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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 also the legacy .lzma format	 and  raw  compressed  streams
       with no container format headers are 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 com‐
       pressed 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, thus
	  they are never considered to be regular files.

       ·  File has more than one hardlink.

       ·  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 permis‐
       sions 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 affect also the memory usage	of  the	 decompressor.
       Typically  the decompressor needs only 5 % to 20 % of the amount of RAM
       that the compressor needed when creating the file.  Still,  the	worst-
       case memory usage of the decompressor is several gigabytes.

       To  prevent uncomfortable surprises caused by huge memory usage, xz has
       a built-in memory usage limiter. The default limit is  40  %  of	 total
       physical	 RAM. While operating systems provide ways to limit the memory
       usage of processes, relying on it wasn't deemed to be flexible enough.

       When compressing, if the selected compression settings exceed the  mem‐
       ory  usage limit, the settings are automatically adjusted downwards and
       a notice about this is displayed. As an exception, if the memory	 usage
       limit  is exceeded when compressing with --format=raw, an error is dis‐
       played and xz will exit with exit status 1.

       If source file cannot be	 decompressed  without	exceeding  the	memory
       usage  limit,  an  error	 message is displayed and the file is skipped.
       Note that compressed files may contain many blocks, which may have been
       compressed  with	 different  settings.  Typically  all blocks will have
       roughly the same memory requirements, but it is possible that  a	 block
       later  in  the  file  will  exceed the memory usage limit, and an error
       about too low memory usage limit gets displayed	after  some  data  has
       already been decompressed.

       The  absolute  value  of the active memory usage limit can be seen near
       the bottom of the output of --long-help.	  The  default	limit  can  be
       overriden with --memory=limit.

   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.

       k or kB
	      The  integer  is	multiplied by 1,000 (10^3). For example, 5k or
	      5kB equals 5000.

       Ki or KiB
	      The integer is multiplied by 1,024 (2^10).

       M or MB
	      The integer is multiplied by 1,000,000 (10^6).

       Mi or MiB
	      The integer is multiplied by 1,048,576 (2^20).

       G or GB
	      The integer is multiplied by 1,000,000,000 (10^9).

       Gi or GiB
	      The integer is multiplied by 1,073,741,824 (2^30).

       A 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.  No files are created or
	      removed.	This  option  is  equivalent  to --decompress --stdout
	      except that the decompressed data is discarded instead of	 being
	      written to standard output.

       -l, --list
	      View  information	 about	the  compressed files. No uncompressed
	      output 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.

	      This feature has not been implemented yet.

   Operation modifiers
       -k, --keep
	      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 not a regular
		 file, has more than one hardlink, or has setuid,  setgid,  or
		 sticky	 bit set.  The setuid, setgid, and sticky bits are not
		 copied to the target file.

	      ·	 If combined with --decompress --stdout and xz doesn't	recog‐
		 nize  the  type  of  the source file, xz will copy the source
		 file as is  to	 standard  output.  This  allows  using	 xzcat
		 --force  like	cat(1) for files that have not been compressed
		 with xz.  Note that in future,	 xz  might  support  new  com‐
		 pressed 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 format.

	      ·	 Allow writing compressed data to a terminal, and reading com‐
		 pressed data from a terminal.

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

       -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 also 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 termi‐
	      nated with the newline character. 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	the  filenames
	      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  autodetected.  Note that raw streams (created with --for‐
		 mat=raw) cannot be autodetected.

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

	      ·	 lzma  or  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 to set not only --format=raw but also specify the filter
		 chain, which would normally be stored in the container format

       -C check, --check=check
	      Specify the type of the integrity	 check,	 which	is  calculated
	      from  the uncompressed data. This option has an effect only when
	      compressing into the .xz format; the .lzma format	 doesn't  sup‐
	      port integrity checks.  The integrity check (if any) is verified
	      when the .xz file is decompressed.

	      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 negli‐

	      ·	 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 compression preset. If a preset level is specified multi‐
	      ple times, the last one takes effect.

