XZ(1) XZ Utils XZ(1)

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. In addition, decompression of the .lz format used by lzip
is 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, .lzma, or .lz 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:

+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,
.tlz, or .lz).

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 or if an error
occurs.

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 (for
example, 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, for
example, 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, for
example, 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, for example, 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.

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, for
example, 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.


Since xz 5.2.6, this option also makes xz 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. In earlier versions this was
only done with --force.

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

+o If the target file already exists, delete it before
compressing 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 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, .tlz, or .lz
suffix. If the source file has the suffix .suf, the suffix is
removed to get the target filename.


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

--files[=file]
Read the filenames to process from file; if file is omitted,
filenames are read from standard input. Filenames must be
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.

lzip Accept only .lz files when decompressing. Compression
is not supported.


The .lz format version 0 and the unextended version 1
are supported. Version 0 files were produced by lzip
1.3 and older. Such files aren't common but may be
found from file archives as a few source packages were
released in this format. People might have old
personal files in this format too. Decompression
support for the format version 0 was removed in lzip
1.18.


lzip 1.4 and later create files in the format version
1. The sync flush marker extension to the format
version 1 was added in lzip 1.6. This extension is
rarely used and isn't supported by xz (diagnosed as
corrupt input).

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:

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).

-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 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.

+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,
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 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 attempt
to adjust the settings downwards so that the limit is no
longer exceeded and display a notice that automatic adjustment
was done. The adjustments are done in this order: reducing
the number of threads, switching to single-threaded mode if
even one thread in multi-threaded mode exceeds the limit, and
finally reducing the LZMA2 dictionary size.


When compressing with --format=raw or if --no-adjust has been
specified, only the number of threads may be reduced since it
can be done without affecting the compressed output.


If the limit cannot be met even with the adjustments described
above, 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 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).


For 32-bit xz there is a special case: if the limit would be
over 4020 MiB, the limit is set to 4020 MiB. On MIPS32
2000 MiB is used instead. (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 (2 GiB on MIPS32) 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.

--memlimit-mt-decompress=limit
Set a memory usage limit for multi-threaded decompression.
This can only affect the number of threads; this will never
make xz refuse to decompress a file. If limit is too low to
allow any multi-threading, the limit is ignored and xz will
continue in single-threaded mode. Note that if also
--memlimit-decompress is used, it will always apply to both
single-threaded and multi-threaded modes, and so the effective
limit for multi-threading will never be higher than the limit
set with --memlimit-decompress.


In contrast to the other memory usage limit options,
--memlimit-mt-decompress=limit has a system-specific default
limit. xz --info-memory can be used to see the current value.


This option and its default value exist because without any
limit the threaded decompressor could end up allocating an
insane amount of memory with some input files. If the default
limit is too low on your system, feel free to increase the
limit but never set it to a value larger than the amount of
usable RAM as with appropriate input files xz will attempt to
use that amount of memory even with a low number of threads.
Running out of memory or swapping will not improve
decompression performance.


See --memlimit-compress=limit for possible ways to specify the
limit. Setting limit to 0 resets the limit to the default
system-specific value.

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

--no-adjust
Display an error and exit if the memory usage limit cannot be
met without adjusting settings that affect the compressed
output. That is, this prevents xz from switching the encoder
from multi-threaded mode to single-threaded mode and from
reducing the LZMA2 dictionary size. Even when this option is
used the number of threads may be reduced to meet the memory
usage limit as that won't affect the compressed output.


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 up to as many threads as the
processor(s) on the system support. The actual number of
threads can be fewer 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.


The single-threaded and multi-threaded compressors produce
different output. Single-threaded compressor will give the
smallest file size but only the output from the multi-threaded
compressor can be decompressed using multiple threads.
Setting threads to 1 will use the single-threaded mode.
Setting threads to any other value, including 0, will use the
multi-threaded compressor even if the system supports only one
hardware thread. (xz 5.2.x used single-threaded mode in this
situation.)


To use multi-threaded mode with only one thread, set threads
to +1. The + prefix has no effect with values other than 1.
A memory usage limit can still make xz switch to single-
threaded mode unless --no-adjust is used. Support for the +
prefix was added in xz 5.4.0.


