Provided by: xz-utils_5.6.2-2ubuntu0.2_amd64 
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:
• When compressing, the suffix of the target file format (.xz or .lzma) is appended to the source
filename to get the target filename.
• 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:
• File is not a regular file. Symbolic links are not followed, and thus they are not considered to be
regular files.
• File has more than one hard link.
• File has setuid, setgid, or sticky bit set.
• The operation mode is set to compress and the file already has a suffix of the target file format (.xz
or .txz when compressing to the .xz format, and .lzma or .tlz when compressing to the .lzma format).
• 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:
• If the target file already exists, delete it before compressing or decompressing.
• Compress or decompress even if the input is a symbolic link to a regular file, has more than
one hard link, or has the setuid, setgid, or sticky bit set. The setuid, setgid, and sticky
bits are not copied to the target file.
• When used with --decompress --stdout and xz cannot recognize the type of the source file, copy
the source file as is to standard output. This allows xzcat --force to be used like cat(1) for
files that have not been compressed with xz. Note that in future, xz might support new
compressed file formats, which may make xz decompress more types of files instead of copying
them as is to standard output. --format=format can be used to restrict xz to decompress only a
single file 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:
• Trying to recover data from a corrupt .xz file.
• Speeding up decompression. This matters mostly with SHA-256 or with files that have compressed
extremely well. It's 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:
• 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.
• CompCPU is a simplified representation of the LZMA2 settings that affect compression speed.
The dictionary size affects speed too, so while CompCPU is the same for levels -6 ... -9,
higher levels still tend to be a little slower. To get even slower and thus possibly better
compression, see --extreme.
• CompMem contains the compressor memory requirements in the single-threaded mode. It may vary
slightly between xz versions.
• 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.
Memory requirements of the multi-threaded mode are significantly higher than that of the single-
threaded mode. With the default value of --block-size, each thread needs 3*3*DictSize plus
CompMem or DecMem. For example, four threads with preset -6 needs 660–670 MiB of memory.
-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. In multi-threaded mode, the sizes of the blocks are stored in
the block headers. This size information is required 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 xz won't be able decompress the files in multi-threaded mode.
--block-list=items
When compressing to the .xz format, start a new block with an optional custom filter chain after
the given intervals of uncompressed data.
The items are a comma-separated list. Each item consists of an optional filter chain number
between 0 and 9 followed by a colon (:) and a required size of uncompressed data. Omitting an
item (two or more consecutive commas) is a shorthand to use the size and filters of the previous
item.
If the input file is bigger than the sum of the sizes in items, the last item is repeated until
the end of the file. A special value of 0 may be used as the last size to indicate that the rest
of the file should be encoded as a single block.
An alternative filter chain for each block can be specified in combination with the
--filters1=filters ... --filters9=filters options. These options define filter chains with an
identifier between 1–9. Filter chain 0 can be used to refer to the default filter chain, which is
the same as not specifying a filter chain. The filter chain identifier can be used before the
uncompressed size, followed by a colon (:). For example, if one specifies
--block-list=1:2MiB,3:2MiB,2:4MiB,,2MiB,0:4MiB then blocks will be created using:
• The filter chain specified by --filters1 and 2 MiB input
• The filter chain specified by --filters3 and 2 MiB input
• The filter chain specified by --filters2 and 4 MiB input
• The filter chain specified by --filters2 and 4 MiB input
• The default filter chain and 2 MiB input
• The default filter chain and 4 MiB input for every block until end of input.
If one specifies a size that exceeds 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 items. 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:
• The limit can be an absolute value in bytes. Using an integer suffix like MiB can be useful.
Example: --memlimit-compress=80MiB
• 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%
• 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.
The default value for threads is 0. In xz 5.4.x and older the default is 1.
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 can be specified in two different ways. The options --filters=filters and
--filters1=filters ... --filters9=filters allow specifying an entire filter chain in one option using the
liblzma filter string syntax. Alternatively, a filter chain can be specified by using one or more
individual filter options in the order they are wanted in the filter chain. That is, the order of the
individual filter options is significant! When decoding raw streams (--format=raw), the filter chain
must be specified in the same order as it was specified when compressing. Any individual filter or
preset options specified before the full chain option (--filters=filters) will be forgotten. Individual
filters specified after the full chain option will reset the filter chain.
Both the full and individual filter options take filter-specific options as a comma-separated list.
Extra commas in options are ignored. Every option has a default value, so specify 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.
--filters=filters
Specify the full filter chain or a preset in a single option. Each filter can be separated by
spaces or two dashes (--). filters may need to be quoted on the shell command line so it is
parsed as a single option. To denote options, use : or =. A preset can be prefixed with a - and
followed with zero or more flags. The only supported flag is e to apply the same options as
--extreme.
--filters1=filters ... --filters9=filters
Specify up to nine additional filter chains that can be used with --block-list.
For example, when compressing an archive with executable files followed by text files, the
executable part could use a filter chain with a BCJ filter and the text part only the LZMA2
filter.
--filters-help
Display a help message describing how to specify presets and custom filter chains in the --filters
and --filters1=filters ... --filters9=filters options, and exit successfully.
--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]
--riscv[=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:
• 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.
• 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
RISC-V 2
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.
Examples:
• IA-64 filter has 16-byte alignment so pb=4,lp=4,lc=0 is good with LZMA2 (2^4=16).
• RISC-V code has 2-byte or 4-byte alignment depending on whether the file contains 16-bit
compressed instructions (the C extension). When 16-bit instructions are used, pb=2,lp=1,lc=3
or pb=1,lp=1,lc=3 is good. When 16-bit instructions aren't present, pb=2,lp=2,lc=2 is the
best. readelf -h can be used to check if "RVC" appears on the "Flags" line.
• ARM64 is always 4-byte aligned so pb=2,lp=2,lc=2 is the best.
• The x86 filter is an exception. It's usually good to stick to LZMA2's defaults
(pb=2,lp=0,lc=3) 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:
• Completion percentage is shown if the size of the input file is known. That is, the percentage
cannot be shown in pipes.
• Amount of compressed data produced (compressing) or consumed (decompressing).
• Amount of uncompressed data consumed (compressing) or produced (decompressing).
• Compression ratio, which is calculated by dividing the amount of compressed data processed so
far by the amount of uncompressed data processed so far.
• 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.
• Elapsed time in the format M:SS or H:MM:SS.
• Estimated remaining time is shown only when the size of the input file is known and a couple of
seconds have already passed since xz started processing the file. The time is shown in a less
precise format which never has any colons, 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 --list, --filters-help, --info-memory, and
--version. It will be supported for compression and decompression in the future.
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.
Filters help
xz --robot --filters-help prints the supported filters in the following format:
filter:option=<value>,option=<value>...
filter Name of the filter
option Name of a filter specific option
value Numeric value ranges appear as <min-max>. String value choices are shown within < > and separated
by a | character.
Each filter is printed on its own line.
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.
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.
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 2024-04-08 XZ(1)