	      The  compression	preset	levels can be categorised roughly into
	      three categories:

	      -0 ... -2
		     Fast presets with relatively low memory usage.  -1 and -2
		     should  give  compression	speed and ratios comparable to
		     bzip2 -1 and bzip2 -9, respectively.  Currently -0 is not
		     very  good	 (not  much faster than -1 but much worse com‐
		     pression). In future, -0 may be indicate some fast	 algo‐
		     rithm instead of LZMA2.

	      -3 ... -5
		     Good  compression	ratio with low to medium memory usage.
		     These are significantly slower than levels 0-2.

	      -6 ... -9
		     Excellent compression with medium to high	memory	usage.
		     These  are	 also slower than the lower preset levels. The
		     default is -6.  Unless you want to maximize the  compres‐
		     sion ratio, you probably don't want a higher preset level
		     than -7 due to speed and memory usage.

	      The exact compression settings (filter chain) used by each  pre‐
	      set  may	vary  between  xz versions. The settings may also vary
	      between files being compressed, if xz determines	that  modified
	      settings	will  probably	give  better compression ratio without
	      significantly affecting compression time or memory usage.

	      Because the settings may vary, the memory usage  may  vary  too.
	      The  following table lists the maximum memory usage of each pre‐
	      set level, which won't be exceeded even in  future  versions  of

	      FIXME: The table below is just a rough idea.

		     Preset   Compression   Decompression
		       -0	  6 MiB		1 MiB
		       -1	  6 MiB		1 MiB
		       -2	 10 MiB		1 MiB
		       -3	 20 MiB		2 MiB
		       -4	 30 MiB		3 MiB
		       -5	 60 MiB		6 MiB
		       -6	100 MiB	       10 MiB
		       -7	200 MiB	       20 MiB
		       -8	400 MiB	       40 MiB
		       -9	800 MiB	       80 MiB

	      When  compressing, xz automatically adjusts the compression set‐
	      tings downwards if the memory usage limit would be exceeded,  so
	      it  is  safe to specify a high preset level even on systems that
	      don't have lots of RAM.

       --fast and --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.

	      Especially the name of --best is misleading, because the defini‐
	      tion  of best depends on the input data, and that usually people
	      don't want the very best compression ratio  anyway,  because  it
	      would be very slow.

       -e, --extreme
	      Modify  the  compression preset (-0 ... -9) so that a little bit
	      better compression ratio can be achieved without increasing mem‐
	      ory usage of the compressor or decompressor (exception: compres‐
	      sor memory usage may increase a little with presets -0 ...  -2).
	      The downside is that the compression time will increase dramati‐
	      cally (it can easily double).

       -M limit, --memory=limit
	      Set the memory usage limit. If this option is  specied  multiple
	      times,  the last one takes effect. The limit can be specified in
	      multiple ways:

	      ·	 The limit can be an absolute value in bytes. Using an integer
		 suffix like MiB can be useful. Example: --memory=80MiB

	      ·	 The  limit  can be specified as a percentage of physical RAM.
		 Example: --memory=70%

	      ·	 The limit can be reset back to its default  value  (currently
		 40 % of physical RAM) by setting it to 0.

	      ·	 The memory usage limiting can be effectively disabled by set‐
		 ting limit to max.  This isn't recommended. It's usually bet‐
		 ter to use, for example, --memory=90%.

	      The  current  limit can be seen near the bottom of the output of
	      the --long-help option.

       -T threads, --threads=threads
	      Specify the maximum number of worker threads to use. The default
	      is  the  number  of available CPU cores. You can see the current
	      value of threads near the end of the output of  the  --long-help

	      The  actual number of worker threads can be less than threads if
	      using more threads would exceed  the  memory  usage  limit.   In
	      addition	to CPU-intensive worker threads, xz may use a few aux‐
	      iliary threads, which don't use a lot of CPU time.

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

   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 UN*X command line.	  When
       compressing,  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

       Many  filters  have  limitations 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 filter 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

		     The default preset is 6, from which  the  default	values
		     for the rest of the LZMA1 or LZMA2 options are taken.

