XZ(1)                              XZ Utils                              XZ(1)



NAME
       xz,  unxz,  xzcat, lzma, unlzma, lzcat - Compress or decompress .xz and
       .lzma files

SYNOPSIS
       xz [option]...  [file]...

       unxz is equivalent to xz --decompress.
       xzcat is equivalent to xz --decompress --stdout.
       lzma is equivalent to xz --format=lzma.
       unlzma is equivalent to xz --format=lzma --decompress.
       lzcat is equivalent to xz --format=lzma --decompress --stdout.

       When writing scripts that need to decompress files, it  is  recommended
       to  always use the name xz with appropriate arguments (xz -d or xz -dc)
       instead of the names unxz and xzcat.

DESCRIPTION
       xz is a general-purpose data compression tool with command line  syntax
       similar  to  gzip(1)  and  bzip2(1).  The native file format is the .xz
       format, but the legacy .lzma format used by LZMA  Utils  and  raw  com-
       pressed streams with no container format headers are also supported.

       xz compresses or decompresses each file according to the selected oper-
       ation mode.  If no files are given or file is -, xz reads from standard
       input and writes the processed data to standard output.  xz will refuse
       (display an error and skip the file) to write compressed data to  stan-
       dard  output  if  it  is a terminal.  Similarly, xz will refuse to read
       compressed data from standard input if it is a terminal.

       Unless --stdout is specified, files other than - are written to  a  new
       file whose name is derived from the source file name:

       o  When  compressing,  the  suffix  of  the  target file format (.xz or
          .lzma) is appended to the source filename to get  the  target  file-
          name.

       o  When  decompressing,  the  .xz  or  .lzma suffix is removed from the
          filename to get the target filename.  xz also  recognizes  the  suf-
          fixes .txz and .tlz, and replaces them with the .tar suffix.

       If  the  target file already exists, an error is displayed and the file
       is skipped.

       Unless writing to standard output, xz will display a warning  and  skip
       the file if any of the following applies:

       o  File  is  not  a regular file.  Symbolic links are not followed, and
          thus they are not considered to be regular files.

       o  File has more than one hard link.

       o  File has setuid, setgid, or sticky bit set.

       o  The operation mode is set to compress and the  file  already  has  a
          suffix  of  the  target file format (.xz or .txz when compressing to
          the .xz format, and .lzma or .tlz when compressing to the .lzma for-
          mat).

       o  The  operation mode is set to decompress and the file doesn't have a
          suffix of any of the supported file formats (.xz,  .txz,  .lzma,  or
          .tlz).

       After successfully compressing or decompressing the file, xz copies the
       owner, group, permissions, access time, and modification time from  the
       source  file  to the target file.  If copying the group fails, the per-
       missions are modified so that the target file doesn't become accessible
       to  users  who  didn't  have  permission to access the source file.  xz
       doesn't support copying other metadata like  access  control  lists  or
       extended attributes yet.

       Once  the  target file has been successfully closed, the source file is
       removed unless --keep was specified.  The source file is never  removed
       if the output is written to standard output.

       Sending  SIGINFO  or  SIGUSR1 to the xz process makes it print progress
       information to standard error.  This has only limited  use  since  when
       standard error is a terminal, using --verbose will display an automati-
       cally updating progress indicator.

   Memory usage
       The memory usage of xz varies from a few hundred kilobytes  to  several
       gigabytes  depending  on  the  compression settings.  The settings used
       when compressing a file determine the memory requirements of the decom-
       pressor.  Typically the decompressor needs 5 % to 20 % of the amount of
       memory that the compressor needed when creating the file.  For example,
       decompressing  a  file  created with xz -9 currently requires 65 MiB of
       memory.  Still, it is possible to have .xz files that  require  several
       gigabytes of memory to decompress.

       Especially  users  of  older  systems  may find the possibility of very
       large memory usage annoying.  To prevent  uncomfortable  surprises,  xz
       has  a  built-in  memory  usage  limiter, which is disabled by default.
       While some operating systems provide ways to limit the memory usage  of
       processes,  relying  on  it  wasn't  deemed to be flexible enough (e.g.
       using ulimit(1) to limit virtual memory tends to cripple mmap(2)).

       The memory usage limiter can be enabled with the  command  line  option
       --memlimit=limit.  Often it is more convenient to enable the limiter by
       default  by  setting  the  environment   variable   XZ_DEFAULTS,   e.g.
       XZ_DEFAULTS=--memlimit=150MiB.   It is possible to set the limits sepa-
       rately for  compression  and  decompression  by  using  --memlimit-com-
       press=limit  and  --memlimit-decompress=limit.  Using these two options
       outside XZ_DEFAULTS is rarely useful because a single run of xz  cannot
       do  both  compression  and  decompression  and  --memlimit=limit (or -M
       limit) is shorter to type on the command line.

       If the specified memory usage limit is exceeded when decompressing,  xz
       will  display  an  error  and decompressing the file will fail.  If the
       limit is exceeded when compressing, xz will try to scale  the  settings
       down  so that the limit is no longer exceeded (except when using --for-
       mat=raw or --no-adjust).  This way the operation won't fail unless  the
       limit is very small.  The scaling of the settings is done in steps that
       don't match the compression level presets, e.g. if the  limit  is  only
       slightly  less than the amount required for xz -9, the settings will be
       scaled down only a little, not all the way down to xz -8.

   Concatenation and padding with .xz files
       It is possible to concatenate .xz files as is.  xz will decompress such
       files as if they were a single .xz file.

       It  is  possible  to  insert  padding between the concatenated parts or
       after the last part.  The padding must consist of null  bytes  and  the
       size of the padding must be a multiple of four bytes.  This can be use-
       ful e.g. if the .xz file is stored on a medium that measures file sizes
       in 512-byte blocks.

       Concatenation  and  padding  are  not  allowed  with .lzma files or raw
       streams.

OPTIONS
   Integer suffixes and special values
       In most places where an integer argument is expected, an optional  suf-
       fix  is  supported to easily indicate large integers.  There must be no
       space between the integer and the suffix.

       KiB    Multiply the integer by 1,024 (2^10).  Ki, k, kB, K, and KB  are
              accepted as synonyms for KiB.

       MiB    Multiply  the integer by 1,048,576 (2^20).  Mi, m, M, and MB are
              accepted as synonyms for MiB.

       GiB    Multiply the integer by 1,073,741,824 (2^30).  Gi, g, G, and  GB
              are accepted as synonyms for GiB.

       The special value max can be used to indicate the maximum integer value
       supported by the option.

   Operation mode
       If multiple operation mode  options  are  given,  the  last  one  takes
       effect.

       -z, --compress
              Compress.   This is the default operation mode when no operation
              mode option is specified and no other operation mode is  implied
              from the command name (for example, unxz implies --decompress).

       -d, --decompress, --uncompress
              Decompress.

