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xz, unxz, xzcat, lzma, unlzma, lzcat - Compress or decompress .xz and .lzma
files
unxz is equivalent to xz --decompress.
xzcat is equivalent to xz --decompress --stdout.
lzma is equivalent to xz --format=lzma.
unlzma is equivalent to xz --format=lzma --decompress.
lzcat is equivalent to xz --format=lzma --decompress --stdout.
When writing scripts that need to decompress files, it is
recommended to always use the name xz with appropriate arguments
(xz -d or xz -dc) instead of the names unxz and
xzcat.
xz is a general-purpose data compression tool with command line syntax
similar to gzip(1) and bzip2(1). The native file format is the
.xz format, but the legacy .lzma format used by LZMA Utils and
raw compressed streams with no container format headers are also supported.
xz compresses or decompresses each file according to
the selected operation mode. If no files are given or file is
-, xz reads from standard input and writes the processed data
to standard output. xz will refuse (display an error and skip the
file) to write compressed data to standard output if it is a
terminal. Similarly, xz will refuse to read compressed data from
standard input if it is a terminal.
Unless --stdout is specified, files other than
- are written to a new file whose name is derived from the source
file name:
- 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 or .lzma suffix is removed from
the filename to get the target filename. xz also recognizes the
suffixes .txz and .tlz, and replaces them with the
.tar suffix.
If the target file already exists, an error is displayed and the
file is skipped.
Unless writing to standard output, xz will display a
warning and skip the file if any of the following applies:
- 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, or .tlz).
After successfully compressing or decompressing the file,
xz copies the owner, group, permissions, access time, and
modification time from the source file to the target file. If copying
the group fails, the permissions are modified so that the target file
doesn't become accessible to users who didn't have permission to access the
source file. xz doesn't support copying other metadata like
access control lists or extended attributes yet.
Once the target file has been successfully closed, the source
file is removed unless --keep was specified. The source
file is never removed if the output is written to standard
output.
Sending SIGINFO or SIGUSR1 to the xz process
makes it print progress information to standard error. This has only limited
use since when standard error is a terminal, using --verbose will
display an automatically updating progress indicator.
The memory usage of xz varies from a few hundred kilobytes to several
gigabytes depending on the compression settings. The settings used when
compressing a file determine the memory requirements of the decompressor.
Typically the decompressor needs 5 % to 20 % of the amount of
memory that the compressor needed when creating the file. For example,
decompressing a file created with xz -9 currently requires
65 MiB of memory. Still, it is possible to have .xz files that
require several gigabytes of memory to decompress.
Especially users of older systems may find the possibility of very
large memory usage annoying. To prevent uncomfortable surprises, xz
has a built-in memory usage limiter, which is disabled by default. While
some operating systems provide ways to limit the memory usage of processes,
relying on it wasn't deemed to be flexible enough (e.g. using
ulimit(1) to limit virtual memory tends to cripple
mmap(2)).
The memory usage limiter can be enabled with the command line
option --memlimit=limit. Often it is more convenient to enable
the limiter by default by setting the environment variable
XZ_DEFAULTS, e.g. XZ_DEFAULTS=--memlimit=150MiB. It is
possible to set the limits separately for compression and decompression by
using --memlimit-compress=limit and
--memlimit-decompress= limit. Using these two options outside
XZ_DEFAULTS is rarely useful because a single run of xz cannot
do both compression and decompression and --memlimit=limit (or
-M limit) is shorter to type on the command line.
If the specified memory usage limit is exceeded when
decompressing, xz will display an error and decompressing the file
will fail. If the limit is exceeded when compressing, xz will try to
scale the settings down so that the limit is no longer exceeded (except when
using --format=raw or --no-adjust). This way the operation
won't fail unless the limit is very small. The scaling of the settings is
done in steps that don't match the compression level presets, e.g. if the
limit is only slightly less than the amount required for xz -9, the
settings will be scaled down only a little, not all the way down to xz
-8.
It is possible to concatenate .xz files as is. xz will decompress
such files as if they were a single .xz file.
It is possible to insert padding between the concatenated parts or
after the last part. The padding must consist of null bytes and the size of
the padding must be a multiple of four bytes. This can be useful e.g. if the
.xz file is stored on a medium that measures file sizes in 512-byte
blocks.
Concatenation and padding are not allowed with .lzma files
or raw streams.
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.
If multiple operation mode options are given, the last one takes effect.
- -z, --compress
- Compress. This is the default operation mode when no operation mode option
is specified and no other operation mode is implied from the command name
(for example, unxz implies --decompress).
- -d, --decompress, --uncompress
- Decompress.