If an automatic number of threads has been requested and no
memory usage limit has been specified, then a system-specific
default soft limit will be used to possibly limit the number
of threads. It is a soft limit in sense that it is ignored if
the number of threads becomes one, thus a soft limit will
never stop xz from compressing or decompressing. This default
soft limit will not make xz switch from multi-threaded mode to
single-threaded mode. The active limits can be seen with xz
--info-memory.


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


Threaded decompression only works on files that contain
multiple blocks with size information in block headers. All
large enough files compressed in multi-threaded mode meet this
condition, but files compressed in single-threaded mode don't
even if --block-size=size has been 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 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 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. For example, 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 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 alignment is known, setting pb accordingly may
reduce the file size a little. For example, 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 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
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]
--arm[=options]
--armthumb[=options]
--arm64[=options]
--powerpc[=options]
--ia64[=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. The BCJ filters are very
fast and use an insignificant amount of memory.


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

+o Some types of files containing executable code (for
example, 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 If a BCJ filter is applied on an archive, it is possible
that it makes the compression ratio worse than not using a
BCJ filter. For example, if there are similar or even
identical executables then filtering will likely make the
files less similar and thus compression is worse. The
contents of non-executable files in the same archive can
matter too. In practice one has to try with and without a
BCJ filter to see which is better in each situation.


Different instruction sets have different alignment: the
executable file must be aligned to a multiple of this value in
the input data to make the filter work.

Filter Alignment Notes
x86 1 32-bit or 64-bit x86
ARM 4
ARM-Thumb 2
ARM64 4 4096-byte alignment is best
PowerPC 4 Big endian only
IA-64 16 Itanium
SPARC 4


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 or even pb=4,lp=4,lc=0 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 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, for example,
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, for example,
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.

+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,
for example, 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, for example, 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) and how many processor threads xz thinks the system has
and the memory usage limits for compression and decompression,
and exit successfully.

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

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

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

ROBOT MODE

The robot mode is activated with the --robot option. It makes the
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 prints 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 multiple tab-
separated columns:

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

2. Memory usage limit for compression in bytes
(--memlimit-compress). A special value of 0 indicates the
default setting which for single-threaded mode is the same as no
limit.

3. Memory usage limit for decompression in bytes
(--memlimit-decompress). A special value of 0 indicates the
default setting which for single-threaded mode is the same as no
limit.

4. Since xz 5.3.4alpha: Memory usage for multi-threaded
decompression in bytes (--memlimit-mt-decompress). This is never
zero because a system-specific default value shown in the column
5 is used if no limit has been specified explicitly. This is
also never greater than the value in the column 3 even if a
larger value has been specified with --memlimit-mt-decompress.

5. Since xz 5.3.4alpha: A system-specific default memory usage limit
that is used to limit the number of threads when compressing with
an automatic number of threads (--threads=0) and no memory usage
limit has been specified (--memlimit-compress). This is also
used as the default value for --memlimit-mt-decompress.

6. Since xz 5.3.4alpha: Number of available processor threads.

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

List mode

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

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

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

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

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

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

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

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

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.

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 when xz is run by a script or tool, for example, GNU
tar(1):

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


Scripts may use XZ_OPT, for example, to set script-specific
default compression options. It is still recommended to allow
users to override XZ_OPT if that is reasonable. For example,
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
-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
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

Create baz.tar.xz with the preset -4e (-4 --extreme), which is slower
than 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=$((123 << 20)) # 123 MiB
OLDLIM=$(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 (for
example, 32 MiB) to compress well, but you want to compress it
quicker than xz -8 would do, a preset with a low CompCPU value (for
example, 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 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, for example, 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. For
example, 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. For example, 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, for example, as
uncompressed TIFF. The distance parameter of the Delta filter is set
to match the number of bytes per pixel in the image. For example,
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 (for example,
.tar), the Delta filter will work on that too as long as all images
have the same number of bytes per pixel.

SEE ALSO

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

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

Tukaani 2023-07-17 XZ(1)