       -t, --test
              Test  the integrity of compressed files.  This option is equiva-
              lent to --decompress --stdout except that the decompressed  data
              is  discarded  instead  of being written to standard output.  No
              files are created or removed.

       -l, --list
              Print information about compressed files.  No uncompressed  out-
              put  is  produced, and no files are created or removed.  In list
              mode, the program cannot read the compressed data from  standard
              input or from other unseekable sources.

              The  default  listing  shows  basic information about files, one
              file per line.  To get more detailed information, use  also  the
              --verbose  option.   For  even  more  information, use --verbose
              twice, but note that this may be slow, because getting  all  the
              extra  information  requires  many  seeks.  The width of verbose
              output exceeds 80 characters,  so  piping  the  output  to  e.g.
              less -S may be convenient if the terminal isn't wide enough.

              The  exact  output  may  vary  between xz versions and different
              locales.  For machine-readable output, --robot --list should  be
              used.

   Operation modifiers
       -k, --keep
              Don't delete the input files.

       -f, --force
              This option has several effects:

              o  If the target file already exists, delete it before compress-
                 ing or decompressing.

              o  Compress or decompress even if the input is a  symbolic  link
                 to  a  regular  file, has more than one hard link, or has the
                 setuid, setgid, or sticky bit set.  The setuid,  setgid,  and
                 sticky bits are not copied to the target file.

              o  When  used with --decompress --stdout and xz cannot recognize
                 the type of the source file, copy the source file  as  is  to
                 standard  output.   This allows xzcat --force to be used like
                 cat(1) for files that have not been compressed with xz.  Note
                 that in future, xz might support new compressed file formats,
                 which may make xz decompress more types of files  instead  of
                 copying  them  as is to standard output.  --format=format can
                 be used to restrict xz to decompress only a single file  for-
                 mat.

       -c, --stdout, --to-stdout
              Write  the  compressed  or  decompressed data to standard output
              instead of a file.  This implies --keep.

       --no-sparse
              Disable creation of sparse files.  By default, if  decompressing
              into  a  regular  file,  xz tries to make the file sparse if the
              decompressed data contains long sequences of binary  zeros.   It
              also  works  when writing to standard output as long as standard
              output is connected to a regular  file  and  certain  additional
              conditions  are  met to make it safe.  Creating sparse files may
              save disk space and speed up the decompression by  reducing  the
              amount of disk I/O.

       -S .suf, --suffix=.suf
              When  compressing,  use  .suf  as the suffix for the target file
              instead of .xz or .lzma.  If not writing to standard output  and
              the  source  file already has the suffix .suf, a warning is dis-
              played and the file is skipped.

              When decompressing, recognize files  with  the  suffix  .suf  in
              addition to files with the .xz, .txz, .lzma, or .tlz suffix.  If
              the source file has the suffix .suf, the suffix  is  removed  to
              get the target filename.

              When  compressing  or  decompressing raw streams (--format=raw),
              the suffix must always be specified unless writing  to  standard
              output, because there is no default suffix for raw streams.

       --files[=file]
              Read  the  filenames  to  process from file; if file is omitted,
              filenames are read from standard input.  Filenames must be  ter-
              minated  with  the  newline character.  A dash (-) is taken as a
              regular filename; it doesn't mean standard input.  If  filenames
              are  given  also  as  command line arguments, they are processed
              before the filenames read from file.

       --files0[=file]
              This is identical to --files[=file] except  that  each  filename
              must be terminated with the null character.

   Basic file format and compression options
       -F format, --format=format
              Specify the file format to compress or decompress:

              auto   This  is  the default.  When compressing, auto is equiva-
                     lent to xz.  When decompressing, the format of the  input
                     file  is  automatically  detected.  Note that raw streams
                     (created with --format=raw) cannot be auto-detected.

              xz     Compress to the .xz file format, or accept only .xz files
                     when decompressing.

              lzma, alone
                     Compress  to the legacy .lzma file format, or accept only
                     .lzma files when  decompressing.   The  alternative  name
                     alone  is  provided for backwards compatibility with LZMA
                     Utils.

              raw    Compress or uncompress a raw stream (no  headers).   This
                     is meant for advanced users only.  To decode raw streams,
                     you need use --format=raw and explicitly specify the fil-
                     ter  chain,  which normally would have been stored in the
                     container headers.

       -C check, --check=check
              Specify the type of the integrity check.  The  check  is  calcu-
              lated  from  the  uncompressed  data and stored in the .xz file.
              This option has an effect only when  compressing  into  the  .xz
              format;  the .lzma format doesn't support integrity checks.  The
              integrity check (if any) is verified when the .xz file is decom-
              pressed.

              Supported check types:

              none   Don't  calculate an integrity check at all.  This is usu-
                     ally a bad idea.  This can be useful  when  integrity  of
                     the data is verified by other means anyway.

              crc32  Calculate  CRC32  using  the  polynomial  from IEEE-802.3
                     (Ethernet).

              crc64  Calculate CRC64 using the polynomial from ECMA-182.  This
                     is the default, since it is slightly better than CRC32 at
                     detecting damaged files and the speed difference is  neg-
                     ligible.

              sha256 Calculate  SHA-256.   This  is somewhat slower than CRC32
                     and CRC64.

              Integrity of the .xz headers is always verified with CRC32.   It
              is not possible to change or disable it.

       -0 ... -9
              Select  a compression preset level.  The default is -6.  If mul-
              tiple preset levels are specified, the last  one  takes  effect.
              If  a  custom filter chain was already specified, setting a com-
              pression preset level clears the custom filter chain.

              The differences between the presets are  more  significant  than
              with  gzip(1)  and  bzip2(1).  The selected compression settings
              determine the memory  requirements  of  the  decompressor,  thus
              using  a  too  high preset level might make it painful to decom-
              press the file on an old system with little RAM.   Specifically,
              it's  not  a  good idea to blindly use -9 for everything like it
              often is with gzip(1) and bzip2(1).

              -0 ... -3
                     These are somewhat fast presets.  -0 is sometimes  faster
                     than  gzip  -9 while compressing much better.  The higher
                     ones often have speed comparable to bzip2(1) with  compa-
                     rable  or  better compression ratio, although the results
                     depend a lot on the type of data being compressed.

              -4 ... -6
                     Good to very good compression while keeping  decompressor
                     memory  usage reasonable even for old systems.  -6 is the
                     default, which is usually a good  choice  e.g.  for  dis-
                     tributing  files  that  need to be decompressible even on
                     systems with only 16 MiB RAM.  (-5e or -6e may  be  worth
                     considering too.  See --extreme.)

              -7 ... -9
                     These  are  like -6 but with higher compressor and decom-
                     pressor memory requirements.  These are useful only  when
                     compressing  files bigger than 8 MiB, 16 MiB, and 32 MiB,
                     respectively.

              On the same hardware, the decompression speed is approximately a
              constant  number  of  bytes  of  compressed data per second.  In
              other words, the better the compression, the faster  the  decom-
              pression  will  usually  be.  This also means that the amount of
              uncompressed output produced per second can vary a lot.

