/usr/man/cat.1/lzcat.1(/usr/man/cat.1/lzcat.1)
XZ(1) XZ Utils XZ(1)
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 argu-
ments (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 con-
tainer format headers are also supported.
xz compresses or decompresses each file according to the
selected operation mode. If no files are given or file is
-, xz reads from standard input and writes the processed
data to standard output. xz will refuse (display an error
and skip the file) to write compressed data to standard out-
put 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:
o When compressing, the suffix of the target file format
(.xz or .lzma) is appended to the source filename to get
the target filename.
o When decompressing, the .xz or .lzma suffix is removed
from the filename to get the target filename. xz also
recognizes the suffixes .txz and .tlz, and replaces them
with the .tar suffix.
If the target file already exists, an error is displayed and
the file is skipped.
Unless writing to standard output, xz will display a warning
and skip the file if any of the following applies:
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o File is not a regular file. Symbolic links are not fol-
lowed, and thus they are not considered to be regular
files.
o File has more than one hard link.
o File has setuid, setgid, or sticky bit set.
o 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).
o The operation mode is set to decompress and the file
doesn't have a suffix of any of the supported file for-
mats (.xz, .txz, .lzma, or .tlz).
After successfully compressing or decompressing the file, xz
copies the owner, group, permissions, access time, and
modification time from the source file to the target file.
If copying the group fails, the permissions are modified so
that the target file doesn't become accessible to users who
didn't have permission to access the source file. xz
doesn't support copying other metadata like access control
lists or extended attributes yet.
Once the target file has been successfully closed, the
source file is removed unless --keep was specified. The
source file is never removed if the output is written to
standard output.
Sending SIGINFO or SIGUSR1 to the xz process makes it print
progress information to standard error. This has only lim-
ited 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 uncomfort-
able surprises, xz has a built-in memory usage limiter,
which is disabled by default. While some operating systems
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provide ways to limit the memory usage of processes, relying
on it wasn't deemed to be flexible enough (e.g. using
ulimit(1) to limit virtual memory tends to cripple mmap(2)).
The memory usage limiter can be enabled with the command
line option --memlimit=limit. Often it is more convenient
to enable the limiter by default by setting the environment
variable XZ_DEFAULTS, e.g. XZ_DEFAULTS=--memlimit=150MiB.
It is possible to set the limits separately for compression
and decompression by using --memlimit-compress=limit and
--memlimit-decompress=limit. Using these two options out-
side 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 compress-
ing, xz will try to scale the settings down so that the
limit is no longer exceeded (except when using --format=raw
or --no-adjust). This way the operation won't fail unless
the limit is very small. The scaling of the settings is
done in steps that don't match the compression level
presets, e.g. if the limit is only slightly less than the
amount required for xz -9, the settings will be scaled down
only a little, not all the way down to xz -8.
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 e.g. if the .xz file is
stored on a medium that measures file sizes in 512-byte
blocks.
Concatenation and padding are not allowed with .lzma files
or raw streams.
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.
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MiB Multiply the integer by 1,048,576 (2^20). Mi, m, M,
and MB are accepted as synonyms for MiB.
GiB Multiply the integer by 1,073,741,824 (2^30). Gi, g,
G, and GB are accepted as synonyms for GiB.
The special value max can be used to indicate the maximum
integer value supported by the option.
Operation mode
If multiple operation mode options are given, the last one
takes effect.
-z, --compress
Compress. This is the default operation mode when no
operation mode option is specified and no other opera-
tion mode is implied from the command name (for exam-
ple, 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 infor-
mation, use also the --verbose option. For even more
information, use --verbose twice, but note that this
may be slow, because getting all the extra information
requires many seeks. The width of verbose output
exceeds 80 characters, so piping the output to e.g.
less -S may be convenient if the terminal isn't wide
enough.
The exact output may vary between xz versions and dif-
ferent locales. For machine-readable output, --robot
--list should be used.
Operation modifiers
-k, --keep
Don't delete the input files.
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-f, --force
This option has several effects:
o If the target file already exists, delete it before
compressing or decompressing.
o Compress or decompress even if the input is a sym-
bolic 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.
o 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 for-
mats, which may make xz decompress more types of
files instead of copying them as is to standard out-
put. --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 con-
nected to a regular file and certain additional condi-
tions 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 stan-
dard output and the source file already has the suffix
.suf, a warning is displayed and the file is skipped.
