Provided by: xz-utils_5.4.1-0.2_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. 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 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.
Custom compressor filter chains
A custom filter chain allows specifying the compression settings in detail instead of
relying on the settings associated to the presets. When a custom filter chain is
specified, preset options (-0 ... -9 and --extreme) earlier on the command line are
forgotten. If a preset option is specified after one or more custom filter chain options,
the new preset takes effect and the custom filter chain options specified earlier are
forgotten.
A filter chain is comparable to piping on the command line. When compressing, the
uncompressed input goes to the first filter, whose output goes to the next filter (if
any). The output of the last filter gets written to the compressed file. The maximum
number of filters in the chain is four, but typically a filter chain has only one or two
filters.
Many filters have limitations on where they can be in the filter chain: some filters can
work only as the last filter in the chain, some only as a non-last filter, and some work
in any position in the chain. Depending on the filter, this limitation is either inherent
to the filter design or exists to prevent security issues.
A custom filter chain is specified by using one or more filter options in the order they
are wanted in the filter chain. That is, the order of filter options is significant!
When decoding raw streams (--format=raw), the filter chain is specified in the same order
as it was specified when compressing.
Filters take filter-specific options as a comma-separated list. Extra commas in options
are ignored. Every option has a default value, so you need to specify only those you want
to change.
To see the whole filter chain and options, use xz -vv (that is, use --verbose twice).
This works also for viewing the filter chain options used by presets.
--lzma1[=options]
--lzma2[=options]
Add LZMA1 or LZMA2 filter to the filter chain. These filters can be used only as
the last filter in the chain.
LZMA1 is a legacy filter, which is supported almost solely due to the legacy .lzma
file format, which supports only LZMA1. LZMA2 is an updated version of LZMA1 to
fix some practical issues of LZMA1. The .xz format uses LZMA2 and doesn't support
LZMA1 at all. Compression speed and ratios of LZMA1 and LZMA2 are practically the
same.
LZMA1 and LZMA2 share the same set of options:
preset=preset
Reset all LZMA1 or LZMA2 options to preset. Preset consist of an integer,
which may be followed by single-letter preset modifiers. The integer can be
from 0 to 9, matching the command line options -0 ... -9. The only
supported modifier is currently e, which matches --extreme. If no preset is
specified, the default values of LZMA1 or LZMA2 options are taken from the
preset 6.
dict=size
Dictionary (history buffer) size indicates how many bytes of the recently
processed uncompressed data is kept in memory. The algorithm tries to find
repeating byte sequences (matches) in the uncompressed data, and replace
them with references to the data currently in the dictionary. The bigger
the dictionary, the higher is the chance to find a match. Thus, increasing
dictionary size usually improves compression ratio, but a dictionary bigger
than the uncompressed file is waste of memory.
Typical dictionary size is from 64 KiB to 64 MiB. The minimum is 4 KiB.
The maximum for compression is currently 1.5 GiB (1536 MiB). The
decompressor already supports dictionaries up to one byte less than 4 GiB,
which is the maximum for the LZMA1 and LZMA2 stream formats.
Dictionary size and match finder (mf) together determine the memory usage of
the LZMA1 or LZMA2 encoder. The same (or bigger) dictionary size is
required for decompressing that was used when compressing, thus the memory
usage of the decoder is determined by the dictionary size used when
compressing. The .xz headers store the dictionary size either as 2^n or 2^n
+ 2^(n-1), so these sizes are somewhat preferred for compression. Other
sizes will get rounded up when stored in the .xz headers.
lc=lc Specify the number of literal context bits. The minimum is 0 and the
maximum is 4; the default is 3. In addition, the sum of lc and lp must not
exceed 4.
All bytes that cannot be encoded as matches are encoded as literals. That
is, literals are simply 8-bit bytes that are encoded one at a time.