              The following table summarises the features of the presets:

                     Preset   DictSize   CompCPU   CompMem   DecMem
                       -0     256 KiB       0        3 MiB    1 MiB
                       -1       1 MiB       1        9 MiB    2 MiB
                       -2       2 MiB       2       17 MiB    3 MiB
                       -3       4 MiB       3       32 MiB    5 MiB
                       -4       4 MiB       4       48 MiB    5 MiB
                       -5       8 MiB       5       94 MiB    9 MiB
                       -6       8 MiB       6       94 MiB    9 MiB
                       -7      16 MiB       6      186 MiB   17 MiB
                       -8      32 MiB       6      370 MiB   33 MiB
                       -9      64 MiB       6      674 MiB   65 MiB

              Column descriptions:

              o  DictSize is the LZMA2 dictionary size.  It is waste of memory
                 to  use a dictionary bigger than the size of the uncompressed
                 file.  This is why it is good to avoid using the  presets  -7
                 ...  -9 when there's no real need for them.  At -6 and lower,
                 the amount of memory wasted is usually low enough to not mat-
                 ter.

              o  CompCPU  is a simplified representation of the LZMA2 settings
                 that affect compression speed.  The dictionary  size  affects
                 speed too, so while CompCPU is the same for levels -6 ... -9,
                 higher levels still tend to be a little slower.  To get  even
                 slower and thus possibly better compression, see --extreme.

              o  CompMem  contains  the  compressor memory requirements in the
                 single-threaded mode.  It may vary slightly between  xz  ver-
                 sions.   Memory  requirements  of  some  of the future multi-
                 threaded modes may be dramatically higher than  that  of  the
                 single-threaded mode.

              o  DecMem  contains  the decompressor memory requirements.  That
                 is, the compression settings determine  the  memory  require-
                 ments  of  the  decompressor.   The exact decompressor memory
                 usage is slighly more than the LZMA2 dictionary size, but the
                 values  in  the  table  have been rounded up to the next full
                 MiB.

       -e, --extreme
              Use a slower variant of the selected  compression  preset  level
              (-0  ...  -9)  to  hopefully get a little bit better compression
              ratio, but with bad luck this can also make  it  worse.   Decom-
              pressor  memory  usage  is  not  affected, but compressor memory
              usage increases a little at preset levels -0 ... -3.

              Since there are two presets  with  dictionary  sizes  4 MiB  and
              8 MiB,  the  presets  -3e  and  -5e use slightly faster settings
              (lower CompCPU) than -4e and -6e, respectively.  That way no two
              presets are identical.

                     Preset   DictSize   CompCPU   CompMem   DecMem
                      -0e     256 KiB       8        4 MiB    1 MiB
                      -1e       1 MiB       8       13 MiB    2 MiB
                      -2e       2 MiB       8       25 MiB    3 MiB
                      -3e       4 MiB       7       48 MiB    5 MiB
                      -4e       4 MiB       8       48 MiB    5 MiB
                      -5e       8 MiB       7       94 MiB    9 MiB
                      -6e       8 MiB       8       94 MiB    9 MiB
                      -7e      16 MiB       8      186 MiB   17 MiB
                      -8e      32 MiB       8      370 MiB   33 MiB
                      -9e      64 MiB       8      674 MiB   65 MiB

              For  example,  there  are a total of four presets that use 8 MiB
              dictionary, whose order from the fastest to the slowest  is  -5,
              -6, -5e, and -6e.

       --fast
       --best These  are  somewhat  misleading  aliases for -0 and -9, respec-
              tively.  These are provided  only  for  backwards  compatibility
              with LZMA Utils.  Avoid using these options.

       --memlimit-compress=limit
              Set  a  memory  usage  limit for compression.  If this option is
              specified multiple times, the last one takes effect.

              If the compression settings exceed the limit, xz will adjust the
              settings  downwards  so that the limit is no longer exceeded and
              display a notice  that  automatic  adjustment  was  done.   Such
              adjustments  are  not made when compressing with --format=raw or
              if --no-adjust has been specified.  In those cases, an error  is
              displayed and xz will exit with exit status 1.

              The limit can be specified in multiple ways:

              o  The  limit can be an absolute value in bytes.  Using an inte-
                 ger suffix like MiB can be useful.  Example:  --memlimit-com-
                 press=80MiB

              o  The  limit can be specified as a percentage of total physical
                 memory (RAM).  This can be useful especially when setting the
                 XZ_DEFAULTS  environment  variable  in a shell initialization
                 script that is shared between different computers.  That  way
                 the  limit  is automatically bigger on systems with more mem-
                 ory.  Example: --memlimit-compress=70%

              o  The limit can be reset back to its default value  by  setting
                 it  to  0.  This is currently equivalent to setting the limit
                 to max (no memory usage limit).  Once multithreading  support
                 has been implemented, there may be a difference between 0 and
                 max for the multithreaded case, so it is recommended to use 0
                 instead of max until the details have been decided.

              See also the section Memory usage.

       --memlimit-decompress=limit
              Set  a  memory usage limit for decompression.  This also affects
              the --list mode.  If  the  operation  is  not  possible  without
              exceeding  the limit, xz will display an error and decompressing
              the file will fail.  See --memlimit-compress=limit for  possible
              ways to specify the limit.

       -M limit, --memlimit=limit, --memory=limit
              This   is  equivalent  to  specifying  --memlimit-compress=limit
              --memlimit-decompress=limit.

       --no-adjust
              Display an error and exit if the compression settings exceed the
              memory usage limit.  The default is to adjust the settings down-
              wards so that the memory usage limit is not exceeded.  Automatic
              adjusting  is  always disabled when creating raw streams (--for-
              mat=raw).

       -T threads, --threads=threads
              Specify the number of worker threads to use.  The actual  number
              of  threads can be less than threads if using more threads would
              exceed the memory usage limit.

              Multithreaded compression and decompression are not  implemented
              yet, so this option has no effect for now.

              As  of  writing  (2010-09-27), it hasn't been decided if threads
              will be used by default on multicore systems  once  support  for
              threading has been implemented.  Comments are welcome.  The com-
              plicating factor is that using many threads  will  increase  the
              memory  usage dramatically.  Note that if multithreading will be
              the default, it will probably be done  so  that  single-threaded
              and  multithreaded modes produce the same output, so compression
              ratio won't be  significantly  affected  if  threading  will  be
              enabled by default.

   Custom compressor filter chains
       A  custom  filter  chain  allows specifying the compression settings in
       detail instead of relying on the settings associated to the preset lev-
       els.   When  a custom filter chain is specified, the compression preset
       level options (-0 ... -9 and --extreme) are silently ignored.