When decompressing, recognize files with the suffix
.suf in addition to files with the .xz, .txz, .lzma, or
.tlz suffix. If the source file has the suffix .suf,
the suffix is removed to get the target filename.
When compressing or decompressing raw streams
(--format=raw), the suffix must always be specified
unless writing to standard output, because there is no
default suffix for raw streams.
--files[=file]
Read the filenames to process from file; if file is
omitted, filenames are read from standard input.
Filenames must be terminated with the newline charac-
ter. 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) can-
not be auto-detected.
xz Compress to the .xz file format, or accept only
.xz files when decompressing.
lzma, alone
Compress to the legacy .lzma file format, or
accept only .lzma files when decompressing. The
alternative name alone is provided for backwards
compatibility with LZMA Utils.
raw Compress or uncompress a raw stream (no headers).
This is meant for advanced users only. To decode
raw streams, you need use --format=raw and expli-
citly specify the filter chain, which normally
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would have been stored in the container headers.
-C check, --check=check
Specify the type of the integrity check. The check is
calculated from the uncompressed data and stored in the
.xz file. This option has an effect only when
compressing into the .xz format; the .lzma format
doesn't support integrity checks. The integrity check
(if any) is verified when the .xz file is decompressed.
Supported check types:
none Don't calculate an integrity check at all. This
is usually a bad idea. This can be useful when
integrity of the data is verified by other means
anyway.
crc32
Calculate CRC32 using the polynomial from IEEE-
802.3 (Ethernet).
crc64
Calculate CRC64 using the polynomial from ECMA-
182. This is the default, since it is slightly
better than CRC32 at detecting damaged files and
the speed difference is negligible.
sha256
Calculate SHA-256. This is somewhat slower than
CRC32 and CRC64.
Integrity of the .xz headers is always verified with
CRC32. It is not possible to change or disable it.
--ignore-check
Don't verify the integrity check of the compressed data
when decompressing. The CRC32 values in the .xz
headers will still be verified normally.
Do not use this option unless you know what you are
doing. Possible reasons to use this option:
o Trying to recover data from a corrupt .xz file.
o 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.
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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 signifi-
cant than with gzip(1) and bzip2(1). The selected
compression settings determine the memory requirements
of the decompressor, thus using a too high preset level
might make it painful to decompress the file on an old
system with little RAM. Specifically, it's not a good
idea to blindly use -9 for everything like it often is
with gzip(1) and bzip2(1).
-0 ... -3
These are somewhat fast presets. -0 is sometimes
faster than gzip -9 while compressing much better.
The higher ones often have speed comparable to
bzip2(1) with comparable or better compression
ratio, although the results depend a lot on the
type of data being compressed.
-4 ... -6
Good to very good compression while keeping
decompressor memory usage reasonable even for old
systems. -6 is the default, which is usually a
good choice e.g. for distributing files that need
to be decompressible even on systems with only
16 MiB RAM. (-5e or -6e may be worth considering
too. See --extreme.)
-7 ... -9
These are like -6 but with higher compressor and
decompressor memory requirements. These are use-
ful only when compressing files bigger than 8 MiB,
16 MiB, and 32 MiB, respectively.
On the same hardware, the decompression speed is
approximately a constant number of bytes of compressed
data per second. In other words, the better the
compression, the faster the decompression will usually
be. This also means that the amount of uncompressed
output produced per second can vary a lot.
The following table summarises the features of the
presets:
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
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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:
o 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.
o CompCPU is a simplified representation of the LZMA2
settings that affect compression speed. The dic-
tionary 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.
o CompMem contains the compressor memory requirements
in the single-threaded mode. It may vary slightly
between xz versions. Memory requirements of some of
the future multithreaded modes may be dramatically
higher than that of the single-threaded mode.
o DecMem contains the decompressor memory require-
ments. That is, the compression settings determine
the memory requirements of the decompressor. The
exact decompressor memory usage is slightly more
than the LZMA2 dictionary size, but the values in
the table have been rounded up to the next full MiB.
-e, --extreme
Use a slower variant of the selected compression preset
level (-0 ... -9) to hopefully get a little bit better
compression ratio, but with bad luck this can also make
it worse. Decompressor memory usage is not affected,
but compressor memory usage increases a little at
preset levels -0 ... -3.
Since there are two presets with dictionary sizes 4 MiB
and 8 MiB, the presets -3e and -5e use slightly faster
settings (lower CompCPU) than -4e and -6e, respec-
tively. 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
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-8e;32 MiB;8;370 MiB;33 MiB -9e;64 MiB;8;674
MiB;65 MiB
For example, there are a total of four presets that use
8 MiB dictionary, whose order from the fastest to the
slowest is -5, -6, -5e, and -6e.
