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Lziprecover Manual

This manual is for Lziprecover (version 1.21-rc1, 23 November 2018).


Copyright © 2009-2018 Antonio Diaz Diaz.

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


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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, produce a correct file by merging the good parts of two or more damaged copies, 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:

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.

For compressible data, multiple lzip-compressed copies have a better chance of surviving intact than one uncompressed copy using the same amount of storage space.

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 damaged media, the combination GNU ddrescue + lziprecover is the best option for recovering data from multiple damaged copies. See ddrescue-example, for an example.

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 member):

     lziprecover -D0 -i -o file -q file.lz

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.


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2 Invoking lziprecover

The format for running lziprecover is:

     lziprecover [options] [files]

When decompressing or testing, '-' 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.

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. 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. See 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 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.
-d
--decompress
Decompress the specified files. If a file does not exist or can't be opened, lziprecover continues decompressing the rest of the files. If a file fails to decompress, or is a terminal, lziprecover exits immediately without decompressing the rest of the files.
-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 produced bytes are sent to standard output unless the '--output' option is used.

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

Make '--list', '--dump', '--remove' and '--strip' ignore format errors.

-k
--keep
Keep (don't delete) input files during decompression.
-l
--list
Print the uncompressed size, compressed size and percentage saved of the specified files. 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'.

'-lq' can be used to verify quickly (without decompressing) the structural integrity of the specified files. (Use '--test' to verify the data integrity). '-alq' additionally verifies that none of the specified files 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. See the chapter 'Merging files' (see 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 from standard input and '--stdout' has not been specified, use 'file' as the name of the decompressed file. If converting a lzma-alone file from standard input and '--stdout' has not been specified, use 'file.lz' as the name of the converted file. (Or plain 'file' if it already ends in '.lz' or '.tlz').
-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. See the chapter 'Repairing files' (see Repairing files) 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 specified files, 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 '--output' option 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.

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 and 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. See 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 '--output' option 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.

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

Table of SI and binary prefixes (unit multipliers):

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


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


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3 Protecting data from accidental loss

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 (see Repairing files), 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.

Lziprecover also protects against multibyte errors (see Merging files), if at least one backup copy of the file is made.

The only remedy for total device failure is storing backup copies in separate media.

How does lzip compare with gzip and bzip2 with respect to data safety? Let's suppose that you made a backup of your valuable scientific data, compressed it, and stored two copies on separate media. Years later you notice that both copies are corrupt.

If you compressed 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 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.


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4 Repairing files

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 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 (see 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, can 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.


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5 Merging files

If you have several copies of a file but all of them are too damaged to repair them (see Repairing files), 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.


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6 Options supporting the tar.lz format

Tarlz is an implementation of the tar archiver which by default creates archives compressed with lzip on a per file basis. Tarlz can append files to the end of such compressed archives because each tar member is compressed in its own lzip member, as well as the end-of-file blocks. Thus tarlz archives are multimember lzip files, which has 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. In this chapter we'll explain the ways in which lziprecover can recover and process multimember tar.lz archives.


6.1 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. 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, you may try to repair the archive, but this has a lower probability of success. 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, 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

6.2 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.


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7 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.


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8 File format

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


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

represents one byte; a box like this:

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

represents a variable number of bytes.


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

Each member has the following structure:

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

All multibyte values are stored in little endian order.

'ID string (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. See Stream format for a complete description.
'CRC32 (4 bytes)'
CRC of the uncompressed original data.
'Data size (8 bytes)'
Size of the uncompressed original data.
'Member size (8 bytes)'
Total size of the member, including header and trailer. This field acts as a distributed index, allows the verification of stream integrity, and facilitates safe recovery of undamaged members from multimember files.


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9 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:

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. See --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
     # 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)


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10 A small tutorial with examples

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

     lziprecover -d file.lz

Example 2: Verify the integrity of the compressed file 'file.lz' and show status.
     lziprecover -tv file.lz

Example 3: The right way of concatenating the decompressed output of two or more compressed files. See 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 4: Decompress 'file.lz' partially until 10 KiB of decompressed data are produced.
     lziprecover -D 0,10KiB file.lz

Example 5: Decompress 'file.lz' partially from decompressed byte 10000 to decompressed byte 15000 (5000 bytes are produced).
     lziprecover -D 10000-15000 file.lz

Example 6: 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 7: 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

Example 8: Recover a compressed backup from two copies on CD-ROM with error-checked merging of copies. (See the ddrescue manual 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 9: 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


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11 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 specified file 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 --block option is given, unzcrash reads the specified file 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 --truncate option is given, unzcrash reads the specified file 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 one provided by 'zutils'. See zcmp

The format for running unzcrash is:

     unzcrash [options] 'lzip -t' file.lz

file.lz must not contain errors and must be correctly decompressed by the decompressor being tested 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-bit
1 1
1,2,3 1, 2 and 3
2-4 2, 3 and 4
1,3-5,8 1, 3, 4, 5 and 8
1-3,5-8 1, 2, 3, 5, 6, 7 and 8

-B[size][,value]
--block[=size][,value]
Test block errors of given size, simulating a whole sector I/O error. Block size defaults to 512 bytes. value defaults to 0. By default, only blocks aligned to a size-byte boundary are tested, but this may be changed with the --delta option.
-d n
--delta=n
Test only one byte, block, or truncation size every n bytes, instead of all of them. If the --block option is given, n defaults to the block size. Else n defaults to 1. Values of n smaller than the block size will result in overlappinng blocks. (Which is convenient for testing because there are usually too few non-overlappinng 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.lz but before the first test call to the decompressor. 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.lz 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.lz

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


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12 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.


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