- -t, --test
- Test the integrity of compressed files. This option is equivalent
to --decompress --stdout except that the decompressed data is
discarded instead of being written to standard output. No files are
created or removed.
- -l, --list
- Print information about compressed files. No uncompressed output is
produced, and no files are created or removed. In list mode, the program
cannot read the compressed data from standard input or from other
unseekable sources.
-
- The default listing shows basic information about files, one file
per line. To get more detailed information, use also the --verbose
option. For even more information, use --verbose twice, but note
that this may be slow, because getting all the extra information requires
many seeks. The width of verbose output exceeds 80 characters, so piping
the output to e.g. less -S may be convenient if the terminal
isn't wide enough.
-
- The exact output may vary between xz versions and different
locales. For machine-readable output, --robot --list should be
used.
- -k, --keep
- Don't delete the input files.
- -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, or
.tlz suffix. If the source file has the suffix .suf, the
suffix is removed to get the target filename.
-
- When compressing or decompressing raw streams (--format=raw), the
suffix must always be specified unless writing to standard output, because
there is no default suffix for raw streams.
- --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.
- -F format, --format=format
- Specify the file format to compress or decompress:
- auto
- This is the default. When compressing, auto is equivalent to
xz. When decompressing, the format of the input file is
automatically detected. Note that raw streams (created with
--format=raw) cannot be auto-detected.
- xz
- Compress to the .xz file format, or accept only .xz files
when decompressing.
- lzma, alone
- Compress to the legacy .lzma file format, or accept only
.lzma files when decompressing. The alternative name alone
is provided for backwards compatibility with LZMA Utils.
- raw
- Compress or uncompress a raw stream (no headers). This is meant for
advanced users only. To decode raw streams, you need use
--format=raw and explicitly specify the filter chain, which
normally would have been stored in the container headers.
- -C check, --check=check
- Specify the type of the integrity check. The check is calculated from the
uncompressed data and stored in the .xz file. This option has an
effect only when compressing into the .xz format; the .lzma
format doesn't support integrity checks. The integrity check (if any) is
verified when the .xz file is decompressed.
-
- Supported check types:
- 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 e.g. for distributing files that need to be
decompressible even on systems with only 16 MiB RAM. (-5e or
-6e may be worth considering too. See --extreme.)
- -7 ... -9
- These are like -6 but with higher compressor and decompressor
memory requirements. These are useful only when compressing files bigger
than 8 MiB, 16 MiB, and 32 MiB, respectively.
-
- On the same hardware, the decompression speed is approximately a constant
number of bytes of compressed data per second. In other words, the better
the compression, the faster the decompression will usually be. This also
means that the amount of uncompressed output produced per second can vary
a lot.
-
- The following table summarises the features of the presets:
Preset |
DictSize |
CompCPU |
CompMem |
DecMem |
-0 |
256 KiB |
0 |
3 MiB |
1 MiB |
-1 |
1 MiB |
1 |
9 MiB |
2 MiB |
-2 |
2 MiB |
2 |
17 MiB |
3 MiB |
-3 |
4 MiB |
3 |
32 MiB |
5 MiB |
-4 |
4 MiB |
4 |
48 MiB |
5 MiB |
-5 |
8 MiB |
5 |
94 MiB |
9 MiB |
-6 |
8 MiB |
6 |
94 MiB |
9 MiB |
-7 |
16 MiB |
6 |
186 MiB |
17 MiB |
-8 |
32 MiB |
6 |
370 MiB |
33 MiB |
-9 |
64 MiB |
6 |
674 MiB |
65 MiB |
-
- Column descriptions:
- 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. Memory requirements
of some of the future multithreaded modes may be dramatically higher than
that of the single-threaded mode.
- 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 adjust
the settings downwards so that the limit is no longer exceeded and display
a notice that automatic adjustment was done. Such adjustments are not made
when compressing with --format=raw or if --no-adjust has
been specified. In those cases, an error is displayed and xz will
exit with exit status 1.
-
- The limit can be specified in multiple ways:
- 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). Once multithreading support has been
implemented, there may be a difference between 0 and max for
the multithreaded case, so it is recommended to use 0 instead of
max until the details have been decided.
-
- For 32-bit xz there is a special case: if the limit would be
over 4020 MiB, the limit is set to
4020 MiB. (The values 0 and max aren't
affected by this. A similar feature doesn't exist for decompression.) This
can be helpful when a 32-bit executable has access to 4 GiB address
space while hopefully doing no harm in other situations.
-
- See also the section Memory usage.