       A filter chain is comparable to piping on the command line.  When  com-
       pressing, the uncompressed input goes to the first filter, whose output
       goes to the next filter (if any).  The output of the last  filter  gets
       written  to  the compressed file.  The maximum number of filters in the
       chain is four, but typically a filter chain has only one  or  two  fil-
       ters.

       Many filters have limitations on where they can be in the filter chain:
       some filters can work only as the last filter in the chain,  some  only
       as  a  non-last  filter,  and  some  work in any position in the chain.
       Depending on the filter, this limitation is either inherent to the fil-
       ter design or exists to prevent security issues.

       A  custom filter chain is specified by using one or more filter options
       in the order they are wanted in the filter chain.  That is,  the  order
       of  filter  options  is significant!  When decoding raw streams (--for-
       mat=raw), the filter chain is specified in the same  order  as  it  was
       specified when compressing.

       Filters  take filter-specific options as a comma-separated list.  Extra
       commas in options are ignored.  Every option has a  default  value,  so
       you need to specify only those you want to change.

       --lzma1[=options]
       --lzma2[=options]
              Add  LZMA1  or  LZMA2 filter to the filter chain.  These filters
              can be used only as the last filter in the chain.

              LZMA1 is a legacy filter, which is supported almost  solely  due
              to  the  legacy  .lzma  file  format, which supports only LZMA1.
              LZMA2 is an updated version  of  LZMA1  to  fix  some  practical
              issues  of LZMA1.  The .xz format uses LZMA2 and doesn't support
              LZMA1 at all.  Compression speed and ratios of LZMA1  and  LZMA2
              are practically the same.

              LZMA1 and LZMA2 share the same set of options:

              preset=preset
                     Reset  all LZMA1 or LZMA2 options to preset.  Preset con-
                     sist of an integer, which may be followed by  single-let-
                     ter  preset  modifiers.   The integer can be from 0 to 9,
                     matching the command line options -0 ...  -9.   The  only
                     supported   modifier   is   currently  e,  which  matches
                     --extreme.  The default  preset  is  6,  from  which  the
                     default values for the rest of the LZMA1 or LZMA2 options
                     are taken.

              dict=size
                     Dictionary (history buffer) size indicates how many bytes
                     of  the  recently  processed uncompressed data is kept in
                     memory.  The  algorithm  tries  to  find  repeating  byte
                     sequences (matches) in the uncompressed data, and replace
                     them with references to the data currently in the dictio-
                     nary.   The  bigger  the  dictionary,  the  higher is the
                     chance to find a match.  Thus, increasing dictionary size
                     usually improves compression ratio, but a dictionary big-
                     ger than the uncompressed file is waste of memory.

                     Typical dictionary size is from 64 KiB  to  64 MiB.   The
                     minimum  is  4 KiB.   The maximum for compression is cur-
                     rently 1.5 GiB (1536 MiB).  The decompressor already sup-
                     ports  dictionaries up to one byte less than 4 GiB, which
                     is the maximum for the LZMA1 and LZMA2 stream formats.

                     Dictionary size and match finder (mf) together  determine
                     the memory usage of the LZMA1 or LZMA2 encoder.  The same
                     (or bigger) dictionary size is required for decompressing
                     that  was used when compressing, thus the memory usage of
                     the decoder is determined by  the  dictionary  size  used
                     when  compressing.   The .xz headers store the dictionary
                     size either as 2^n or 2^n + 2^(n-1), so these  sizes  are
                     somewhat preferred for compression.  Other sizes will get
                     rounded up when stored in the .xz headers.

              lc=lc  Specify the number of literal context bits.  The  minimum
                     is  0  and  the maximum is 4; the default is 3.  In addi-
                     tion, the sum of lc and lp must not exceed 4.

                     All bytes that cannot be encoded as matches  are  encoded
                     as  literals.   That  is, literals are simply 8-bit bytes
                     that are encoded one at a time.

                     The literal coding makes an assumption that  the  highest
                     lc  bits of the previous uncompressed byte correlate with
                     the next byte.  E.g. in typical English text,  an  upper-
                     case letter is often followed by a lower-case letter, and
                     a lower-case letter is usually followed by another lower-
                     case  letter.  In the US-ASCII character set, the highest
                     three bits are 010 for upper-case  letters  and  011  for
                     lower-case  letters.   When lc is at least 3, the literal
                     coding can take advantage of this property in the  uncom-
                     pressed data.

                     The default value (3) is usually good.  If you want maxi-
                     mum compression, test lc=4.  Sometimes it helps a little,
                     and sometimes it makes compression worse.  If it makes it
                     worse, test e.g. lc=2 too.

              lp=lp  Specify the number of literal position bits.  The minimum
                     is 0 and the maximum is 4; the default is 0.

                     Lp  affects  what  kind  of alignment in the uncompressed
                     data is assumed when encoding literals.  See pb below for
                     more information about alignment.

              pb=pb  Specify  the  number  of position bits.  The minimum is 0
                     and the maximum is 4; the default is 2.

                     Pb affects what kind of  alignment  in  the  uncompressed
                     data  is assumed in general.  The default means four-byte
                     alignment (2^pb=2^2=4), which is often a good choice when
                     there's no better guess.

                     When  the  aligment  is known, setting pb accordingly may
                     reduce the file size a little.  E.g. with text files hav-
                     ing  one-byte  alignment  (US-ASCII,  ISO-8859-*, UTF-8),
                     setting  pb=0  can  improve  compression  slightly.   For
                     UTF-16  text, pb=1 is a good choice.  If the alignment is
                     an odd number like  3  bytes,  pb=0  might  be  the  best
                     choice.

                     Even though the assumed alignment can be adjusted with pb
                     and lp, LZMA1 and  LZMA2  still  slightly  favor  16-byte
                     alignment.   It  might  be worth taking into account when
                     designing file formats that are likely to be  often  com-
                     pressed with LZMA1 or LZMA2.

              mf=mf  Match  finder has a major effect on encoder speed, memory
                     usage, and compression ratio.  Usually Hash  Chain  match
                     finders  are  faster than Binary Tree match finders.  The
                     default depends on the preset: 0 uses hc3, 1-3  use  hc4,
                     and the rest use bt4.

                     The  following  match  finders are supported.  The memory
                     usage formulas below are rough approximations, which  are
                     closest to the reality when dict is a power of two.

                     hc3    Hash Chain with 2- and 3-byte hashing
                            Minimum value for nice: 3
                            Memory usage:
                            dict * 7.5 (if dict <= 16 MiB);
                            dict * 5.5 + 64 MiB (if dict > 16 MiB)

                     hc4    Hash Chain with 2-, 3-, and 4-byte hashing
                            Minimum value for nice: 4
                            Memory usage:
                            dict * 7.5 (if dict <= 32 MiB);
                            dict * 6.5 (if dict > 32 MiB)

                     bt2    Binary Tree with 2-byte hashing
                            Minimum value for nice: 2
                            Memory usage: dict * 9.5

                     bt3    Binary Tree with 2- and 3-byte hashing
                            Minimum value for nice: 3
                            Memory usage:
                            dict * 11.5 (if dict <= 16 MiB);
                            dict * 9.5 + 64 MiB (if dict > 16 MiB)

                     bt4    Binary Tree with 2-, 3-, and 4-byte hashing
                            Minimum value for nice: 4
                            Memory usage:
                            dict * 11.5 (if dict <= 32 MiB);
                            dict * 10.5 (if dict > 32 MiB)

              mode=mode
                     Compression mode specifies the method to analyze the data
                     produced by the match finder.  Supported modes  are  fast
                     and normal.  The default is fast for presets 0-3 and nor-
                     mal for presets 4-9.

