xz(1) 맨 페이지 - 윈디하나의 솔라나라


맨 페이지 이름


XZ(1)                              XZ Utils                              XZ(1)

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

       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.

       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:

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

       ·  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:

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

       ·  File has more than one hard link.

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

       ·  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‐

       ·  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

       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

   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

       -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

       -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

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

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

              ·  If the target file already exists, delete it before compress‐
                 ing or decompressing.

              ·  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.

              ·  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‐

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

              Decompress only the first .xz stream, and silently ignore possi‐
              ble  remaining  input  data following the stream.  Normally such
              trailing garbage makes xz display an error.

              xz never decompresses more than one stream from .lzma  files  or
              raw  streams, but this option still makes xz ignore the possible
              trailing data after the .lzma file or raw stream.

              This option has no effect if the operation mode is not  --decom‐
              press or --test.

              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.

              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.

              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

              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‐

              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

              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‐

              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.

              Don't verify the integrity check of  the  compressed  data  when
              decompressing.   The  CRC32 values in the .xz headers will still
              be verified normally.

              Do not use this option unless you know what you are doing.  Pos‐
              sible reasons to use this option:

              ·  Trying to recover data from a corrupt .xz file.

              ·  Speeding  up decompression.  This matters mostly with SHA-256
                 or with files that have compressed extremely well.  It's rec‐
                 ommended  to  not use this option for this purpose unless the
                 file integrity is verified externally in some other way.

       -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,

              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:

                     tab(;);  c  c  c  c  c  n  n n n n.  Preset;DictSize;Com‐
                     pCPU;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:

              ·  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‐

              ·  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.

              ·  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.

              ·  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 slightly more than the LZMA2  dictionary  size,  but
                 the values in the table have been rounded up to the next full

       -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.

                     tab(;);  c  c  c  c  c  n  n n n n.  Preset;DictSize;Com‐
                     pCPU;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.

       --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.

              When  compressing  to  the .xz format, split the input data into
              blocks of size bytes.  The blocks are  compressed  independently
              from each other, which helps with multi-threading and makes lim‐
              ited random-access decompression possible.  This option is typi‐
              cally  used to override the default block size in multi-threaded
              mode, but this option can be used in single-threaded mode too.

              In multi-threaded mode about three  times  size  bytes  will  be
              allocated  in  each  thread for buffering input and output.  The
              default size is three times the LZMA2 dictionary size or 1  MiB,
              whichever is more.  Typically a good value is 2-4 times the size
              of the LZMA2 dictionary or at least 1 MiB.  Using size less than
              the LZMA2 dictionary size is waste of RAM because then the LZMA2
              dictionary buffer will never get fully used.  The sizes  of  the
              blocks  are  stored in the block headers, which a future version
              of xz will use for multi-threaded decompression.

              In single-threaded mode no block splitting is done  by  default.
              Setting this option doesn't affect memory usage.  No size infor‐
              mation is stored in block headers, thus files created in single-
              threaded  mode  won't  be  identical  to files created in multi-
              threaded mode.  The lack of size information also means  that  a
              future  version  of  xz  won't  be  able decompress the files in
              multi-threaded mode.

              When compressing to the .xz format, start a new block after  the
              given intervals of uncompressed data.

              The  uncompressed  sizes of the blocks are specified as a comma-
              separated list.  Omitting a size (two or more  consecutive  com‐
              mas) is a shorthand to use the size of the previous block.

              If  the  input  file  is  bigger than the sum of sizes, the last
              value in sizes is repeated until the end of the file.  A special
              value  of  0  may be used as the last value to indicate that the
              rest of the file should be encoded as a single block.

              If one specifies sizes that  exceed  the  encoder's  block  size
              (either  the  default value in threaded mode or the value speci‐
              fied with --block-size=size), the encoder will create additional
              blocks  while  keeping  the  boundaries specified in sizes.  For
              example,      if      one      specifies      --block-size=10MiB
              --block-list=5MiB,10MiB,8MiB,12MiB,24MiB  and  the input file is
              80 MiB, one will get 11 blocks: 5, 10, 8, 10, 2, 10, 10, 4,  10,
              10, and 1 MiB.

