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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
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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
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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.
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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
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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.
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-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
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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
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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
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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:
tab(;); c c c c c n n n n n.
Preset;DictSize;CompCPU;CompMem;DecMem -0;256 KiB;0;3 MiB;1
MiB -1;1 MiB;1;9 MiB;2 MiB -2;2 MiB;2;17 MiB;3 MiB -3;4
MiB;3;32 MiB;5 MiB -4;4 MiB;4;48 MiB;5 MiB -5;8 MiB;5;94
MiB;9 MiB -6;8 MiB;6;94 MiB;9 MiB -7;16 MiB;6;186 MiB;17 MiB
-8;32 MiB;6;370 MiB;33 MiB -9;64 MiB;6;674 MiB;65 MiB
Column descriptions:
+ DictSize is the LZMA2 dictionary size. It is waste of memory
to use a dictionary bigger than the size of the uncompressed
file. This is why it is good to avoid using the presets -7
... -9 when there's no real need for them. At -6 and lower,
the amount of memory wasted is usually low enough to not
matter.
+ CompCPU is a simplified representation of the LZMA2 settings
that affect compression speed. The dictionary size affects
speed too, so while CompCPU is the same for levels -6 ... -9,
higher levels still tend to be a little slower. To get even
slower and thus possibly better compression, see --extreme.
+ CompMem contains the compressor memory requirements in the
single-threaded mode. It may vary slightly between xz
versions.
+ DecMem contains the decompressor memory requirements. That
is, the compression settings determine the memory requirements
of the decompressor. The exact decompressor memory usage is
slightly more than the LZMA2 dictionary size, but the values
in the table have been rounded up to the next full MiB.
Memory requirements of the multi-threaded mode are significantly
higher than that of the single-threaded mode. With the default
value of --block-size, each thread needs 3*3*DictSize plus
CompMem or DecMem. For example, four threads with preset -6
needs 660670 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
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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.
tab(;); c c c c c n n n n n.
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 24 times the size
of the LZMA2 dictionary or at least 1 MiB. Using size less than
the LZMA2 dictionary size is waste of RAM because then the LZMA2
dictionary buffer will never get fully used. In multi-threaded
mode, the sizes of the blocks are stored in the block headers.
This size information is required for multi-threaded
decompression.
In single-threaded mode no block splitting is done by default.
Setting this option doesn't affect memory usage. No size
information is stored in block headers, thus files created in
single-threaded mode won't be identical to files created in
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multi-threaded mode. The lack of size information also means
that xz won't be able decompress the files in multi-threaded
mode.
--block-list=items
When compressing to the .xz format, start a new block with an
optional custom filter chain after the given intervals of
uncompressed data.
The items are a comma-separated list. Each item consists of an
optional filter chain number between 0 and 9 followed by a colon
(:) and a required size of uncompressed data. Omitting an item
(two or more consecutive commas) is a shorthand to use the size
and filters of the previous item.
If the input file is bigger than the sum of the sizes in items,
the last item is repeated until the end of the file. A special
value of 0 may be used as the last size to indicate that the rest
of the file should be encoded as a single block.
An alternative filter chain for each block can be specified in
combination with the --filters1=filters ... --filters9=filters
options. These options define filter chains with an identifier
between 19. Filter chain 0 can be used to refer to the default
filter chain, which is the same as not specifying a filter chain.
The filter chain identifier can be used before the uncompressed
size, followed by a colon (:). For example, if one specifies
--block-list=1:2MiB,3:2MiB,2:4MiB,,2MiB,0:4MiB then blocks will
be created using:
+ The filter chain specified by --filters1 and 2 MiB input
+ The filter chain specified by --filters3 and 2 MiB input
+ The filter chain specified by --filters2 and 4 MiB input
+ The filter chain specified by --filters2 and 4 MiB input
+ The default filter chain and 2 MiB input
+ The default filter chain and 4 MiB input for every block until
end of input.