                     Usually fast is used with Hash Chain  match  finders  and
                     normal with Binary Tree match finders.  This is also what
                     the presets do.

              nice=nice
                     Specify what is considered to be  a  nice  length  for  a
                     match.  Once a match of at least nice bytes is found, the
                     algorithm stops looking for possibly better matches.

                     Nice can be 2-273 bytes.  Higher values tend to give bet-
                     ter  compression  ratio  at  the  expense  of speed.  The
                     default depends on the preset.

              depth=depth
                     Specify the maximum search depth  in  the  match  finder.
                     The  default  is  the special value of 0, which makes the
                     compressor determine a reasonable depth from mf and nice.

                     Reasonable depth for Hash Chains is 4-100 and 16-1000 for
                     Binary  Trees.  Using very high values for depth can make
                     the encoder extremely slow with some files.   Avoid  set-
                     ting  the  depth  over  1000  unless  you are prepared to
                     interrupt the compression in case it is  taking  far  too
                     long.

              When  decoding  raw streams (--format=raw), LZMA2 needs only the
              dictionary size.  LZMA1 needs also lc, lp, and pb.

       --x86[=options]
       --powerpc[=options]
       --ia64[=options]
       --arm[=options]
       --armthumb[=options]
       --sparc[=options]
              Add a branch/call/jump (BCJ) filter to the filter chain.   These
              filters  can  be  used  only  as a non-last filter in the filter
              chain.

              A BCJ filter converts relative addresses in the machine code  to
              their  absolute  counterparts.   This doesn't change the size of
              the data, but it increases redundancy, which can help  LZMA2  to
              produce  0-15 %  smaller  .xz  file.  The BCJ filters are always
              reversible, so using a BCJ filter for wrong type of data doesn't
              cause  any data loss, although it may make the compression ratio
              slightly worse.

              It is fine to apply a BCJ filter on a whole executable;  there's
              no  need to apply it only on the executable section.  Applying a
              BCJ filter on an archive that contains both executable and  non-
              executable  files may or may not give good results, so it gener-
              ally isn't good to blindly apply a BCJ filter  when  compressing
              binary packages for distribution.

              These  BCJ filters are very fast and use insignificant amount of
              memory.  If a BCJ filter improves compression ratio of  a  file,
              it  can  improve  decompression speed at the same time.  This is
              because, on the same hardware, the decompression speed of  LZMA2
              is  roughly  a fixed number of bytes of compressed data per sec-
              ond.

              These BCJ filters have known problems related to the compression
              ratio:

              o  Some  types  of files containing executable code (e.g. object
                 files, static libraries, and Linux kernel modules)  have  the
                 addresses  in  the  instructions  filled  with filler values.
                 These BCJ filters will still do the address conversion, which
                 will make the compression worse with these files.

              o  Applying a BCJ filter on an archive containing multiple simi-
                 lar executables can make the compression ratio worse than not
                 using  a  BCJ filter.  This is because the BCJ filter doesn't
                 detect the boundaries of the executable  files,  and  doesn't
                 reset the address conversion counter for each executable.

              Both  of the above problems will be fixed in the future in a new
              filter.  The old BCJ filters will still be  useful  in  embedded
              systems,  because  the  decoder of the new filter will be bigger
              and use more memory.

              Different instruction sets have have different alignment:

                     Filter      Alignment   Notes
                     x86             1       32-bit or 64-bit x86
                     PowerPC         4       Big endian only
                     ARM             4       Little endian only
                     ARM-Thumb       2       Little endian only
                     IA-64          16       Big or little endian
                     SPARC           4       Big or little endian

              Since the BCJ-filtered data is usually  compressed  with  LZMA2,
              the  compression  ratio  may  be  improved slightly if the LZMA2
              options are set to match the alignment of the selected BCJ  fil-
              ter.   For example, with the IA-64 filter, it's good to set pb=4
              with LZMA2 (2^4=16).  The x86 filter is an exception; it's  usu-
              ally  good  to stick to LZMA2's default four-byte alignment when
              compressing x86 executables.

              All BCJ filters support the same options:

              start=offset
                     Specify the start offset that  is  used  when  converting
                     between relative and absolute addresses.  The offset must
                     be a multiple of the alignment of the filter (see the ta-
                     ble  above).   The  default  is  zero.   In practice, the
                     default is good; specifying a  custom  offset  is  almost
                     never useful.

       --delta[=options]
              Add  the Delta filter to the filter chain.  The Delta filter can
              be only used as a non-last filter in the filter chain.

              Currently only simple byte-wise delta calculation is  supported.
              It  can  be  useful  when  compressing  e.g. uncompressed bitmap
              images or uncompressed  PCM  audio.   However,  special  purpose
              algorithms  may  give  significantly better results than Delta +
              LZMA2.  This is true especially  with  audio,  which  compresses
              faster and better e.g. with flac(1).

              Supported options:

              dist=distance
                     Specify  the  distance of the delta calculation in bytes.
                     distance must be 1-256.  The default is 1.

                     For example, with dist=2 and eight-byte input A1 B1 A2 B3
                     A3 B5 A4 B7, the output will be A1 B1 01 02 01 02 01 02.

   Other options
       -q, --quiet
              Suppress  warnings  and notices.  Specify this twice to suppress
              errors too.  This option has no effect on the exit status.  That
              is,  even  if a warning was suppressed, the exit status to indi-
              cate a warning is still used.

       -v, --verbose
              Be verbose.  If standard error is connected to  a  terminal,  xz
              will  display  a progress indicator.  Specifying --verbose twice
              will give even more verbose output.

              The progress indicator shows the following information:

              o  Completion percentage is shown if the size of the input  file
                 is known.  That is, the percentage cannot be shown in pipes.

              o  Amount  of compressed data produced (compressing) or consumed
                 (decompressing).

              o  Amount of uncompressed data consumed  (compressing)  or  pro-
                 duced (decompressing).

              o  Compression ratio, which is calculated by dividing the amount
                 of compressed data processed so far by the amount  of  uncom-
                 pressed data processed so far.

              o  Compression  or decompression speed.  This is measured as the
                 amount of uncompressed data consumed  (compression)  or  pro-
                 duced  (decompression)  per  second.  It is shown after a few
                 seconds have passed since xz started processing the file.

              o  Elapsed time in the format M:SS or H:MM:SS.

              o  Estimated remaining time is shown only when the size  of  the
                 input  file  is  known  and  a couple of seconds have already
                 passed since xz started processing the  file.   The  time  is
                 shown  in  a  less precise format which never has any colons,
                 e.g. 2 min 30 s.