The literal coding makes an assumption that the highest lc bits of the
previous uncompressed byte correlate with the next byte. For example, in
typical English text, an upper-case letter is often followed by a lower-case
letter, and a lower-case letter is usually followed by another lower-case
letter. In the US-ASCII character set, the highest three bits are 010 for
upper-case letters and 011 for lower-case letters. When lc is at least 3,
the literal coding can take advantage of this property in the uncompressed
data.
The default value (3) is usually good. If you want maximum compression,
test lc=4. Sometimes it helps a little, and sometimes it makes compression
worse. If it makes it worse, test lc=2 too.
lp=lp Specify the number of literal position bits. The minimum is 0 and the
maximum is 4; the default is 0.
Lp affects what kind of alignment in the uncompressed data is assumed when
encoding literals. See pb below for more information about alignment.
pb=pb Specify the number of position bits. The minimum is 0 and the maximum is 4;
the default is 2.
Pb affects what kind of alignment in the uncompressed data is assumed in
general. The default means four-byte alignment (2^pb=2^2=4), which is often
a good choice when there's no better guess.
When the alignment is known, setting pb accordingly may reduce the file size
a little. For example, with text files having one-byte alignment (US-ASCII,
ISO-8859-*, UTF-8), setting pb=0 can improve compression slightly. For
UTF-16 text, pb=1 is a good choice. If the alignment is an odd number like
3 bytes, pb=0 might be the best choice.
Even though the assumed alignment can be adjusted with pb and lp, LZMA1 and
LZMA2 still slightly favor 16-byte alignment. It might be worth taking into
account when designing file formats that are likely to be often compressed
with LZMA1 or LZMA2.
mf=mf Match finder has a major effect on encoder speed, memory usage, and
compression ratio. Usually Hash Chain match finders are faster than Binary
Tree match finders. The default depends on the preset: 0 uses hc3, 1–3 use
hc4, and the rest use bt4.
The following match finders are supported. The memory usage formulas below
are rough approximations, which are closest to the reality when dict is a
power of two.
hc3 Hash Chain with 2- and 3-byte hashing
Minimum value for nice: 3
Memory usage:
dict * 7.5 (if dict <= 16 MiB);
dict * 5.5 + 64 MiB (if dict > 16 MiB)
hc4 Hash Chain with 2-, 3-, and 4-byte hashing
Minimum value for nice: 4
Memory usage:
dict * 7.5 (if dict <= 32 MiB);
dict * 6.5 (if dict > 32 MiB)
bt2 Binary Tree with 2-byte hashing
Minimum value for nice: 2
Memory usage: dict * 9.5
bt3 Binary Tree with 2- and 3-byte hashing
Minimum value for nice: 3
Memory usage:
dict * 11.5 (if dict <= 16 MiB);
dict * 9.5 + 64 MiB (if dict > 16 MiB)
bt4 Binary Tree with 2-, 3-, and 4-byte hashing
Minimum value for nice: 4
Memory usage:
dict * 11.5 (if dict <= 32 MiB);
dict * 10.5 (if dict > 32 MiB)
mode=mode
Compression mode specifies the method to analyze the data produced by the
match finder. Supported modes are fast and normal. The default is fast for
presets 0–3 and normal for presets 4–9.
Usually fast is used with Hash Chain match finders and normal with Binary
Tree match finders. This is also what the presets do.
nice=nice
Specify what is considered to be a nice length for a match. Once a match of
at least nice bytes is found, the algorithm stops looking for possibly
better matches.
Nice can be 2–273 bytes. Higher values tend to give better compression
ratio at the expense of speed. The default depends on the preset.
depth=depth
Specify the maximum search depth in the match finder. The default is the
special value of 0, which makes the compressor determine a reasonable depth
from mf and nice.
Reasonable depth for Hash Chains is 4–100 and 16–1000 for Binary Trees.
Using very high values for depth can make the encoder extremely slow with
some files. Avoid setting the depth over 1000 unless you are prepared to
interrupt the compression in case it is taking far too long.
When decoding raw streams (--format=raw), LZMA2 needs only the dictionary size.