              In multi-threaded mode the sizes of the blocks are stored in the
              block headers.  This isn't done in single-threaded mode, so  the
              encoded  output won't be identical to that of the multi-threaded

              When compressing, if more than timeout milliseconds (a  positive
              integer)  has  passed  since the previous flush and reading more
              input would block, all the pending input data  is  flushed  from
              the  encoder  and made available in the output stream.  This can
              be useful if xz is used to compress data that is streamed over a
              network.   Small  timeout  values make the data available at the
              receiving end with a small delay, but large timeout values  give
              better compression ratio.

              This  feature  is disabled by default.  If this option is speci‐
              fied more than once, the last one  takes  effect.   The  special
              timeout  value  of 0 can be used to explicitly disable this fea‐

              This feature is not available on non-POSIX systems.

              This feature is still experimental.  Currently xz is  unsuitable
              for  decompressing  the  stream  in real time due to how xz does

              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:

              ·  The limit can be an absolute value in bytes.  Using an  inte‐
                 ger  suffix like MiB can be useful.  Example: --memlimit-com‐

              ·  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%

              ·  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.

              For 32-bit xz there is a special case: if  the  limit  would  be
              over  4020 MiB, the limit is set to 4020 MiB.  (The values 0 and
              max aren't affected by this.  A similar  feature  doesn't  exist
              for  decompression.)   This  can  be  helpful when a 32-bit exe‐
              cutable has access to 4 GiB address space while hopefully  doing
              no harm in other situations.

              See also the section Memory usage.

              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

              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‐

       -T threads, --threads=threads
              Specify the number of worker threads to use.  Setting threads to
              a  special value 0 makes xz use as many threads as there are CPU
              cores on the system.  The actual number of threads can  be  less
              than  threads  if the input file is not big enough for threading
              with the given settings or if using more  threads  would  exceed
              the memory usage limit.

              Currently  the  only threading method is to split the input into
              blocks and compress them independently  from  each  other.   The
              default  block  size depends on the compression level and can be
              overridden with the --block-size=size option.

              Threaded decompression hasn't been  implemented  yet.   It  will
              only work on files that contain multiple blocks with size infor‐
              mation in block headers.  All files compressed in multi-threaded
              mode  meet  this  condition,  but  files  compressed  in single-
              threaded mode don't even if --block-size=size is used.

   Custom compressor filter chains
       A custom filter chain allows specifying  the  compression  settings  in
       detail  instead  of  relying on the settings associated to the presets.
       When a custom filter chain is specified, preset options (-0 ... -9  and
       --extreme)  earlier  on  the  command  line are forgotten.  If a preset
       option is specified after one or more custom filter chain options,  the
       new  preset  takes effect and the custom filter chain options specified
       earlier are forgotten.

       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‐

       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.

       To  see  the  whole  filter chain and options, use xz -vv (that is, use
       --verbose twice).  This works also for viewing the filter chain options
       used by presets.

              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:

                     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.  If no preset is specified, the default values
                     of LZMA1 or LZMA2 options are taken from the preset 6.

                     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

                     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)

                     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.

                     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.

                     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

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

              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

              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‐

              These BCJ filters have known problems related to the compression

              ·  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.

              ·  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 different alignment:

                     tab(;); l n l l n l.  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:

                     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.

              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:

                     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:

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

              ·  Amount  of compressed data produced (compressing) or consumed

              ·  Amount of uncompressed data consumed  (compressing)  or  pro‐
                 duced (decompressing).

              ·  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.

              ·  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.

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

              ·  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.

              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.

              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

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

       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 compression and decompression in the future.

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


       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

       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.

              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

       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
              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
              6.  Uncompressed start offset relative to the beginning  of  the
              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
              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
              Since xz 5.1.2alpha:
              4.  Minimum xz version required to decompress the file

       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
              Since xz 5.1.2alpha:
              12. Minimum xz version required to decompress the file

       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.

       0      All is good.

       1      An error occurred.

       2      Something  worth  a  warning  occurred,  but  no  actual  errors

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

       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

              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_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:

                     export XZ_OPT

       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.

              tab(;);  c  c  c c n n.  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:

              tab(;); c c c c n n.  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

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

   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

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

   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 once --rsyncable has been implemented, the result‐
       ing files won't necessarily be rsyncable unless both old and new  files
       have  been  compressed  with  the same xz version.  This problem can be
       fixed if a part of the encoder implementation is frozen to keep rsynca‐
       ble output stable across xz versions.

   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‐

       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."
              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

   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:

              tab(;); c c n n.  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.

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

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

Tukaani                           2020-02-01                             XZ(1)
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