--fast
--best
These are somewhat misleading aliases for -0 and -9,
respectively. These are provided only for backwards
compatibility with LZMA Utils. Avoid using these
options.
--block-size=size
When compressing to the .xz format, split the input
data into blocks of size bytes. The blocks are
compressed independently from each other, which helps
with multi-threading and makes limited random-access
decompression possible. This option is typically used
to override the default block size in multi-threaded
mode, but this option can be used in single-threaded
mode too.
In multi-threaded mode about three times size bytes
will be allocated in each thread for buffering input
and output. The default size is three times the LZMA2
dictionary size or 1 MiB, whichever is more. Typically
a good value is 2-4 times the size of the LZMA2 dic-
tionary or at least 1 MiB. Using size less than the
LZMA2 dictionary size is waste of RAM because then the
LZMA2 dictionary buffer will never get fully used. The
sizes of the blocks are stored in the block headers,
which a future version of xz will use for multi-
threaded decompression.
In single-threaded mode no block splitting is done by
default. Setting this option doesn't affect memory
usage. No size information is stored in block headers,
thus files created in single-threaded mode won't be
identical to files created in multi-threaded mode. The
lack of size information also means that a future ver-
sion of xz won't be able decompress the files in
multi-threaded mode.
--block-list=sizes
When compressing to the .xz format, start a new block
after the given intervals of uncompressed data.
The uncompressed sizes of the blocks are specified as a
comma-separated list. Omitting a size (two or more
consecutive commas) is a shorthand to use the size of
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the previous block.
If the input file is bigger than the sum of sizes, the
last value in sizes is repeated until the end of the
file. A special value of 0 may be used as the last
value to indicate that the rest of the file should be
encoded as a single block.
If one specifies sizes that exceed the encoder's block
size (either the default value in threaded mode or the
value specified with --block-size=size), the encoder
will create additional blocks while keeping the boun-
daries specified in sizes. For example, if one speci-
fies --block-size=10MiB
--block-list=5MiB,10MiB,8MiB,12MiB,24MiB and the input
file is 80 MiB, one will get 11 blocks: 5, 10, 8, 10,
2, 10, 10, 4, 10, 10, and 1 MiB.
In multi-threaded mode the sizes of the blocks are
stored in the block headers. This isn't done in
single-threaded mode, so the encoded output won't be
identical to that of the multi-threaded mode.
--flush-timeout=timeout
When compressing, if more than timeout milliseconds (a
positive integer) has passed since the previous flush
and reading more input would block, all the pending
input data is flushed from the encoder and made avail-
able 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 expli-
citly disable this feature.
This feature is not available on non-POSIX systems.
This feature is still experimental. Currently xz is
unsuitable for decompressing the stream in real time
due to how xz does buffering.
--memlimit-compress=limit
Set a memory usage limit for compression. If this
option is specified multiple times, the last one takes
effect.
If the compression settings exceed the limit, xz will
adjust the settings downwards so that the limit is no
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longer exceeded and display a notice that automatic
adjustment was done. Such adjustments are not made
when compressing with --format=raw or if --no-adjust
has been specified. In those cases, an error is
displayed and xz will exit with exit status 1.
The limit can be specified in multiple ways:
o The limit can be an absolute value in bytes. Using
an integer suffix like MiB can be useful. Example:
--memlimit-compress=80MiB
o The limit can be specified as a percentage of total
physical memory (RAM). This can be useful espe-
cially 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%
o The limit can be reset back to its default value by
setting it to 0. This is currently equivalent to
setting the limit to max (no memory usage limit).
Once multithreading support has been implemented,
there may be a difference between 0 and max for the
multithreaded case, so it is recommended to use 0
instead of max until the details have been decided.
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 pos-
sible without exceeding the limit, xz will display an
error and decompressing the file will fail. See
--memlimit-compress=limit for possible ways to specify
the limit.
-M limit, --memlimit=limit, --memory=limit
This is equivalent to specifying
--memlimit-compress=limit --memlimit-decompress=limit.
--no-adjust
Display an error and exit if the compression settings
exceed the memory usage limit. The default is to
adjust the settings downwards so that the memory usage
limit is not exceeded. Automatic adjusting is always
disabled when creating raw streams (--format=raw).