              When standard error is not a terminal, --verbose  will  make  xz
              print the filename, compressed size, uncompressed size, compres-
              sion ratio, and possibly also the speed and elapsed  time  on  a
              single line to standard error after compressing or decompressing
              the file.  The speed and elapsed time are included only when the
              operation  took at least a few seconds.  If the operation didn't
              finish, e.g. due to user interruption, also the completion  per-
              centage is printed if the size of the input file is known.

       -Q, --no-warn
              Don't set the exit status to 2 even if a condition worth a warn-
              ing was detected.  This  option  doesn't  affect  the  verbosity
              level,  thus  both  --quiet and --no-warn have to be used to not
              display warnings and to not alter the exit status.

       --robot
              Print messages in a machine-parsable format.  This  is  intended
              to  ease  writing  frontends  that  want  to  use  xz instead of
              liblzma, which may be the case with various scripts.  The output
              with  this  option  enabled  is  meant  to  be  stable across xz
              releases.  See the section ROBOT MODE for details.

       --info-memory
              Display, in human-readable  format,  how  much  physical  memory
              (RAM)  xz  thinks the system has and the memory usage limits for
              compression and decompression, and exit successfully.

       -h, --help
              Display  a  help  message  describing  the  most  commonly  used
              options, and exit successfully.

       -H, --long-help
              Display  a  help message describing all features of xz, and exit
              successfully

       -V, --version
              Display the version number of xz and liblzma in  human  readable
              format.   To get machine-parsable output, specify --robot before
              --version.

ROBOT MODE
       The robot mode is activated with the --robot option.  It makes the out-
       put of xz easier to parse by other programs.  Currently --robot is sup-
       ported only together with --version,  --info-memory,  and  --list.   It
       will  be  supported  for  normal  compression  and decompression in the
       future.

   Version
       xz --robot --version will print the version number of xz and liblzma in
       the following format:

       XZ_VERSION=XYYYZZZS
       LIBLZMA_VERSION=XYYYZZZS

       X      Major version.

       YYY    Minor  version.  Even numbers are stable.  Odd numbers are alpha
              or beta versions.

       ZZZ    Patch level for stable releases or just a counter  for  develop-
              ment releases.

       S      Stability.  0 is alpha, 1 is beta, and 2 is stable.  S should be
              always 2 when YYY is even.

       XYYYZZZS are the same on both lines if xz and liblzma are from the same
       XZ Utils release.

       Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.

   Memory limit information
       xz  --robot --info-memory prints a single line with three tab-separated
       columns:

       1.  Total amount of physical memory (RAM) in bytes

       2.  Memory usage limit for compression in bytes.  A  special  value  of
           zero  indicates the default setting, which for single-threaded mode
           is the same as no limit.

       3.  Memory usage limit for decompression in bytes.  A special value  of
           zero  indicates the default setting, which for single-threaded mode
           is the same as no limit.

       In the future, the output of xz --robot  --info-memory  may  have  more
       columns, but never more than a single line.

   List mode
       xz --robot --list uses tab-separated output.  The first column of every
       line has a string that indicates the type of the information  found  on
       that line:

       name   This is always the first line when starting to list a file.  The
              second column on the line is the filename.

       file   This line contains overall information about the .xz file.  This
              line is always printed after the name line.

       stream This line type is used only when --verbose was specified.  There
              are as many stream lines as there are streams in the .xz file.

       block  This line type is used only when --verbose was specified.  There
              are  as  many  block  lines as there are blocks in the .xz file.
              The block lines are shown after all the stream lines;  different
              line types are not interleaved.

       summary
              This  line type is used only when --verbose was specified twice.
              This line is printed after all block lines.  Like the file line,
              the  summary  line  contains  overall  information about the .xz
              file.

       totals This line is always the very last line of the list  output.   It
              shows the total counts and sizes.

       The columns of the file lines:
              2.  Number of streams in the file
              3.  Total number of blocks in the stream(s)
              4.  Compressed size of the file
              5.  Uncompressed size of the file
              6.  Compression  ratio,  for  example  0.123.   If ratio is over
                  9.999, three dashes  (---)  are  displayed  instead  of  the
                  ratio.
              7.  Comma-separated  list of integrity check names.  The follow-
                  ing strings are used for the known check types: None, CRC32,
                  CRC64,  and  SHA-256.  For unknown check types, Unknown-N is
                  used, where N is the Check ID as a decimal  number  (one  or
                  two digits).
              8.  Total size of stream padding in the file

       The columns of the stream lines:
              2.  Stream number (the first stream is 1)
              3.  Number of blocks in the stream
              4.  Compressed start offset
              5.  Uncompressed start offset
              6.  Compressed size (does not include stream padding)
              7.  Uncompressed size
              8.  Compression ratio
              9.  Name of the integrity check
              10. Size of stream padding

       The columns of the block lines:
              2.  Number of the stream containing this block
              3.  Block  number  relative  to the beginning of the stream (the
                  first block is 1)
              4.  Block number relative to the beginning of the file
              5.  Compressed start offset relative to  the  beginning  of  the
                  file
              6.  Uncompressed  start  offset relative to the beginning of the
                  file
              7.  Total compressed size of the block (includes headers)
              8.  Uncompressed size
              9.  Compression ratio
              10. Name of the integrity check

       If --verbose was specified twice, additional columns  are  included  on
       the  block  lines.   These  are  not displayed with a single --verbose,
       because getting this information requires many seeks and  can  thus  be
       slow:
              11. Value of the integrity check in hexadecimal
              12. Block header size
              13. Block  flags:  c  indicates that compressed size is present,
                  and u indicates that uncompressed size is present.   If  the
                  flag  is  not  set,  a dash (-) is shown instead to keep the
                  string length fixed.  New flags may be added to the  end  of
                  the string in the future.
              14. Size  of  the  actual  compressed  data  in  the block (this
                  excludes the block header, block padding, and check fields)
              15. Amount of memory (in  bytes)  required  to  decompress  this
                  block with this xz version
              16. Filter  chain.   Note  that most of the options used at com-
                  pression time cannot be known, because only the options that
                  are needed for decompression are stored in the .xz headers.

       The columns of the summary lines:
              2.  Amount of memory (in bytes) required to decompress this file
                  with this xz version
              3.  yes or no indicating if all block  headers  have  both  com-
                  pressed size and uncompressed size stored in them

       The columns of the totals line:
              2.  Number of streams
              3.  Number of blocks
              4.  Compressed size
              5.  Uncompressed size
              6.  Average compression ratio
              7.  Comma-separated  list  of  integrity  check  names that were
                  present in the files
              8.  Stream padding size
              9.  Number of files.  This is here to keep the order of the ear-
                  lier columns the same as on file lines.