- --memlimit-decompress=limit
- Set a memory usage limit for decompression. This also affects the
--list mode. If the operation is not possible without exceeding the
limit, xz will display an error and decompressing the file
will fail. See --memlimit-compress=limit for possible ways
to specify the limit.
- -M limit, --memlimit=limit,
--memory=limit
- This is equivalent to specifying --memlimit-compress=limit
--memlimit-decompress=limit.
- --no-adjust
- Display an error and exit if the compression settings exceed the memory
usage limit. The default is to adjust the settings downwards so that the
memory usage limit is not exceeded. Automatic adjusting is always disabled
when creating raw streams (--format=raw).
- -T threads, --threads=threads
- Specify the number of worker threads to use. Setting threads to a
special value 0 makes xz use as many threads as there are
CPU cores on the system. The actual number of threads can be less than
threads if the input file is not big enough for threading with the
given settings or if using more threads would exceed the memory usage
limit.
-
- Currently the only threading method is to split the input into blocks and
compress them independently from each other. The default block size
depends on the compression level and can be overridden with the
--block-size=size option.
-
- Threaded decompression hasn't been implemented yet. It will only work on
files that contain multiple blocks with size information in block headers.
All files compressed in multi-threaded mode meet this condition, but files
compressed in single-threaded mode don't even if
--block-size=size is used.
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. E.g. in
typical English text, an upper-case letter is often followed by a
lower-case letter, and a lower-case letter is usually followed by another
lower-case letter. In the US-ASCII character set, the highest three bits
are 010 for upper-case letters and 011 for lower-case letters. When
lc is at least 3, the literal coding can take advantage of this
property in the uncompressed data.
-
- The default value (3) is usually good. If you want maximum compression,
test lc=4. Sometimes it helps a little, and sometimes it makes
compression worse. If it makes it worse, test e.g. lc=2 too.
- lp=lp
- Specify the number of literal position bits. The minimum is 0 and the
maximum is 4; the default is 0.
-
- Lp affects what kind of alignment in the uncompressed data is
assumed when encoding literals. See pb below for more information
about alignment.
- pb=pb
- Specify the number of position bits. The minimum is 0 and the maximum is
4; the default is 2.
-
- Pb affects what kind of alignment in the uncompressed data is
assumed in general. The default means four-byte alignment
(2^pb=2^2=4), which is often a good choice when there's no better
guess.
-
- When the aligment is known, setting pb accordingly may reduce the
file size a little. E.g. with text files having one-byte alignment
(US-ASCII, ISO-8859-*, UTF-8), setting pb=0 can improve compression
slightly. For UTF-16 text, pb=1 is a good choice. If the alignment
is an odd number like 3 bytes, pb=0 might be the best choice.
-
- Even though the assumed alignment can be adjusted with pb and
lp, LZMA1 and LZMA2 still slightly favor 16-byte alignment. It
might be worth taking into account when designing file formats that are
likely to be often compressed with LZMA1 or LZMA2.
- mf=mf
- 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]
- --powerpc[=options]
- --ia64[=options]
- --arm[=options]
- --armthumb[=options]
- --sparc[=options]
- Add a branch/call/jump (BCJ) filter to the filter chain. These filters can
be used only as a non-last filter in the filter chain.
-
- A BCJ filter converts relative addresses in the machine code to their
absolute counterparts. This doesn't change the size of the data, but it
increases redundancy, which can help LZMA2 to produce 0-15 %
smaller .xz file. The BCJ filters are always reversible, so using a
BCJ filter for wrong type of data doesn't cause any data loss, although it
may make the compression ratio slightly worse.
-
- It is fine to apply a BCJ filter on a whole executable; there's no need to
apply it only on the executable section. Applying a BCJ filter on an
archive that contains both executable and non-executable files may or may
not give good results, so it generally isn't good to blindly apply a BCJ
filter when compressing binary packages for distribution.
-
- These BCJ filters are very fast and use insignificant amount of memory. If
a BCJ filter improves compression ratio of a file, it can improve
decompression speed at the same time. This is because, on the same
hardware, the decompression speed of LZMA2 is roughly a fixed number of
bytes of compressed data per second.
-
- These BCJ filters have known problems related to the compression
ratio:
- Some types of files containing executable code (e.g. object files, static
libraries, and Linux kernel modules) have the addresses in the
instructions filled with filler values. These BCJ filters will still do
the address conversion, which will make the compression worse with these
files.
- Applying a BCJ filter on an archive containing multiple similar
executables can make the compression ratio worse than not using a BCJ
filter. This is because the BCJ filter doesn't detect the boundaries of
the executable files, and doesn't reset the address conversion counter for
each executable.