-T threads, --threads=threads
Specify the number of worker threads to use. Setting
threads to a special value 0 makes xz use as many
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threads as there are CPU cores on the system. The
actual number of threads can be less than threads if
the input file is not big enough for threading with the
given settings or if using more threads would exceed
the memory usage limit.
Currently the only threading method is to split the
input into blocks and compress them independently from
each other. The default block size depends on the
compression level and can be overriden with the
--block-size=size option.
Threaded decompression hasn't been implemented yet. It
will only work on files that contain multiple blocks
with size information in block headers. All files
compressed in multi-threaded mode meet this condition,
but files compressed in single-threaded mode don't even
if --block-size=size is used.
Custom compressor filter chains
A custom filter chain allows specifying the compression set-
tings in detail instead of relying on the settings associ-
ated to the presets. When a custom filter chain is speci-
fied, preset options (-0 ... -9 and --extreme) earlier on
the command line are forgotten. If a preset option is
specified after one or more custom filter chain options, the
new preset takes effect and the custom filter chain options
specified earlier are forgotten.
A filter chain is comparable to piping on the command line.
When compressing, the uncompressed input goes to the first
filter, whose output goes to the next filter (if any). The
output of the last filter gets written to the compressed
file. The maximum number of filters in the chain is four,
but typically a filter chain has only one or two filters.
Many filters have limitations on where they can be in the
filter chain: some filters can work only as the last filter
in the chain, some only as a non-last filter, and some work
in any position in the chain. Depending on the filter, this
limitation is either inherent to the filter design or exists
to prevent security issues.
A custom filter chain is specified by using one or more
filter options in the order they are wanted in the filter
chain. That is, the order of filter options is significant!
When decoding raw streams (--format=raw), the filter chain
is specified in the same order as it was specified when
compressing.
Filters take filter-specific options as a comma-separated
list. Extra commas in options are ignored. Every option
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has a default value, so you need to specify only those you
want to change.
To see the whole filter chain and options, use xz -vv (that
is, use --verbose twice). This works also for viewing the
filter chain options used by presets.
--lzma1[=options]
--lzma2[=options]
Add LZMA1 or LZMA2 filter to the filter chain. These
filters can be used only as the last filter in the
chain.
LZMA1 is a legacy filter, which is supported almost
solely due to the legacy .lzma file format, which sup-
ports only LZMA1. LZMA2 is an updated version of LZMA1
to fix some practical issues of LZMA1. The .xz format
uses LZMA2 and doesn't support LZMA1 at all. Compres-
sion speed and ratios of LZMA1 and LZMA2 are practi-
cally 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 fol-
lowed by single-letter preset modifiers. The
integer can be from 0 to 9, matching the command
line options -0 ... -9. The only supported modif-
ier 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 refer-
ences to the data currently in the dictionary.
The bigger the dictionary, the higher is the
chance to find a match. Thus, increasing diction-
ary 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
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LZMA2 stream formats.
Dictionary size and match finder (mf) together
determine the memory usage of the LZMA1 or LZMA2
encoder. The same (or bigger) dictionary size is
required for decompressing that was used when
compressing, thus the memory usage of the decoder
is determined by the dictionary size used when
compressing. The .xz headers store the dictionary
size either as 2^n or 2^n + 2^(n-1), so these
sizes are somewhat preferred for compression.
Other sizes will get rounded up when stored in the
.xz headers.
lc=lc
Specify the number of literal context bits. The
minimum is 0 and the maximum is 4; the default is
3. In addition, the sum of lc and lp must not
exceed 4.
All bytes that cannot be encoded as matches are
encoded as literals. That is, literals are simply
8-bit bytes that are encoded one at a time.
The literal coding makes an assumption that the
highest lc bits of the previous uncompressed byte
correlate with the next byte. E.g. in typical
English text, an upper-case letter is often fol-
lowed by a lower-case letter, and a lower-case
letter is usually followed by another lower-case
letter. In the US-ASCII character set, the
highest three bits are 010 for upper-case letters
and 011 for lower-case letters. When lc is at
least 3, the literal coding can take advantage of
this property in the uncompressed data.
The default value (3) is usually good. If you
want maximum compression, test lc=4. Sometimes it
helps a little, and sometimes it makes compression
worse. If it makes it worse, test e.g. lc=2 too.
lp=lp
Specify the number of literal position bits. The
minimum is 0 and the maximum is 4; the default is
0.
Lp affects what kind of alignment in the
uncompressed data is assumed when encoding
literals. See pb below for more information about
alignment.
pb=pb
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Specify the number of position bits. The minimum
is 0 and the maximum is 4; the default is 2.