       If  --verbose  was  specified twice, additional columns are included on
       the totals line:
              10. Maximum amount of memory (in bytes) required  to  decompress
                  the files with this xz version
              11. yes  or  no  indicating  if all block headers have both com-
                  pressed size and uncompressed size stored in them

       Future versions may add new line types and new columns can be added  to
       the existing line types, but the existing columns won't be changed.

EXIT STATUS
       0      All is good.

       1      An error occurred.

       2      Something  worth  a  warning  occurred,  but  no  actual  errors
              occurred.

       Notices (not warnings or errors) printed on standard error don't affect
       the exit status.

ENVIRONMENT
       xz  parses  space-separated lists of options from the environment vari-
       ables XZ_DEFAULTS and XZ_OPT, in this order, before parsing the options
       from  the  command  line.   Note  that only options are parsed from the
       environment variables; all non-options are silently  ignored.   Parsing
       is  done  with  getopt_long(3)  which is used also for the command line
       arguments.

       XZ_DEFAULTS
              User-specific or system-wide default options.  Typically this is
              set in a shell initialization script to enable xz's memory usage
              limiter by default.  Excluding shell initialization scripts  and
              similar   special   cases,  scripts  must  never  set  or  unset
              XZ_DEFAULTS.

       XZ_OPT This is for passing options to xz when it is not possible to set
              the  options  directly on the xz command line.  This is the case
              e.g. when xz is run by a script or tool, e.g. GNU tar(1):

                     XZ_OPT=-2v tar caf foo.tar.xz foo

              Scripts may use XZ_OPT e.g. to set script-specific default  com-
              pression  options.   It  is  still recommended to allow users to
              override XZ_OPT if that is reasonable, e.g. in sh(1) scripts one
              may use something like this:

                     XZ_OPT=${XZ_OPT-"-7e"}
                     export XZ_OPT

LZMA UTILS COMPATIBILITY
       The  command  line  syntax  of  xz  is  practically a superset of lzma,
       unlzma, and lzcat as found from LZMA Utils 4.32.x.  In most  cases,  it
       is possible to replace LZMA Utils with XZ Utils without breaking exist-
       ing scripts.  There are some incompatibilities though, which may  some-
       times cause problems.

   Compression preset levels
       The  numbering  of the compression level presets is not identical in xz
       and LZMA Utils.  The most important difference is how dictionary  sizes
       are  mapped  to different presets.  Dictionary size is roughly equal to
       the decompressor memory usage.

              Level     xz      LZMA Utils
               -0     256 KiB      N/A
               -1       1 MiB     64 KiB
               -2       2 MiB      1 MiB
               -3       4 MiB    512 KiB
               -4       4 MiB      1 MiB

               -5       8 MiB      2 MiB
               -6       8 MiB      4 MiB
               -7      16 MiB      8 MiB
               -8      32 MiB     16 MiB
               -9      64 MiB     32 MiB

       The dictionary size differences affect the compressor memory usage too,
       but  there  are some other differences between LZMA Utils and XZ Utils,
       which make the difference even bigger:

              Level     xz      LZMA Utils 4.32.x
               -0       3 MiB          N/A
               -1       9 MiB          2 MiB
               -2      17 MiB         12 MiB
               -3      32 MiB         12 MiB
               -4      48 MiB         16 MiB
               -5      94 MiB         26 MiB
               -6      94 MiB         45 MiB
               -7     186 MiB         83 MiB
               -8     370 MiB        159 MiB
               -9     674 MiB        311 MiB

       The default preset level in LZMA Utils is -7 while in XZ  Utils  it  is
       -6, so both use an 8 MiB dictionary by default.

   Streamed vs. non-streamed .lzma files
       The  uncompressed  size  of the file can be stored in the .lzma header.
       LZMA Utils does that when compressing regular files.   The  alternative
       is  to  mark  that  uncompressed size is unknown and use end-of-payload
       marker to indicate where the decompressor should stop.  LZMA Utils uses
       this  method  when uncompressed size isn't known, which is the case for
       example in pipes.

       xz supports decompressing .lzma files with  or  without  end-of-payload
       marker,  but  all  .lzma  files  created  by xz will use end-of-payload
       marker and have uncompressed  size  marked  as  unknown  in  the  .lzma
       header.   This may be a problem in some uncommon situations.  For exam-
       ple, a .lzma decompressor in an embedded device might  work  only  with
       files  that have known uncompressed size.  If you hit this problem, you
       need to use LZMA Utils or LZMA SDK to create  .lzma  files  with  known
       uncompressed size.

   Unsupported .lzma files
       The .lzma format allows lc values up to 8, and lp values up to 4.  LZMA
       Utils can decompress files with any lc and lp, but always creates files
       with  lc=3  and  lp=0.  Creating files with other lc and lp is possible
       with xz and with LZMA SDK.

       The implementation of the LZMA1 filter in liblzma requires that the sum
       of  lc  and lp must not exceed 4.  Thus, .lzma files, which exceed this
       limitation, cannot be decompressed with xz.

       LZMA Utils creates only .lzma files which have a dictionary size of 2^n
       (a  power  of  2)  but accepts files with any dictionary size.  liblzma
       accepts only .lzma files which have a dictionary size of 2^n or  2^n  +
       2^(n-1).   This  is  to  decrease  false positives when detecting .lzma
       files.

       These limitations shouldn't be a problem in practice, since practically
       all  .lzma  files  have been compressed with settings that liblzma will
       accept.

   Trailing garbage
       When decompressing, LZMA Utils silently  ignore  everything  after  the
       first  .lzma  stream.   In  most  situations, this is a bug.  This also
       means that LZMA Utils don't support  decompressing  concatenated  .lzma
       files.

       If  there  is  data left after the first .lzma stream, xz considers the
       file to be corrupt.  This may break obscure scripts which have  assumed
       that trailing garbage is ignored.

NOTES
   Compressed output may vary
       The  exact  compressed output produced from the same uncompressed input
       file may vary between XZ Utils versions even if compression options are
       identical.  This is because the encoder can be improved (faster or bet-
       ter compression) without affecting the file  format.   The  output  can
       vary  even  between  different  builds of the same XZ Utils version, if
       different build options are used.

       The above means that implementing --rsyncable to create  rsyncable  .xz
       files  is  not  going  to happen without freezing a part of the encoder
       implementation, which can then be used with --rsyncable.

   Embedded .xz decompressors
       Embedded .xz decompressor implementations like XZ Embedded don't neces-
       sarily support files created with integrity check types other than none
       and  crc32.   Since  the  default  is  --check=crc64,  you   must   use
       --check=none or --check=crc32 when creating files for embedded systems.

       Outside  embedded systems, all .xz format decompressors support all the
       check types, or at least are able to decompress the file without  veri-
       fying the integrity check if the particular check is not supported.