-
- Both of the above problems will be fixed in the future in a new filter.
The old BCJ filters will still be useful in embedded systems, because the
decoder of the new filter will be bigger and use more memory.
-
- Different instruction sets have different alignment:
Filter |
Alignment |
Notes |
x86 |
1 |
32-bit or 64-bit x86 |
PowerPC |
4 |
Big endian only |
ARM |
4 |
Little endian only |
ARM-Thumb |
2 |
Little endian only |
IA-64 |
16 |
Big or little endian |
SPARC |
4 |
Big or little endian |
-
- Since the BCJ-filtered data is usually compressed with LZMA2, the
compression ratio may be improved slightly if the LZMA2 options are set to
match the alignment of the selected BCJ filter. For example, with the
IA-64 filter, it's good to set pb=4 with LZMA2 (2^4=16). The x86
filter is an exception; it's usually good to stick to LZMA2's default
four-byte alignment when compressing x86 executables.
-
- All BCJ filters support the same options:
- start=offset
- Specify the start offset that is used when converting between
relative and absolute addresses. The offset must be a multiple of
the alignment of the filter (see the table above). The default is zero. In
practice, the default is good; specifying a custom offset is almost
never useful.
- --delta[=options]
- Add the Delta filter to the filter chain. The Delta filter can be only
used as a non-last filter in the filter chain.
-
- Currently only simple byte-wise delta calculation is supported. It can be
useful when compressing e.g. uncompressed bitmap images or uncompressed
PCM audio. However, special purpose algorithms may give significantly
better results than Delta + LZMA2. This is true especially with audio,
which compresses faster and better e.g. with flac(1).
-
- Supported options:
- dist=distance
- Specify the distance of the delta calculation in bytes.
distance must be 1-256. The default is 1.
-
- For example, with dist=2 and eight-byte input A1 B1 A2 B3 A3 B5 A4
B7, the output will be A1 B1 01 02 01 02 01 02.
- -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, e.g. 2 min 30 s.
-
- When standard error is not a terminal, --verbose will make
xz print the filename, compressed size, uncompressed size,
compression ratio, and possibly also the speed and elapsed time on a
single line to standard error after compressing or decompressing the file.
The speed and elapsed time are included only when the operation took at
least a few seconds. If the operation didn't finish, e.g. due to user
interruption, also the completion percentage is printed if the size of the
input file is known.
- -Q, --no-warn
- Don't set the exit status to 2 even if a condition worth a warning was
detected. This option doesn't affect the verbosity level, thus both
--quiet and --no-warn have to be used to not display
warnings and to not alter the exit status.
- --robot
- Print messages in a machine-parsable format. This is intended to ease
writing frontends that want to use xz instead of liblzma, which may
be the case with various scripts. The output with this option enabled is
meant to be stable across xz releases. See the section ROBOT
MODE for details.
- --info-memory
- Display, in human-readable format, how much physical memory (RAM)
xz thinks the system has and the memory usage limits for
compression and decompression, and exit successfully.
- -h, --help
- Display a help message describing the most commonly used options, and exit
successfully.
- -H, --long-help
- Display a help message describing all features of xz, and exit
successfully
- -V, --version
- Display the version number of xz and liblzma in human readable
format. To get machine-parsable output, specify --robot before
--version.
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.
xz --robot --version will print the version number of xz and
liblzma in the following format:
XZ_VERSION=XYYYZZZS
LIBLZMA_VERSION=XYYYZZZS
- X
- Major version.
- YYY
- Minor version. Even numbers are stable. Odd numbers are alpha or beta
versions.
- ZZZ
- Patch level for stable releases or just a counter for development
releases.
- S
- Stability. 0 is alpha, 1 is beta, and 2 is stable. S should be
always 2 when YYY is even.
XYYYZZZS are the same on both lines if xz and
liblzma are from the same XZ Utils release.
Examples: 4.999.9beta is 49990091 and 5.0.0 is
50000002.
xz --robot --info-memory prints a single line with three tab-separated
columns:
- 1.
- Total amount of physical memory (RAM) in bytes
- 2.
- Memory usage limit for compression in bytes. A special value of zero
indicates the default setting, which for single-threaded mode is the same
as no limit.
- 3.
- Memory usage limit for decompression in bytes. A special value of zero
indicates the default setting, which for single-threaded mode is the same
as no limit.
In the future, the output of xz --robot --info-memory may
have more columns, but never more than a single line.
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.
- 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.
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 e.g.
when xz is run by a script or tool, e.g. GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
-
- Scripts may use XZ_OPT e.g. to set script-specific default
compression options. It is still recommended to allow users to override
XZ_OPT if that is reasonable, e.g. in sh(1) scripts one may
use something like this:
XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT
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.