If one specifies a size that exceeds the encoder's block size
(either the default value in threaded mode or the value specified
with --block-size=size), the encoder will create additional
blocks while keeping the boundaries specified in items. For
example, if one specifies --block-size=10MiB
--block-list=5MiB,10MiB,8MiB,12MiB,24MiB and the input file is 80
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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:
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+ 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
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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.
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If an automatic number of threads has been requested and no
memory usage limit has been specified, then a system-specific
default soft limit will be used to possibly limit the number of
threads. It is a soft limit in sense that it is ignored if the
number of threads becomes one, thus a soft limit will never stop
xz from compressing or decompressing. This default soft limit
will not make xz switch from multi-threaded mode to single-
threaded mode. The active limits can be seen with xz
--info-memory.
Currently the only threading method is to split the input into
blocks and compress them independently from each other. The
default block size depends on the compression level and can be
overridden with the --block-size=size option.
Threaded decompression only works on files that contain multiple
blocks with size information in block headers. All large enough
files compressed in multi-threaded mode meet this condition, but
files compressed in single-threaded mode don't even if
--block-size=size has been used.
The default value for threads is 0. In xz 5.4.x and older the
default is 1.
Custom compressor filter chains
A custom filter chain allows specifying the compression settings in
detail instead of relying on the settings associated to the presets.
When a custom filter chain is specified, preset options (-0 ... -9 and
--extreme) earlier on the command line are forgotten. If a preset
option is specified after one or more custom filter chain options, the
new preset takes effect and the custom filter chain options specified
earlier are forgotten.
A filter chain is comparable to piping on the command line. When
compressing, the uncompressed input goes to the first filter, whose
output goes to the next filter (if any). The output of the last
filter gets written to the compressed file. The maximum number of
filters in the chain is four, but typically a filter chain has only
one or two filters.
Many filters have limitations on where they can be in the filter
chain: some filters can work only as the last filter in the chain,
some only as a non-last filter, and some work in any position in the
chain. Depending on the filter, this limitation is either inherent to
the filter design or exists to prevent security issues.
A custom filter chain can be specified in two different ways. The
options --filters=filters and --filters1=filters ...
--filters9=filters allow specifying an entire filter chain in one
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option using the liblzma filter string syntax. Alternatively, a
filter chain can be specified by using one or more individual filter
options in the order they are wanted in the filter chain. That is,
the order of the individual filter options is significant! When
decoding raw streams (--format=raw), the filter chain must be
specified in the same order as it was specified when compressing. Any
individual filter or preset options specified before the full chain
option (--filters=filters) will be forgotten. Individual filters
specified after the full chain option will reset the filter chain.
Both the full and individual filter options take filter-specific
options as a comma-separated list. Extra commas in options are
ignored. Every option has a default value, so specify those you want
to change.
To see the whole filter chain and options, use xz -vv (that is, use
--verbose twice). This works also for viewing the filter chain
options used by presets.
--filters=filters
Specify the full filter chain or a preset in a single option.
Each filter can be separated by spaces or two dashes (--).
filters may need to be quoted on the shell command line so it is
parsed as a single option. To denote options, use : or =. A
preset can be prefixed with a - and followed with zero or more
flags. The only supported flag is e to apply the same options as
--extreme.
--filters1=filters ... --filters9=filters
Specify up to nine additional filter chains that can be used with
--block-list.
For example, when compressing an archive with executable files
followed by text files, the executable part could use a filter
chain with a BCJ filter and the text part only the LZMA2 filter.
--filters-help
Display a help message describing how to specify presets and
custom filter chains in the --filters and --filters1=filters ...
--filters9=filters options, and exit successfully.
--lzma1[=options]
--lzma2[=options]
Add LZMA1 or LZMA2 filter to the filter chain. These filters can
be used only as the last filter in the chain.
LZMA1 is a legacy filter, which is supported almost solely due to
the legacy .lzma file format, which supports only LZMA1. LZMA2
is an updated version of LZMA1 to fix some practical issues of
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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
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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
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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, 13 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 03 and normal for
presets 49.
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.
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Once a match of at least nice bytes is found, the algorithm
stops looking for possibly better matches.