Pb affects what kind of alignment in the
uncompressed data is assumed in general. The
default means four-byte alignment (2^pb=2^2=4),
which is often a good choice when there's no
better guess.
When the aligment is known, setting pb accordingly
may reduce the file size a little. E.g. with text
files having one-byte alignment (US-ASCII, ISO-
8859-*, UTF-8), setting pb=0 can improve compres-
sion slightly. For UTF-16 text, pb=1 is a good
choice. If the alignment is an odd number like 3
bytes, pb=0 might be the best choice.
Even though the assumed alignment can be adjusted
with pb and lp, LZMA1 and LZMA2 still slightly
favor 16-byte alignment. It might be worth taking
into account when designing file formats that are
likely to be often compressed with LZMA1 or LZMA2.
mf=mf
Match finder has a major effect on encoder speed,
memory usage, and compression ratio. Usually Hash
Chain match finders are faster than Binary Tree
match finders. The default depends on the preset:
0 uses hc3, 1-3 use hc4, and the rest use bt4.
The following match finders are supported. The
memory usage formulas below are rough approxima-
tions, 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
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Minimum value for nice: 3
Memory usage:
dict * 11.5 (if dict <= 16 MiB);
dict * 9.5 + 64 MiB (if dict > 16 MiB)
bt4 Binary Tree with 2-, 3-, and 4-byte hashing
Minimum value for nice: 4
Memory usage:
dict * 11.5 (if dict <= 32 MiB);
dict * 10.5 (if dict > 32 MiB)
mode=mode
Compression mode specifies the method to analyze
the data produced by the match finder. Supported
modes are fast and normal. The default is fast
for presets 0-3 and normal for presets 4-9.
Usually fast is used with Hash Chain match finders
and normal with Binary Tree match finders. This
is also what the presets do.
nice=nice
Specify what is considered to be a nice length for
a match. Once a match of at least nice bytes is
found, the algorithm stops looking for possibly
better matches.
Nice can be 2-273 bytes. Higher values tend to
give better compression ratio at the expense of
speed. The default depends on the preset.
depth=depth
Specify the maximum search depth in the match
finder. The default is the special value of 0,
which makes the compressor determine a reasonable
depth from mf and nice.
Reasonable depth for Hash Chains is 4-100 and
16-1000 for Binary Trees. Using very high values
for depth can make the encoder extremely slow with
some files. Avoid setting the depth over 1000
unless you are prepared to interrupt the compres-
sion in case it is taking far too long.
When decoding raw streams (--format=raw), LZMA2 needs
only the dictionary size. LZMA1 needs also lc, lp, and
pb.
--x86[=options]
--powerpc[=options]
--ia64[=options]
--arm[=options]
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--armthumb[=options]
--sparc[=options]
Add a branch/call/jump (BCJ) filter to the filter
chain. These filters can be used only as a non-last
filter in the filter chain.
A BCJ filter converts relative addresses in the machine
code to their absolute counterparts. This doesn't
change the size of the data, but it increases redun-
dancy, which can help LZMA2 to produce 0-15 % smaller
.xz file. The BCJ filters are always reversible, so
using a BCJ filter for wrong type of data doesn't cause
any data loss, although it may make the compression
ratio slightly worse.
It is fine to apply a BCJ filter on a whole executable;
there's no need to apply it only on the executable sec-
tion. Applying a BCJ filter on an archive that con-
tains both executable and non-executable files may or
may not give good results, so it generally isn't good
to blindly apply a BCJ filter when compressing binary
packages for distribution.
These BCJ filters are very fast and use insignificant
amount of memory. If a BCJ filter improves compression
ratio of a file, it can improve decompression speed at
the same time. This is because, on the same hardware,
the decompression speed of LZMA2 is roughly a fixed
number of bytes of compressed data per second.
These BCJ filters have known problems related to the
compression ratio:
o Some types of files containing executable code (e.g.
object files, static libraries, and Linux kernel
modules) have the addresses in the instructions
filled with filler values. These BCJ filters will
still do the address conversion, which will make the
compression worse with these files.
o Applying a BCJ filter on an archive containing mul-
tiple similar executables can make the compression
ratio worse than not using a BCJ filter. This is
because the BCJ filter doesn't detect the boundaries
of the executable files, and doesn't reset the
address conversion counter for each executable.
Both of the above problems will be fixed in the future
in a new filter. The old BCJ filters will still be
useful in embedded systems, because the decoder of the
new filter will be bigger and use more memory.
