '--zcmp=false' to disable comparisons. If testing a decompressor
     different from the one used by default by zcmp, it is needed to force
     unzcrash and zcmp to use the same decompressor with a command like
     'unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' FILE'


   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 (e.g., bug) which caused
unzcrash to panic.


File: lziprecover.info,  Node: Problems,  Next: Concept index,  Prev: Unzcrash,  Up: Top

13 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)
* data safety:                           Data safety.               (line 6)
* examples:                              Examples.                  (line 6)
* file format:                           File format.               (line 6)
* file names:                            File names.                (line 6)
* getting help:                          Problems.                  (line 6)
* introduction:                          Introduction.              (line 6)
* invoking:                              Invoking lziprecover.      (line 6)
* merging files:                         Merging files.             (line 6)
* merging with a backup:                 Merging with a backup.     (line 6)
* options:                               Invoking lziprecover.      (line 6)
* repairing one byte:                    Repairing one byte.        (line 6)
* reproducing a mailbox:                 Reproducing a mailbox.     (line 6)
* reproducing one sector:                Reproducing one sector.    (line 6)
* tarlz:                                 Tarlz.                     (line 6)
* trailing data:                         Trailing data.             (line 6)
* unzcrash:                              Unzcrash.                  (line 6)
* usage:                                 Invoking lziprecover.      (line 6)
* version:                               Invoking lziprecover.      (line 6)



Tag Table:
Node: Top226
Node: Introduction1406
Node: Invoking lziprecover5398
Ref: --trailing-error6265
Ref: range-format8644
Ref: --reproduce8979
Ref: --repair13278
Node: Data safety25584
Node: Merging with a backup27572
Node: Reproducing a mailbox28836
Node: Repairing one byte31337
Node: Merging files33402
Ref: performance-of-merge34572
Ref: ddrescue-example36181
Node: Reproducing one sector37468
Ref: performance-of-reproduce41351
Ref: ddrescue-example244026
Node: Tarlz46446
Node: File names50110
Node: File format50567
Node: Trailing data53258
Node: Examples56499
Ref: concat-example57075
Node: Unzcrash58467
Node: Problems64739
Node: Concept index65291

End Tag Table


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