     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

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

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

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

          xz compresses or decompresses each file according to the selected
          operation 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 standard 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
             filename.

          o  When decompressing, the .xz or .lzma suffix is removed from the
             filename to get the target filename.  xz also recognizes the
             suffixes .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:

                                        - 1 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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
             format).

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

          After successfully compressing or decompressing the file, xz copies
          the owner, group, permissions, access time, and modification time from
          the source file to the target file.  If copying the group fails, the
          permissions 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
          automatically 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
          decompressor.  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)).

                                        - 2 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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
          separately for compression and decompression by using
          --memlimit-compress=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
          --format=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
          useful e.g. if the .xz file is stored on a medium that measures file
          sizes in 512-byte blocks.

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

     OPTIONS
        Integer suffixes and special values
          In most places where an integer argument is expected, an optional
          suffix 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.

                                        - 3 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

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

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

          -t, --test
               Test the integrity of compressed files.  This option is
               equivalent 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 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.

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

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

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

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

               o  If the target file already exists, delete it before
                  compressing or decompressing.

                                        - 4 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               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 format.

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

          --single-stream
               Decompress only the first .xz stream, and silently ignore
               possible remaining input data following the stream.  Normally
               such trailing garbage makes xz display an error.

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

               This option has no effect if the operation mode is not
               --decompress or --test.

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

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

                                        - 5 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               the target filename.

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

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

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

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

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

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

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

               raw  Compress or uncompress a raw stream (no headers).  This is
                    meant for advanced users only.  To decode raw streams, you
                    need use --format=raw and explicitly specify the filter
                    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 calculated
               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 decompressed.

               Supported check types:

                                        - 6 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

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

               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.

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

               Do not use this option unless you know what you are doing.
               Possible reasons to use this option:

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

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

          -0 ... -9
               Select a compression preset level.  The default is -6.  If
               multiple preset levels are specified, the last one takes effect.
               If a custom filter chain was already specified, setting a
               compression 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 decompress 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).

                                        - 7 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               -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 comparable 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
                    distributing 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
                    decompressor memory requirements.  These are useful only
                    when compressing files bigger than 8 MiB, 16 MiB, and
                    32 MiB, respectively.

               On the same hardware, the decompression speed is approximately a
               constant number of bytes of compressed data per second.  In other
               words, the better the compression, the faster the decompression
               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
                  matter.

                                        - 8 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               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
                  versions.  Memory requirements of some of the future
                  multithreaded 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 requirements
                  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 MiB.

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

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

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

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

          --fast
          --best
               These are somewhat misleading aliases for -0 and -9,

                                        - 9 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               respectively.  These are provided only for backwards
               compatibility with LZMA Utils.  Avoid using these options.

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

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

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

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

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

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

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

                                       - 10 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               MiB, one will get 11 blocks: 5, 10, 8, 10, 2, 10, 10, 4, 10, 10,
               and 1 MiB.

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

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

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

               This feature is not available on non-POSIX systems.

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

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

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

               The limit can be specified in multiple ways:

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

               o  The limit can be specified as a percentage of total physical
                  memory (RAM).  This can be useful especially when setting the
                  XZ_DEFAULTS environment variable in a shell initialization
                  script that is shared between different computers.  That way

                                       - 11 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

                  the limit is automatically bigger on systems with more memory.
                  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.

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

               See also the section Memory usage.

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

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

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

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

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

                                       - 12 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

          A filter chain is comparable to piping on the command line.  When
          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 filters.

          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 filter 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
          (--format=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.

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

               LZMA1 is a legacy filter, which is supported almost solely due to
               the legacy .lzma file format, which supports only LZMA1.  LZMA2

                                       - 13 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               is an updated version of LZMA1 to fix some practical issues of
               LZMA1.  The .xz format uses LZMA2 and doesn't support LZMA1 at
               all.  Compression speed and ratios of LZMA1 and LZMA2 are
               practically the same.

               LZMA1 and LZMA2 share the same set of options:

               preset=preset
                    Reset all LZMA1 or LZMA2 options to preset.  Preset consist
                    of an integer, which may be followed by single-letter 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.

               dict=size
                    Dictionary (history buffer) size indicates how many bytes of
                    the recently processed uncompressed data is kept in memory.
                    The algorithm tries to find repeating byte sequences
                    (matches) in the uncompressed data, and replace them with
                    references to the data currently in the dictionary.  The
                    bigger the dictionary, the higher is the chance to find a
                    match.  Thus, increasing dictionary size usually improves
                    compression ratio, but a dictionary bigger 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 currently
                    1.5 GiB (1536 MiB).  The decompressor already supports
                    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 addition, the
                    sum of lc and lp must not exceed 4.

                    All bytes that cannot be encoded as matches are encoded as

                                       - 14 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

                    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 uncompressed data.

                    The default value (3) is usually good.  If you want maximum
                    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 having
                    one-byte alignment (US-ASCII, ISO-8859-*, UTF-8), setting
                    pb=0 can improve compression slightly.  For UTF-16 text,
                    pb=1 is a good choice.  If the alignment is an odd number
                    like 3 bytes, pb=0 might be the best choice.

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

               mf=mf

                                       - 15 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

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

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

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

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

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

               mode=mode
                    Compression mode specifies the method to analyze the data
                    produced by the match finder.  Supported modes are fast and
                    normal.  The default is fast for presets 0-3 and normal for
                    presets 4-9.

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

               nice=nice

                                       - 16 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

                    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 better
                    compression ratio at the expense of speed.  The default
                    depends on the preset.

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

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

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

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

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

               It is fine to apply a BCJ filter on a whole executable; there's
               no need to apply it only on the executable section.  Applying a
               BCJ filter on an archive that contains both executable and non-
               executable files may or may not give good results, so it
               generally 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

                                       - 17 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               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 second.