LZMA1 needs also lc, lp, and pb.
--x86[=options]
--arm[=options]
--armthumb[=options]
--arm64[=options]
--powerpc[=options]
--ia64[=options]
--sparc[=options]
Add a branch/call/jump (BCJ) filter to the filter chain. These filters can be used
only as a non-last filter in the filter chain.
A BCJ filter converts relative addresses in the machine code to their absolute
counterparts. This doesn't change the size of the data but it increases
redundancy, which can help LZMA2 to produce 0–15 % smaller .xz file. The BCJ
filters are always reversible, so using a BCJ filter for wrong type of data doesn't
cause any data loss, although it may make the compression ratio slightly worse.
The BCJ filters are very fast and use an insignificant amount of memory.
These BCJ filters have known problems related to the compression ratio:
• 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
Since the BCJ-filtered data is usually compressed with LZMA2, the compression ratio
may be improved slightly if the LZMA2 options are set to match the alignment of the
selected BCJ filter. For example, with the IA-64 filter, it's good to set pb=4 or
even pb=4,lp=4,lc=0 with LZMA2 (2^4=16). The x86 filter is an exception; it's
usually good to stick to LZMA2's default four-byte alignment when compressing x86
executables.
All BCJ filters support the same options:
start=offset
Specify the start offset that is used when converting between relative and
absolute addresses. The offset must be a multiple of the alignment of the
filter (see the table above). The default is zero. In practice, the
default is good; specifying a custom offset is almost never useful.
--delta[=options]
Add the Delta filter to the filter chain. The Delta filter can be only used as a
non-last filter in the filter chain.
Currently only simple byte-wise delta calculation is supported. It can be useful
when compressing, for example, uncompressed bitmap images or uncompressed PCM
audio. However, special purpose algorithms may give significantly better results
than Delta + LZMA2. This is true especially with audio, which compresses faster
and better, for example, with flac(1).
Supported options:
dist=distance
Specify the distance of the delta calculation in bytes. distance must be
1–256. The default is 1.
For example, with dist=2 and eight-byte input A1 B1 A2 B3 A3 B5 A4 B7, the
output will be A1 B1 01 02 01 02 01 02.
Other options
-q, --quiet
Suppress warnings and notices. Specify this twice to suppress errors too. This
option has no effect on the exit status. That is, even if a warning was
suppressed, the exit status to indicate a warning is still used.
-v, --verbose
Be verbose. If standard error is connected to a terminal, xz will display a
progress indicator. Specifying --verbose twice will give even more verbose output.
The progress indicator shows the following information:
• 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 --version,
--info-memory, and --list. It will be supported for compression and decompression in the
future.
Version
xz --robot --version will print the version number of xz and liblzma in the following
format:
XZ_VERSION=XYYYZZZS
LIBLZMA_VERSION=XYYYZZZS
X Major version.
YYY Minor version. Even numbers are stable. Odd numbers are alpha or beta versions.
ZZZ Patch level for stable releases or just a counter for development releases.
S Stability. 0 is alpha, 1 is beta, and 2 is stable. S should be always 2 when YYY
is even.
XYYYZZZS are the same on both lines if xz and liblzma are from the same XZ Utils release.
Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.
Memory limit information
xz --robot --info-memory prints a single line with three tab-separated columns:
1. Total amount of physical memory (RAM) in bytes.
2. Memory usage limit for compression in bytes (--memlimit-compress). A special value of
0 indicates the default setting which for single-threaded mode is the same as no
limit.
3. Memory usage limit for decompression in bytes (--memlimit-decompress). A special
value of 0 indicates the default setting which for single-threaded mode is the same as
no limit.
4. Since xz 5.3.4alpha: Memory usage for multi-threaded decompression in bytes
(--memlimit-mt-decompress). This is never zero because a system-specific default
value shown in the column 5 is used if no limit has been specified explicitly. This
is also never greater than the value in the column 3 even if a larger value has been
specified with --memlimit-mt-decompress.