Nice can be 2273 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 4100 and 161000 for
Binary Trees. Using very high values for depth can make the
encoder extremely slow with some files. Avoid setting the
depth over 1000 unless you are prepared to interrupt the
compression in case it is taking far too long.
When decoding raw streams (--format=raw), LZMA2 needs only the
dictionary size. LZMA1 needs also lc, lp, and pb.
--x86[=options]
--arm[=options]
--armthumb[=options]
--arm64[=options]
--powerpc[=options]
--ia64[=options]
--sparc[=options]
--riscv[=options]
Add a branch/call/jump (BCJ) filter to the filter chain. These
filters can be used only as a non-last filter in the filter
chain.
A BCJ filter converts relative addresses in the machine code to
their absolute counterparts. This doesn't change the size of the
data but it increases redundancy, which can help LZMA2 to produce
015 % 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
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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.
tab(;); l n l l n l. Filter;Alignment;Notes x86;1;32-bit or
64-bit x86 ARM;4; ARM-Thumb;2; ARM64;4;4096-byte alignment
is best PowerPC;4;Big endian only IA-64;16;Itanium SPARC;4;
RISC-V;2;
Since the BCJ-filtered data is usually compressed with LZMA2, the
compression ratio may be improved slightly if the LZMA2 options
are set to match the alignment of the selected BCJ filter.
Examples:
+ IA-64 filter has 16-byte alignment so pb=4,lp=4,lc=0 is good
with LZMA2 (2^4=16).
+ RISC-V code has 2-byte or 4-byte alignment depending on
whether the file contains 16-bit compressed instructions (the
C extension). When 16-bit instructions are used,
pb=2,lp=1,lc=3 or pb=1,lp=1,lc=3 is good. When 16-bit
instructions aren't present, pb=2,lp=2,lc=2 is the best.
readelf -h can be used to check if "RVC" appears on the
"Flags" line.
+ ARM64 is always 4-byte aligned so pb=2,lp=2,lc=2 is the best.
+ The x86 filter is an exception. It's usually good to stick to
LZMA2's defaults (pb=2,lp=0,lc=3) when compressing x86
executables.
All BCJ filters support the same options:
start=offset
Specify the start offset that is used when converting
between relative and absolute addresses. The offset must be
a multiple of the alignment of the filter (see the table
above). The default is zero. In practice, the default is
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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 1256. 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.
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+ 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
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-V, --version
Display the version number of xz and liblzma in human readable
format. To get machine-parsable output, specify --robot before
--version.
ROBOT MODE
The robot mode is activated with the --robot option. It makes the
output of xz easier to parse by other programs. Currently --robot is
supported only together with --list, --filters-help, --info-memory,
and --version. It will be supported for compression and decompression
in the future.
List mode
xz --robot --list uses tab-separated output. The first column of
every line has a string that indicates the type of the information
found on that line:
name This is always the first line when starting to list a file. The
second column on the line is the filename.
file This line contains overall information about the .xz file. This
line is always printed after the name line.
stream
This line type is used only when --verbose was specified. There
are as many stream lines as there are streams in the .xz file.
block
This line type is used only when --verbose was specified. There
are as many block lines as there are blocks in the .xz file. The
block lines are shown after all the stream lines; different line
types are not interleaved.
summary
This line type is used only when --verbose was specified twice.
This line is printed after all block lines. Like the file line,
the summary line contains overall information about the .xz file.
totals
This line is always the very last line of the list output. It
shows the total counts and sizes.
The columns of the file lines:
2. Number of streams in the file
3. Total number of blocks in the stream(s)
4. Compressed size of the file
5. Uncompressed size of the file
6. Compression ratio, for example, 0.123. If ratio is over
9.999, three dashes (---) are displayed instead of the ratio.
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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
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headers.