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

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

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

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

               Different instruction sets have different alignment:

                    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 filter.  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 usually good to
               stick to LZMA2's default four-byte alignment when compressing x86
               executables.

               All BCJ filters support the same options:

               start=offset
                    Specify the start offset that is used when converting
                    between relative and absolute addresses.  The offset must be
                    a multiple of the alignment of the filter (see the table

                                       - 18 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

                    above).  The default is zero.  In practice, the default is
                    good; specifying a custom offset is almost never useful.

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

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

               Supported options:

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

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

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

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

               The progress indicator shows the following information:

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

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

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

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

                                       - 19 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               o  Compression or decompression speed.  This is measured as the
                  amount of uncompressed data consumed (compression) or produced
                  (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,
               compression 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 percentage 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
               warning was detected.  This option doesn't affect the verbosity
               level, thus both --quiet and --no-warn have to be used to not
               display warnings and to not alter the exit status.

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

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

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

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

                                       - 20 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

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

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

          XZ_VERSION=XYYYZZZS
          LIBLZMA_VERSION=XYYYZZZS

          X    Major version.

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

          ZZZ  Patch level for stable releases or just a counter for development
               releases.

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

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

          Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.

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

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

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

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

          In the future, the output of xz --robot --info-memory may have more

                                       - 21 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          columns, but never more than a single line.

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

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

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

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

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

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

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

          The columns of the file lines:
               2.  Number of streams in the file
               3.  Total number of blocks in the stream(s)
               4.  Compressed size of the file
               5.  Uncompressed size of the file
               6.  Compression ratio, for example 0.123.  If ratio is over
                   9.999, three dashes (---) are displayed instead of the ratio.
               7.  Comma-separated list of integrity check names.  The following
                   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

                                       - 22 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

               4.  Compressed start offset
               5.  Uncompressed start offset
               6.  Compressed size (does not include stream padding)
               7.  Uncompressed size
               8.  Compression ratio
               9.  Name of the integrity check
               10. Size of stream padding

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

          If --verbose was specified twice, additional columns are included on
          the block lines.  These are not displayed with a single --verbose,
          because getting this information requires many seeks and can thus be
          slow:
               11. Value of the integrity check in hexadecimal
               12. Block header size
               13. Block flags: c indicates that compressed size is present, and
                   u indicates that uncompressed size is present.  If the flag
                   is not set, a dash (-) is shown instead to keep the string
                   length fixed.  New flags may be added to the end of the
                   string in the future.
               14. Size of the actual compressed data in the block (this
                   excludes the block header, block padding, and check fields)
               15. Amount of memory (in bytes) required to decompress this block
                   with this xz version
               16. Filter chain.  Note that most of the options used at
                   compression 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
                   compressed size and uncompressed size stored in them
               Since xz 5.1.2alpha:
               4.  Minimum xz version required to decompress the file

                                       - 23 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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
                   earlier 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
                   compressed size and uncompressed size stored in them
               Since xz 5.1.2alpha:
               12. Minimum xz version required to decompress the file

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

     EXIT STATUS
          0    All is good.

          1    An error occurred.

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

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

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

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

                                       - 24 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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
               compression options.  It is still recommended to allow users to
               override XZ_OPT if that is reasonable, e.g. in sh(1) scripts one
               may use something like this:

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

     LZMA UTILS COMPATIBILITY
          The command line syntax of xz is practically a superset of lzma,
          unlzma, and lzcat as found from LZMA Utils 4.32.x.  In most cases, it
          is possible to replace LZMA Utils with XZ Utils without breaking
          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
                -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

                                       - 25 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

                -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
          example, a .lzma decompressor in an embedded device might work only
          with files that have known uncompressed size.  If you hit this
          problem, you need to use LZMA Utils or LZMA SDK to create .lzma files
          with known uncompressed size.

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

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

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

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

        Trailing garbage
          When decompressing, LZMA Utils silently ignore everything after the

                                       - 26 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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 unless --single-stream was used.  This may break
          obscure scripts which have assumed that trailing garbage is ignored.

     NOTES
        Compressed output may vary
          The exact compressed output produced from the same uncompressed input
          file may vary between XZ Utils versions even if compression options
          are identical.  This is because the encoder can be improved (faster or
          better 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
          resulting 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
          rsyncable output stable across xz versions.

        Embedded .xz decompressors
          Embedded .xz decompressor implementations like XZ Embedded don't
          necessarily 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
          verifying the integrity check if the particular check is not
          supported.

          XZ Embedded supports BCJ filters, but only with the default start
          offset.

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

               xz foo

          Decompress bar.xz into bar and don't remove bar.xz even if
          decompression is successful:

               xz -dk bar.xz

                                       - 27 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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
          standard 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
          compression 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
          following sh(1) script checks that the version number of the xz tool
          is at least 5.0.0.  This method is compatible with old beta versions,
          which didn't support the --robot option:

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

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

               NEWLIM=$((123 << 20))  # 123 MiB

                                       - 28 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

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

        Custom compressor filter chains
          The simplest use for custom filter chains is customizing a LZMA2
          preset.  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

                                       - 29 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          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 uncompressed file is waste of memory, so the above command
          isn't useful for small files.

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

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

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

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

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

               xz --x86 --lzma2 libfoo.so

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

          The Delta filter together with LZMA2 can give good results with bitmap
          images.  It should usually beat PNG, which has a few more advanced
          filters 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),

                                       - 30 -      Formatted:  November 12, 2020

     XZ(1)                             Tukaani                             XZ(1)
     XZ Utils                                                           XZ Utils

                                     2020-02-01

          the Delta filter will work on that too as long as all images have the
          same number of bytes per pixel.

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

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

                                       - 31 -      Formatted:  November 12, 2020