5. Since xz 5.3.4alpha: A system-specific default memory usage limit that is used to
limit the number of threads when compressing with an automatic number of threads
(--threads=0) and no memory usage limit has been specified (--memlimit-compress).
This is also used as the default value for --memlimit-mt-decompress.
6. Since xz 5.3.4alpha: Number of available processor threads.
In the future, the output of xz --robot --info-memory may have more columns, but never
more than a single line.
List mode
xz --robot --list uses tab-separated output. The first column of every line has a string
that indicates the type of the information found on that line:
name This is always the first line when starting to list a file. The second column on
the line is the filename.
file This line contains overall information about the .xz file. This line is always
printed after the name line.
stream This line type is used only when --verbose was specified. There are as many stream
lines as there are streams in the .xz file.
block This line type is used only when --verbose was specified. There are as many block
lines as there are blocks in the .xz file. The block lines are shown after all the
stream lines; different line types are not interleaved.
summary
This line type is used only when --verbose was specified twice. This line is
printed after all block lines. Like the file line, the summary line contains
overall information about the .xz file.
totals This line is always the very last line of the list output. It shows the total
counts and sizes.
The columns of the file lines:
2. Number of streams in the file
3. Total number of blocks in the stream(s)
4. Compressed size of the file
5. Uncompressed size of the file
6. Compression ratio, for example, 0.123. If ratio is over 9.999, three dashes
(---) are displayed instead of the ratio.
7. Comma-separated list of integrity check names. The following strings are used
for the known check types: None, CRC32, CRC64, and SHA-256. For unknown check
types, Unknown-N is used, where N is the Check ID as a decimal number (one or
two digits).
8. Total size of stream padding in the file
The columns of the stream lines:
2. Stream number (the first stream is 1)
3. Number of blocks in the stream
4. Compressed start offset
5. Uncompressed start offset
6. Compressed size (does not include stream padding)
7. Uncompressed size
8. Compression ratio
9. Name of the integrity check
10. Size of stream padding
The columns of the block lines:
2. Number of the stream containing this block
3. Block number relative to the beginning of the stream (the first block is 1)
4. Block number relative to the beginning of the file
5. Compressed start offset relative to the beginning of the file
6. Uncompressed start offset relative to the beginning of the file
7. Total compressed size of the block (includes headers)
8. Uncompressed size
9. Compression ratio
10. Name of the integrity check
If --verbose was specified twice, additional columns are included on the block lines.
These are not displayed with a single --verbose, because getting this information requires
many seeks and can thus be slow:
11. Value of the integrity check in hexadecimal
12. Block header size
13. Block flags: c indicates that compressed size is present, and u indicates that
uncompressed size is present. If the flag is not set, a dash (-) is shown
instead to keep the string length fixed. New flags may be added to the end of
the string in the future.
14. Size of the actual compressed data in the block (this excludes the block
header, block padding, and check fields)
15. Amount of memory (in bytes) required to decompress this block with this xz
version
16. Filter chain. Note that most of the options used at compression time cannot be
known, because only the options that are needed for decompression are stored in
the .xz headers.
The columns of the summary lines:
2. Amount of memory (in bytes) required to decompress this file with this xz
version
3. yes or no indicating if all block headers have both compressed size and
uncompressed size stored in them
Since xz 5.1.2alpha:
4. Minimum xz version required to decompress the file
The columns of the totals line:
2. Number of streams
3. Number of blocks
4. Compressed size
5. Uncompressed size
6. Average compression ratio
7. Comma-separated list of integrity check names that were present in the files
8. Stream padding size
9. Number of files. This is here to keep the order of the earlier columns the
same as on file lines.
If --verbose was specified twice, additional columns are included on the totals line:
10. Maximum amount of memory (in bytes) required to decompress the files with this
xz version
11. yes or no indicating if all block headers have both compressed size and
uncompressed size stored in them
Since xz 5.1.2alpha:
12. Minimum xz version required to decompress the file
Future versions may add new line types and new columns can be added to the existing line
types, but the existing columns won't be changed.