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.
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.
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.
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.
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 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.
Compress the file foo into foo.xz using the default compression
level (-6), and remove foo if compression is successful:
Decompress bar.xz into bar and don't remove
bar.xz even if decompression is successful:
Create baz.tar.xz with the preset -4e (-4
--extreme), which is slower than e.g. the default -6, but needs
less memory for compression and decompression (48 MiB and
5 MiB, respectively):
tar cf - baz | xz -4e > baz.tar.xz
A mix of compressed and uncompressed files can be decompressed to
standard output with a single command:
xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt
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.
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
The simplest use for custom filter chains is customizing a LZMA2 preset. This
can be useful, because the presets cover only a subset of the potentially
useful combinations of compression settings.
The CompCPU columns of the tables from the descriptions of the
options -0 ... -9 and --extreme are useful when
customizing LZMA2 presets. Here are the relevant parts collected from those
two tables:
Preset |
CompCPU |
-0 |
0 |
-1 |
1 |
-2 |
2 |
-3 |
3 |
-4 |
4 |
-5 |
5 |
-6 |
6 |
-5e |
7 |
-6e |
8 |
If you know that a file requires somewhat big dictionary (e.g. 32
MiB) to compress well, but you want to compress it quicker than xz -8
would do, a preset with a low CompCPU value (e.g. 1) can be modified to use
a bigger dictionary:
xz --lzma2=preset=1,dict=32MiB foo.tar
With certain files, the above command may be faster than xz
-6 while compressing significantly better. However, it must be
emphasized that only some files benefit from a big dictionary while keeping
the CompCPU value low. The most obvious situation, where a big dictionary
can help a lot, is an archive containing very similar files of at least a
few megabytes each. The dictionary size has to be significantly bigger than
any individual file to allow LZMA2 to take full advantage of the
similarities between consecutive files.
If very high compressor and decompressor memory usage is fine, and
the file being compressed is at least several hundred megabytes, it may be
useful to use an even bigger dictionary than the 64 MiB that xz -9
would use:
xz -vv --lzma2=dict=192MiB big_foo.tar
Using -vv (--verbose --verbose) like in the above
example can be useful to see the memory requirements of the compressor and
decompressor. Remember that using a dictionary bigger than the size of the
uncompressed file is waste of memory, so the above command isn't useful for
small files.
Sometimes the compression time doesn't matter, but the
decompressor memory usage has to be kept low e.g. to make it possible to
decompress the file on an embedded system. The following command uses
-6e (-6 --extreme) as a base and sets the dictionary to only
64 KiB. The resulting file can be decompressed with XZ Embedded
(that's why there is --check=crc32) using about 100 KiB of
memory.
xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo
If you want to squeeze out as many bytes as possible, adjusting
the number of literal context bits (lc) and number of position bits
(pb) can sometimes help. Adjusting the number of literal position
bits (lp) might help too, but usually lc and pb are
more important. E.g. a source code archive contains mostly US-ASCII text, so
something like the following might give slightly (like 0.1 %) smaller
file than xz -6e (try also without lc=4):
xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar
Using another filter together with LZMA2 can improve compression
with certain file types. E.g. to compress a x86-32 or x86-64 shared library
using the x86 BCJ filter:
xz --x86 --lzma2 libfoo.so
Note that the order of the filter options is significant. If
--x86 is specified after --lzma2, xz will give an
error, because there cannot be any filter after LZMA2, and also because the
x86 BCJ filter cannot be used as the last filter in the chain.
The Delta filter together with LZMA2 can give good results with
bitmap images. It should usually beat PNG, which has a few more advanced
filters than simple delta but uses Deflate for the actual compression.
The image has to be saved in uncompressed format, e.g. as
uncompressed TIFF. The distance parameter of the Delta filter is set to
match the number of bytes per pixel in the image. E.g. 24-bit RGB bitmap
needs dist=3, and it is also good to pass pb=0 to LZMA2 to
accommodate the three-byte alignment:
xz --delta=dist=3 --lzma2=pb=0 foo.tiff
If multiple images have been put into a single archive (e.g.
.tar), the Delta filter will work on that too as long as all images
have the same number of bytes per pixel.
xzdec(1), xzdiff(1), xzgrep(1), xzless(1),
xzmore(1), gzip(1), bzip2(1), 7z(1)
XZ Utils: <https://tukaani.org/xz/>
XZ Embedded: <https://tukaani.org/xz/embedded.html>
LZMA SDK: <http://7-zip.org/sdk.html>
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