The columns of the summary lines:
2. Amount of memory (in bytes) required to decompress this file
with this xz version
3. yes or no indicating if all block headers have both
compressed size and uncompressed size stored in them
Since xz 5.1.2alpha:
4. Minimum xz version required to decompress the file
The columns of the totals line:
2. Number of streams
3. Number of blocks
4. Compressed size
5. Uncompressed size
6. Average compression ratio
7. Comma-separated list of integrity check names that were
present in the files
8. Stream padding size
9. Number of files. This is here to keep the order of the
earlier columns the same as on file lines.
If --verbose was specified twice, additional columns are included on
the totals line:
10. Maximum amount of memory (in bytes) required to decompress
the files with this xz version
11. yes or no indicating if all block headers have both
compressed size and uncompressed size stored in them
Since xz 5.1.2alpha:
12. Minimum xz version required to decompress the file
Future versions may add new line types and new columns can be added to
the existing line types, but the existing columns won't be changed.
Filters help
xz --robot --filters-help prints the supported filters in the
following format:
filter:option=<value>,option=<value>...
filter
Name of the filter
option
Name of a filter specific option
value
Numeric value ranges appear as <min-max>. String value choices
are shown within < > and separated by a | character.
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Each filter is printed on its own line.
Memory limit information
xz --robot --info-memory prints a single line with multiple tab-
separated columns:
1. Total amount of physical memory (RAM) in bytes.
2. Memory usage limit for compression in bytes (--memlimit-compress).
A special value of 0 indicates the default setting which for
single-threaded mode is the same as no limit.
3. Memory usage limit for decompression in bytes
(--memlimit-decompress). A special value of 0 indicates the
default setting which for single-threaded mode is the same as no
limit.
4. Since xz 5.3.4alpha: Memory usage for multi-threaded decompression
in bytes (--memlimit-mt-decompress). This is never zero because a
system-specific default value shown in the column 5 is used if no
limit has been specified explicitly. This is also never greater
than the value in the column 3 even if a larger value has been
specified with --memlimit-mt-decompress.
5. Since xz 5.3.4alpha: A system-specific default memory usage limit
that is used to limit the number of threads when compressing with
an automatic number of threads (--threads=0) and no memory usage
limit has been specified (--memlimit-compress). This is also used
as the default value for --memlimit-mt-decompress.
6. Since xz 5.3.4alpha: Number of available processor threads.
In the future, the output of xz --robot --info-memory may have more
columns, but never more than a single line.
Version
xz --robot --version prints the version number of xz and liblzma in
the following format:
XZ_VERSION=XYYYZZZS
LIBLZMA_VERSION=XYYYZZZS
X Major version.
YYY Minor version. Even numbers are stable. Odd numbers are alpha
or beta versions.
ZZZ Patch level for stable releases or just a counter for development
releases.
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S Stability. 0 is alpha, 1 is beta, and 2 is stable. S should be
always 2 when YYY is even.
XYYYZZZS are the same on both lines if xz and liblzma are from the
same XZ Utils release.
Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.
EXIT STATUS
0 All is good.
1 An error occurred.
2 Something worth a warning occurred, but no actual errors
occurred.
Notices (not warnings or errors) printed on standard error don't
affect the exit status.
ENVIRONMENT
xz parses space-separated lists of options from the environment
variables XZ_DEFAULTS and XZ_OPT, in this order, before parsing the
options from the command line. Note that only options are parsed from
the environment variables; all non-options are silently ignored.
Parsing is done with getopt_long(3) which is used also for the command
line arguments.
XZ_DEFAULTS
User-specific or system-wide default options. Typically this is
set in a shell initialization script to enable xz's memory usage
limiter by default. Excluding shell initialization scripts and
similar special cases, scripts must never set or unset
XZ_DEFAULTS.
XZ_OPT
This is for passing options to xz when it is not possible to set
the options directly on the xz command line. This is the case
when xz is run by a script or tool, for example, GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
Scripts may use XZ_OPT, for example, to set script-specific
default compression options. It is still recommended to allow
users to override XZ_OPT if that is reasonable. For example, in
sh(1) scripts one may use something like this:
XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT
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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.
tab(;); c c c c n n. 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:
tab(;); c c c c n n. 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.
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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.
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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
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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:
tab(;); c c n n. 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
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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
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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>
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