EXIT STATUS
0 All is good.
1 An error occurred.
2 Something worth a warning occurred, but no actual errors occurred.
Notices (not warnings or errors) printed on standard error don't affect the exit status.
ENVIRONMENT
xz parses space-separated lists of options from the environment variables XZ_DEFAULTS and
XZ_OPT, in this order, before parsing the options from the command line. Note that only
options are parsed from the environment variables; all non-options are silently ignored.
Parsing is done with getopt_long(3) which is used also for the command line arguments.
XZ_DEFAULTS
User-specific or system-wide default options. Typically this is set in a shell
initialization script to enable xz's memory usage limiter by default. Excluding
shell initialization scripts and similar special cases, scripts must never set or
unset XZ_DEFAULTS.
XZ_OPT This is for passing options to xz when it is not possible to set the options
directly on the xz command line. This is the case when xz is run by a script or
tool, for example, GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
Scripts may use XZ_OPT, for example, to set script-specific default compression
options. It is still recommended to allow users to override XZ_OPT if that is
reasonable. For example, in sh(1) scripts one may use something like this:
XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT
LZMA UTILS COMPATIBILITY
The command line syntax of xz is practically a superset of lzma, unlzma, and lzcat as
found from LZMA Utils 4.32.x. In most cases, it is possible to replace LZMA Utils with XZ
Utils without breaking existing scripts. There are some incompatibilities though, which
may sometimes cause problems.
Compression preset levels
The numbering of the compression level presets is not identical in xz and LZMA Utils. The
most important difference is how dictionary sizes are mapped to different presets.
Dictionary size is roughly equal to the decompressor memory usage.
Level xz LZMA Utils
-0 256 KiB N/A
-1 1 MiB 64 KiB
-2 2 MiB 1 MiB
-3 4 MiB 512 KiB
-4 4 MiB 1 MiB
-5 8 MiB 2 MiB
-6 8 MiB 4 MiB
-7 16 MiB 8 MiB
-8 32 MiB 16 MiB
-9 64 MiB 32 MiB
The dictionary size differences affect the compressor memory usage too, but there are some
other differences between LZMA Utils and XZ Utils, which make the difference even bigger:
Level xz LZMA Utils 4.32.x
-0 3 MiB N/A
-1 9 MiB 2 MiB
-2 17 MiB 12 MiB
-3 32 MiB 12 MiB
-4 48 MiB 16 MiB
-5 94 MiB 26 MiB
-6 94 MiB 45 MiB
-7 186 MiB 83 MiB
-8 370 MiB 159 MiB
-9 674 MiB 311 MiB
The default preset level in LZMA Utils is -7 while in XZ Utils it is -6, so both use an 8
MiB dictionary by default.
Streamed vs. non-streamed .lzma files
The uncompressed size of the file can be stored in the .lzma header. LZMA Utils does that
when compressing regular files. The alternative is to mark that uncompressed size is
unknown and use end-of-payload marker to indicate where the decompressor should stop.
LZMA Utils uses this method when uncompressed size isn't known, which is the case, for
example, in pipes.
xz supports decompressing .lzma files with or without end-of-payload marker, but all .lzma
files created by xz will use end-of-payload marker and have uncompressed size marked as
unknown in the .lzma header. This may be a problem in some uncommon situations. For
example, a .lzma decompressor in an embedded device might work only with files that have
known uncompressed size. If you hit this problem, you need to use LZMA Utils or LZMA SDK
to create .lzma files with known uncompressed size.
Unsupported .lzma files
The .lzma format allows lc values up to 8, and lp values up to 4. LZMA Utils can
decompress files with any lc and lp, but always creates files with lc=3 and lp=0.
Creating files with other lc and lp is possible with xz and with LZMA SDK.
The implementation of the LZMA1 filter in liblzma requires that the sum of lc and lp must
not exceed 4. Thus, .lzma files, which exceed this limitation, cannot be decompressed
with xz.
