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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 the legacy .lzma format used	by LZMA	 Utils	and  raw  com-
       pressed streams with no container format	headers	are also supported.

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

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

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

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

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

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

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

       o  File has more	than one hard link.

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

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

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

       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:

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

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

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

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

	      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.

       -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:

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

	      Column descriptions:

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

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

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

	      o	 DecMem	contains the decompressor memory  requirements.	  That
		 is,  the  compression	settings determine the memory require-
		 ments of the decompressor.   The  exact  decompressor	memory
		 usage	is  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.

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

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

       --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.

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

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

	      The limit	can be specified in multiple ways:

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

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

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

	      See also the section Memory usage.

	      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.  The actual	number
	      of threads can be	less than threads if using more	threads	 would
	      exceed the memory	usage limit.

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

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

   Custom compressor filter chains
       A custom	filter chain allows specifying	the  compression  settings  in
       detail  instead	of  relying on the settings associated to the 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

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

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

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

	      Different	instruction sets have have different alignment:

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

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

	      All BCJ filters support the same options:

		     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:

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

	      o	 Amount	 of compressed data produced (compressing) or consumed

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

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

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

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

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

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

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

	      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

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

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

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

       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.

	      Level	xz	LZMA Utils
	       -0     256 KiB	   N/A
	       -1	1 MiB	  64 KiB
	       -2	2 MiB	   1 MiB
	       -3	4 MiB	 512 KiB
	       -4	4 MiB	   1 MiB
	       -5	8 MiB	   2 MiB
	       -6	8 MiB	   4 MiB
	       -7      16 MiB	   8 MiB
	       -8      32 MiB	  16 MiB
	       -9      64 MiB	  32 MiB

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

	      Level	xz	LZMA Utils 4.32.x
	       -0	3 MiB	       N/A
	       -1	9 MiB	       2 MiB
	       -2      17 MiB	      12 MiB

	       -3      32 MiB	      12 MiB
	       -4      48 MiB	      16 MiB
	       -5      94 MiB	      26 MiB
	       -6      94 MiB	      45 MiB
	       -7     186 MiB	      83 MiB
	       -8     370 MiB	     159 MiB
	       -9     674 MiB	     311 MiB

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

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

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

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

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

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

       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.  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."

       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:

	      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

       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: <>
       XZ Embedded: <>
       LZMA SDK: <>

Tukaani				  2013-06-21				 XZ(1)


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