Manpages

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:

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:

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.

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.

Operation modifiers
-k, --keep

Don’t delete the input files.

-f, --force

This option has several effects:

-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.

--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.

--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:

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.

-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.

none

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.

•

-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.

-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.

•

Memory requirements of the multi-threaded mode are significantly higher than that of the single-threaded mode. With the default value of --block-size, each thread needs 3*3*DictSize plus CompMem or DecMem. For example, four threads with preset -6 needs 660–670 MiB of memory.

-e, --extreme

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

--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.

--block-list=items

When compressing to the .xz format, start a new block with an optional custom filter chain after the given intervals of uncompressed data.

•

If one specifies a size that exceeds the encoder’s block size (either the default value in threaded mode or the value specified with --block-size=size), the encoder will create additional blocks while keeping the boundaries specified in items. For example, if one specifies --block-size=10MiB --block-list=5MiB,10MiB,8MiB,12MiB,24MiB and the input file is 80 MiB, one will get 11 blocks: 5, 10, 8, 10, 2, 10, 10, 4, 10, 10, and 1 MiB.

--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.

--memlimit-compress=limit

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

•

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.

--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.

-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.

-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.

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

A filter chain is comparable to piping on the command line. When compressing, the uncompressed input goes to the first filter, whose output goes to the next filter (if any). The output of the last filter gets written to the compressed file. The maximum number of filters in the chain is four, but typically a filter chain has only one or two filters.

Many filters have limitations on where they can be in the filter chain: some filters can work only as the last filter in the chain, some only as a non-last filter, and some work in any position in the chain. Depending on the filter, this limitation is either inherent to the filter design or exists to prevent security issues.

A custom filter chain can be specified in two different ways. The options --filters=filters and --filters1=filters ... --filters9=filters allow specifying an entire filter chain in one option using the liblzma filter string syntax. Alternatively, a filter chain can be specified by using one or more individual filter options in the order they are wanted in the filter chain. That is, the order of the individual filter options is significant! When decoding raw streams (--format=raw), the filter chain must be specified in the same order as it was specified when compressing. Any individual filter or preset options specified before the full chain option (--filters=filters) will be forgotten. Individual filters specified after the full chain option will reset the filter chain.

Both the full and individual filter options take filter-specific options as a comma-separated list. Extra commas in options are ignored. Every option has a default value, so specify those you want to change.

To see the whole filter chain and options, use xz -vv (that is, use --verbose twice). This works also for viewing the filter chain options used by presets.
--filters=filters

Specify the full filter chain or a preset in a single option. Each filter can be separated by spaces or two dashes (--). filters may need to be quoted on the shell command line so it is parsed as a single option. To denote options, use : or =. A preset can be prefixed with a - and followed with zero or more flags. The only supported flag is e to apply the same options as --extreme.

--filters1=filters ... --filters9=filters

Specify up to nine additional filter chains that can be used with --block-list.

--filters-help

Display a help message describing how to specify presets and custom filter chains in the --filters and --filters1=filters ... --filters9=filters options, and exit successfully.

--lzma1[=options]
--lzma2[=options]

Add LZMA1 or LZMA2 filter to the filter chain. These filters can be used only as the last filter in the chain.

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.

hc3

Minimum value for nice: 3
Memory usage:
dict * 7.5 (if dict <= 16 MiB);
dict * 5.5 + 64 MiB (if dict > 16 MiB)

Minimum value for nice: 4
Memory usage:
dict * 7.5 (if dict <= 32 MiB);
dict * 6.5 (if dict > 32 MiB)

Minimum value for nice: 2
Memory usage: dict * 9.5

Minimum value for nice: 3
Memory usage:
dict * 11.5 (if dict <= 16 MiB);
dict * 9.5 + 64 MiB (if dict > 16 MiB)

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.

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.

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.

--x86[=options]
--arm[=options]
--armthumb[=options]
--arm64[=options]
--powerpc[=options]
--ia64[=options]
--sparc[=options]
--riscv[=options]

Add a branch/call/jump (BCJ) filter to the filter chain. These filters can be used only as a non-last filter in the filter chain.

•

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.

•

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.

dist=distance

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

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.

•

When standard error is not a terminal, --verbose will make xz print the filename, compressed size, uncompressed size, compression ratio, and possibly also the speed and elapsed time on a single line to standard error after compressing or decompressing the file. The speed and elapsed time are included only when the operation took at least a few seconds. If the operation didn’t finish, for example, due to user interruption, also the completion percentage is printed if the size of the input file is known.

-Q, --no-warn

Don’t set the exit status to 2 even if a condition worth a warning was detected. This option doesn’t affect the verbosity level, thus both --quiet and --no-warn have to be used to not display warnings and to not alter the exit status.

--robot

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

--info-memory

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

-h, --help

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

-H, --long-help

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

-V, --version

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

ROBOT MODE

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

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

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.

The columns of the file lines:

The columns of the stream lines:

The columns of the block lines:

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:

The columns of the summary lines:

Since xz 5.1.2alpha:

The columns of the totals line:

If --verbose was specified twice, additional columns are included on the totals line:

Since xz 5.1.2alpha:

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

Filters help
xz --robot --filters-help prints the supported filters in the following format:

filter:option=<value>,option=<value>...

Each filter is printed on its own line.

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

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

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

XZ_VERSION=XYYYZZZS
LIBLZMA_VERSION=XYYYZZZS

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

Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.

EXIT STATUS

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

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.

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:

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:

If you know that a file requires somewhat big dictionary (for example, 32 MiB) to compress well, but you want to compress it quicker than xz -8 would do, a preset with a low CompCPU value (for example, 1) can be modified to use a bigger dictionary:

xz --lzma2=preset=1,dict=32MiB foo.tar

With certain files, the above command may be faster than xz -6 while compressing significantly better. However, it must be emphasized that only some files benefit from a big dictionary while keeping the CompCPU value low. The most obvious situation, where a big dictionary can help a lot, is an archive containing very similar files of at least a few megabytes each. The dictionary size has to be significantly bigger than any individual file to allow LZMA2 to take full advantage of the similarities between consecutive files.

If very high compressor and decompressor memory usage is fine, and the file being compressed is at least several hundred megabytes, it may be useful to use an even bigger dictionary than the 64 MiB that xz -9 would use:

xz -vv --lzma2=dict=192MiB big_foo.tar

Using -vv (--verbose --verbose) like in the above example can be useful to see the memory requirements of the compressor and decompressor. Remember that using a dictionary bigger than the size of the uncompressed file is waste of memory, so the above command isn’t useful for small files.

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

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

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

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

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

xz --x86 --lzma2 libfoo.so

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

The Delta filter together with LZMA2 can give good results with bitmap images. It should usually beat PNG, which has a few more advanced filters than simple delta but uses Deflate for the actual compression.

The image has to be saved in uncompressed format, for example, as uncompressed TIFF. The distance parameter of the Delta filter is set to match the number of bytes per pixel in the image. For example, 24-bit RGB bitmap needs dist=3, and it is also good to pass pb=0 to LZMA2 to accommodate the three-byte alignment:

xz --delta=dist=3 --lzma2=pb=0 foo.tiff

If multiple images have been put into a single archive (for example, .tar), the Delta filter will work on that too as long as all images have the same number of bytes per pixel.

SEE ALSO

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

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