LZMA Utils creates only .lzma files which have a dictionary size of 2^n (a power of 2) but
accepts files with any dictionary size. liblzma accepts only .lzma files which have a
dictionary size of 2^n or 2^n + 2^(n-1). This is to decrease false positives when
detecting .lzma files.
These limitations shouldn't be a problem in practice, since practically all .lzma files
have been compressed with settings that liblzma will accept.
Trailing garbage
When decompressing, LZMA Utils silently ignore everything after the first .lzma stream.
In most situations, this is a bug. This also means that LZMA Utils don't support
decompressing concatenated .lzma files.
If there is data left after the first .lzma stream, xz considers the file to be corrupt
unless --single-stream was used. This may break obscure scripts which have assumed that
trailing garbage is ignored.
NOTES
Compressed output may vary
The exact compressed output produced from the same uncompressed input file may vary
between XZ Utils versions even if compression options are identical. This is because the
encoder can be improved (faster or better compression) without affecting the file format.
The output can vary even between different builds of the same XZ Utils version, if
different build options are used.
The above means that once --rsyncable has been implemented, the resulting files won't
necessarily be rsyncable unless both old and new files have been compressed with the same
xz version. This problem can be fixed if a part of the encoder implementation is frozen
to keep rsyncable output stable across xz versions.
Embedded .xz decompressors
Embedded .xz decompressor implementations like XZ Embedded don't necessarily support files
created with integrity check types other than none and crc32. Since the default is
--check=crc64, you must use --check=none or --check=crc32 when creating files for embedded
systems.
Outside embedded systems, all .xz format decompressors support all the check types, or at
least are able to decompress the file without verifying the integrity check if the
particular check is not supported.
XZ Embedded supports BCJ filters, but only with the default start offset.
EXAMPLES
Basics
Compress the file foo into foo.xz using the default compression level (-6), and remove foo
if compression is successful:
xz foo
Decompress bar.xz into bar and don't remove bar.xz even if decompression is successful:
xz -dk bar.xz
Create baz.tar.xz with the preset -4e (-4 --extreme), which is slower than the default -6,
but needs less memory for compression and decompression (48 MiB and 5 MiB, respectively):
tar cf - baz | xz -4e > baz.tar.xz
A mix of compressed and uncompressed files can be decompressed to standard output with a
single command:
xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt
Parallel compression of many files
On GNU and *BSD, find(1) and xargs(1) can be used to parallelize compression of many
files:
find . -type f \! -name '*.xz' -print0 \
| xargs -0r -P4 -n16 xz -T1
The -P option to xargs(1) sets the number of parallel xz processes. The best value for
the -n option depends on how many files there are to be compressed. If there are only a
couple of files, the value should probably be 1; with tens of thousands of files, 100 or
even more may be appropriate to reduce the number of xz processes that xargs(1) will
eventually create.
The option -T1 for xz is there to force it to single-threaded mode, because xargs(1) is
used to control the amount of parallelization.
Robot mode
Calculate how many bytes have been saved in total after compressing multiple files:
xz --robot --list *.xz | awk '/^totals/{print $5-$4}'
A script may want to know that it is using new enough xz. The following sh(1) script
checks that the version number of the xz tool is at least 5.0.0. This method is
compatible with old beta versions, which didn't support the --robot option:
if ! eval "$(xz --robot --version 2> /dev/null)" ||
[ "$XZ_VERSION" -lt 50000002 ]; then
echo "Your xz is too old."
fi
unset XZ_VERSION LIBLZMA_VERSION
Set a memory usage limit for decompression using XZ_OPT, but if a limit has already been
set, don't increase it:
NEWLIM=$((123 << 20)) # 123 MiB
OLDLIM=$(xz --robot --info-memory | cut -f3)
if [ $OLDLIM -eq 0 -o $OLDLIM -gt $NEWLIM ]; then
XZ_OPT="$XZ_OPT --memlimit-decompress=$NEWLIM"
export XZ_OPT
fi
Custom compressor filter chains
The simplest use for custom filter chains is customizing a LZMA2 preset. This can be
useful, because the presets cover only a subset of the potentially useful combinations of
compression settings.