       XZ  Embedded supports BCJ filters, but only with the default start off-
       set.

EXAMPLES
   Basics
       Compress the file foo into foo.xz using the default  compression  level
       (-6), and remove foo if compression is successful:

              xz foo

       Decompress  bar.xz  into bar and don't remove bar.xz even if decompres-
       sion is successful:

              xz -dk bar.xz

       Create baz.tar.xz with the preset -4e (-4 --extreme), which  is  slower
       than  e.g.  the  default  -6, but needs less memory for compression and
       decompression (48 MiB and 5 MiB, respectively):

              tar cf - baz | xz -4e > baz.tar.xz

       A mix of compressed and uncompressed files can be decompressed to stan-
       dard output with a single command:

              xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt

   Parallel compression of many files
       On  GNU  and *BSD, find(1) and xargs(1) can be used to parallelize com-
       pression of many files:

              find . -type f \! -name '*.xz' -print0 \
                  | xargs -0r -P4 -n16 xz -T1

       The -P option to xargs(1) sets the number  of  parallel  xz  processes.
       The best value for the -n option depends on how many files there are to
       be compressed.  If there are only a couple of files, the  value  should
       probably be 1; with tens of thousands of files, 100 or even more may be
       appropriate to reduce the number of xz  processes  that  xargs(1)  will
       eventually create.

       The  option  -T1  for  xz is there to force it to single-threaded mode,
       because xargs(1) is used to control the amount of parallelization.

   Robot mode
       Calculate how many bytes have been saved  in  total  after  compressing
       multiple files:

              xz --robot --list *.xz | awk '/^totals/{print $5-$4}'

       A  script may want to know that it is using new enough xz.  The follow-
       ing sh(1) script checks that the version number of the xz  tool  is  at
       least  5.0.0.   This method is compatible with old beta versions, which
       didn't support the --robot option:

              if ! eval "$(xz --robot --version 2> /dev/null)" ||
                      [ "$XZ_VERSION" -lt 50000002 ]; then
                  echo "Your xz is too old."
              fi
              unset XZ_VERSION LIBLZMA_VERSION

       Set a memory usage limit for decompression using XZ_OPT, but if a limit
       has already been set, don't increase it:

              NEWLIM=$((123 << 20))  # 123 MiB
              OLDLIM=$(xz --robot --info-memory | cut -f3)
              if [ $OLDLIM -eq 0 -o $OLDLIM -gt $NEWLIM ]; then
                  XZ_OPT="$XZ_OPT --memlimit-decompress=$NEWLIM"
                  export XZ_OPT
              fi

   Custom compressor filter chains
       The  simplest  use for custom filter chains is customizing a LZMA2 pre-
       set.  This can be useful, because the presets cover only  a  subset  of
       the potentially useful combinations of compression settings.

       The  CompCPU columns of the tables from the descriptions of the options
       -0 ... -9 and --extreme are  useful  when  customizing  LZMA2  presets.
       Here are the relevant parts collected from those two tables:

              Preset   CompCPU
               -0         0
               -1         1
               -2         2
               -3         3
               -4         4
               -5         5
               -6         6
               -5e        7
               -6e        8

       If  you know that a file requires somewhat big dictionary (e.g. 32 MiB)
       to compress well, but you want to compress it quicker than xz -8  would
       do, a preset with a low CompCPU value (e.g. 1) can be modified to use a
       bigger dictionary:

              xz --lzma2=preset=1,dict=32MiB foo.tar

       With certain files, the above command may be faster than  xz  -6  while
       compressing  significantly better.  However, it must be emphasized that
       only some files benefit from a big dictionary while keeping the CompCPU
       value low.  The most obvious situation, where a big dictionary can help
       a lot, is an archive containing very similar files of at  least  a  few
       megabytes  each.   The  dictionary  size has to be significantly bigger
       than any individual file to allow LZMA2 to take full advantage  of  the
       similarities between consecutive files.

       If  very high compressor and decompressor memory usage is fine, and the
       file being compressed is at least several hundred megabytes, it may  be
       useful  to  use  an  even  bigger dictionary than the 64 MiB that xz -9
       would use:

              xz -vv --lzma2=dict=192MiB big_foo.tar

       Using -vv (--verbose --verbose) like in the above example can be useful
       to  see  the  memory  requirements  of the compressor and decompressor.
       Remember that using a dictionary bigger than the  size  of  the  uncom-
       pressed  file is waste of memory, so the above command isn't useful for
       small files.

       Sometimes the compression time doesn't  matter,  but  the  decompressor
       memory  usage has to be kept low e.g. to make it possible to decompress
       the file on an embedded system.  The following  command  uses  -6e  (-6
       --extreme)  as  a  base  and  sets  the dictionary to only 64 KiB.  The
       resulting file can be decompressed with XZ Embedded (that's  why  there
       is --check=crc32) using about 100 KiB of memory.

              xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo

       If  you  want  to  squeeze out as many bytes as possible, adjusting the
       number of literal context bits (lc) and number of  position  bits  (pb)
       can sometimes help.  Adjusting the number of literal position bits (lp)
       might help too, but usually lc and  pb  are  more  important.   E.g.  a
       source  code  archive  contains mostly US-ASCII text, so something like
       the following might give slightly (like 0.1 %) smaller file than xz -6e
       (try also without lc=4):

              xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar

       Using  another  filter together with LZMA2 can improve compression with
       certain file types.  E.g. to compress a x86-32 or x86-64 shared library
       using the x86 BCJ filter:

              xz --x86 --lzma2 libfoo.so

       Note  that the order of the filter options is significant.  If --x86 is
       specified after --lzma2, xz will give an error, because there cannot be
       any  filter  after LZMA2, and also because the x86 BCJ filter cannot be
       used as the last filter in the chain.

       The Delta filter together with LZMA2 can give good results with  bitmap
       images.  It should usually beat PNG, which has a few more advanced fil-
       ters than simple delta but uses Deflate for the actual compression.

       The image has to be saved in uncompressed format, e.g. as  uncompressed
       TIFF.   The  distance parameter of the Delta filter is set to match the
       number of bytes per pixel in the image.  E.g. 24-bit RGB  bitmap  needs
       dist=3,  and  it  is also good to pass pb=0 to LZMA2 to accommodate the
       three-byte alignment:

              xz --delta=dist=3 --lzma2=pb=0 foo.tiff

       If multiple images have been put into a single archive (e.g. .tar), the
       Delta  filter will work on that too as long as all images have the same
       number of bytes per pixel.

SEE ALSO
       xzdec(1),  xzdiff(1),   xzgrep(1),   xzless(1),   xzmore(1),   gzip(1),
       bzip2(1), 7z(1)

       XZ Utils: <http://tukaani.org/xz/>
       XZ Embedded: <http://tukaani.org/xz/embedded.html>
       LZMA SDK: <http://7-zip.org/sdk.html>



Tukaani                           2012-05-27                             XZ(1)