		     Dictionary (history buffer) size indicates how many bytes
		     of the recently processed uncompressed data  is  kept  in
		     memory.  One  method  to  reduce size of the uncompressed
		     data is to store distance-length  pairs,  which  indicate
		     what  data to repeat from the dictionary buffer. The big‐
		     ger the dictionary, the better the compression ratio usu‐
		     ally  is,	but  dictionaries bigger than the uncompressed
		     data are waste of RAM.

		     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. The decompressor already supports dictio‐
		     naries up to one byte less than 4 GiB, which is the maxi‐
		     mum for LZMA1 and LZMA2 stream formats.

		     Dictionary size has the  biggest  effect  on  compression
		     ratio.   Dictionary size and match finder together deter‐
		     mine the memory usage of the LZMA1 or LZMA2 encoder.  The
		     same  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

	      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.

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

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

		     Compression mode specifies the function used  to  analyze
		     the  data	produced by the match finder.  Supported modes
		     are fast and normal.  The default is fast for presets 0-2
		     and normal for presets 3-9.

	      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. Hash
		     Chains are	 usually  used	together  with	mode=fast  and
		     Binary Trees with mode=normal.  The memory usage formulas
		     are only rough estimates, which are  closest  to  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

		     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

		     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 expense of  speed.  The  default
		     depends on the preset level.

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

		     Using  very  high	values	for depth can make the encoder
		     extremely slow with carefully crafted files.  Avoid  set‐
		     ting  the	depth  over  1000  unless  you are prepared to
		     interrupt the compression in case it is taking too long.

	      When decoding raw streams (--format=raw), LZMA2 needs  only  the
	      value of dict.  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 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 allows  e.g.  LZMA2  to
	      get better compression ratio.

	      The BCJ filters are always reversible, so using a BCJ filter for
	      wrong type of data doesn't cause any data loss. However,	apply‐
	      ing  a  BCJ filter for wrong type of data is a bad idea, because
	      it tends to make the compression ratio worse.

	      Different instruction sets have have different alignment:

		     Filter	 Alignment   Notes
		     x86	     1	     32-bit and 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.

		     Specifying	 a  non-zero  start  offset is probably useful
		     only if the executable has multiple sections,  and	 there
		     are  many	cross-section  jumps  or calls. Applying a BCJ
		     filter separately for each section with proper start off‐
		     set and then compressing the result as a single chunk may
		     give some improvement in compression  ratio  compared  to
		     applying  the  BCJ filter with the default offset for the
		     whole executable.

	      Add Delta filter to the filter chain. The Delta  filter  can  be
	      used only as 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 algo‐
	      rithms may give significantly better results than Delta + LZMA2.
	      This  is true especially with audio, which compresses faster and
	      better e.g. with FLAC.

	      Supported options:

		     Specify the distance of the delta calculation  as	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 (useful mostly for debug‐

       -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.

       -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.

       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_OPT A	 space-separated  list of options is parsed from XZ_OPT before
	      parsing the options given on the command line.  Note  that  only
	      options  are  parsed  from  XZ_OPT; all non-options are silently
	      ignored. Parsing is done with getopt_long(3) which is used  also
	      for the command line arguments.

       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 existing
       scripts. There are some incompatibilities though, which	may  sometimes
       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
	       -1      64 KiB	  64 KiB
	       -2     512 KiB	   1 MiB
	       -3	1 MiB	 512 KiB
	       -4	2 MiB	   1 MiB
	       -5	4 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
	       -1	2 MiB	       2 MiB
	       -2	5 MiB	      12 MiB
	       -3      13 MiB	      12 MiB
	       -4      25 MiB	      16 MiB
	       -5      48 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 8 MiB dictionary by default.

   Streamed vs. non-streamed .lzma files
       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 exam‐
       ple 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 dictionary size  of  2^n
       (a  power  of  2), but accepts files with any dictionary size.  liblzma
       accepts only .lzma files which have dictionary size of  2^n  or	2^n  +
       2^(n-1).	  This is to decrease false positives when autodetecting .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 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.

   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  differ‐
       ent build options are used or if the endianness of the hardware is dif‐
       ferent for different builds.

       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  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‐

       xzdec(1), gzip(1), bzip2(1)

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

Tukaani				  2009-08-27				 XZ(1)

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