The CompCPU columns of the tables from the descriptions of the options -0 ... -9 and
--extreme are useful when customizing LZMA2 presets. Here are the relevant parts
collected from those two tables:
Preset CompCPU
-0 0
-1 1
-2 2
-3 3
-4 4
-5 5
-6 6
-5e 7
-6e 8
If you know that a file requires somewhat big dictionary (for example, 32 MiB) to compress
well, but you want to compress it quicker than xz -8 would do, a preset with a low CompCPU
value (for example, 1) can be modified to use a bigger dictionary:
xz --lzma2=preset=1,dict=32MiB foo.tar
With certain files, the above command may be faster than xz -6 while compressing
significantly better. However, it must be emphasized that only some files benefit from a
big dictionary while keeping the CompCPU value low. The most obvious situation, where a
big dictionary can help a lot, is an archive containing very similar files of at least a
few megabytes each. The dictionary size has to be significantly bigger than any
individual file to allow LZMA2 to take full advantage of the similarities between
consecutive files.
If very high compressor and decompressor memory usage is fine, and the file being
compressed is at least several hundred megabytes, it may be useful to use an even bigger
dictionary than the 64 MiB that xz -9 would use:
xz -vv --lzma2=dict=192MiB big_foo.tar
Using -vv (--verbose --verbose) like in the above example can be useful to see the memory
requirements of the compressor and decompressor. Remember that using a dictionary bigger
than the size of the uncompressed file is waste of memory, so the above command isn't
useful for small files.
Sometimes the compression time doesn't matter, but the decompressor memory usage has to be
kept low, for example, to make it possible to decompress the file on an embedded system.
The following command uses -6e (-6 --extreme) as a base and sets the dictionary to only
64 KiB. The resulting file can be decompressed with XZ Embedded (that's why there is
--check=crc32) using about 100 KiB of memory.
xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo
If you want to squeeze out as many bytes as possible, adjusting the number of literal
context bits (lc) and number of position bits (pb) can sometimes help. Adjusting the
number of literal position bits (lp) might help too, but usually lc and pb are more
important. For example, a source code archive contains mostly US-ASCII text, so something
like the following might give slightly (like 0.1 %) smaller file than xz -6e (try also
without lc=4):
xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar
Using another filter together with LZMA2 can improve compression with certain file types.
For example, to compress a x86-32 or x86-64 shared library using the x86 BCJ filter:
xz --x86 --lzma2 libfoo.so
Note that the order of the filter options is significant. If --x86 is specified after
--lzma2, xz will give an error, because there cannot be any filter after LZMA2, and also
because the x86 BCJ filter cannot be used as the last filter in the chain.
The Delta filter together with LZMA2 can give good results with bitmap images. It should
usually beat PNG, which has a few more advanced filters than simple delta but uses Deflate
for the actual compression.
The image has to be saved in uncompressed format, for example, as uncompressed TIFF. The
distance parameter of the Delta filter is set to match the number of bytes per pixel in
the image. For example, 24-bit RGB bitmap needs dist=3, and it is also good to pass pb=0
to LZMA2 to accommodate the three-byte alignment:
xz --delta=dist=3 --lzma2=pb=0 foo.tiff
If multiple images have been put into a single archive (for example, .tar), the Delta
filter will work on that too as long as all images have the same number of bytes per
pixel.
SEE ALSO
xzdec(1), xzdiff(1), xzgrep(1), xzless(1), xzmore(1), gzip(1), bzip2(1), 7z(1)
XZ Utils: <https://tukaani.org/xz/>
XZ Embedded: <https://tukaani.org/xz/embedded.html>
LZMA SDK: <http://7-zip.org/sdk.html>