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Different instruction sets have have different align-
ment:
tab(;); l n l l n l. Filter;Alignment;Notes
x86;1;32-bit or 64-bit x86 PowerPC;4;Big endian
only ARM;4;Little endian only ARM-Thumb;2;Little
endian only IA-64;16;Big or little endian
SPARC;4;Big or little endian
Since the BCJ-filtered data is usually compressed with
LZMA2, the compression ratio may be improved slightly
if the LZMA2 options are set to match the alignment of
the selected BCJ filter. For example, with the IA-64
filter, it's good to set pb=4 with LZMA2 (2^4=16). The
x86 filter is an exception; it's usually good to stick
to LZMA2's default four-byte alignment when compressing
x86 executables.
All BCJ filters support the same options:
start=offset
Specify the start offset that is used when con-
verting 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; specify-
ing a custom offset is almost never useful.
--delta[=options]
Add the Delta filter to the filter chain. The Delta
filter can be only used as a non-last filter in the
filter chain.
Currently only simple byte-wise delta calculation is
supported. It can be useful when compressing e.g.
uncompressed bitmap images or uncompressed PCM audio.
However, special purpose algorithms may give signifi-
cantly better results than Delta + LZMA2. This is true
especially with audio, which compresses faster and
better e.g. with flac(1).
Supported options:
dist=distance
Specify the distance of the delta calculation in
bytes. distance must be 1-256. The default is 1.
For example, with dist=2 and eight-byte input A1
B1 A2 B3 A3 B5 A4 B7, the output will be A1 B1 01
02 01 02 01 02.
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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 termi-
nal, xz will display a progress indicator. Specifying
--verbose twice will give even more verbose output.
The progress indicator shows the following information:
o Completion percentage is shown if the size of the
input file is known. That is, the percentage cannot
be shown in pipes.
o Amount of compressed data produced (compressing) or
consumed (decompressing).
o Amount of uncompressed data consumed (compressing)
or produced (decompressing).
o Compression ratio, which is calculated by dividing
the amount of compressed data processed so far by
the amount of uncompressed data processed so far.
o Compression or decompression speed. This is meas-
ured 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.
o Elapsed time in the format M:SS or H:MM:SS.
o Estimated remaining time is shown only when the size
of the input file is known and a couple of seconds
have already passed since xz started processing the
file. The time is shown in a less precise format
which never has any colons, e.g. 2 min 30 s.
When standard error is not a terminal, --verbose will
make xz print the filename, compressed size,
uncompressed size, compression ratio, and possibly also
the speed and elapsed time on a single line to standard
error after compressing or decompressing the file. The
speed and elapsed time are included only when the
operation took at least a few seconds. If the opera-
tion didn't finish, e.g. due to user interruption, also
the completion percentage is printed if the size of the
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input file is known.
-Q, --no-warn
Don't set the exit status to 2 even if a condition
worth a warning was detected. This option doesn't
affect the verbosity level, thus both --quiet and
--no-warn have to be used to not display warnings and
to not alter the exit status.
--robot
Print messages in a machine-parsable format. This is
intended to ease writing frontends that want to use xz
instead of liblzma, which may be the case with various
scripts. The output with this option enabled is meant
to be stable across xz releases. See the section ROBOT
MODE for details.
--info-memory
Display, in human-readable format, how much physical
memory (RAM) xz thinks the system has and the memory
usage limits for compression and decompression, and
exit successfully.
-h, --help
Display a help message describing the most commonly
used options, and exit successfully.
-H, --long-help
Display a help message describing all features of xz,
and exit successfully
-V, --version
Display the version number of xz and liblzma in human
readable format. To get machine-parsable output,
specify --robot before --version.
ROBOT MODE
The robot mode is activated with the --robot option. It
makes the output of xz easier to parse by other programs.
Currently --robot is supported only together with --version,
--info-memory, and --list. It will be supported for
compression and decompression in the future.
Version
xz --robot --version will print the version number of xz and
liblzma in the following format:
XZ_VERSION=XYYYZZZS
LIBLZMA_VERSION=XYYYZZZS
X Major version.
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YYY Minor version. Even numbers are stable. Odd numbers
are alpha or beta versions.
ZZZ Patch level for stable releases or just a counter for
development releases.
S Stability. 0 is alpha, 1 is beta, and 2 is stable. S
should be always 2 when YYY is even.
XYYYZZZS are the same on both lines if xz and liblzma are
from the same XZ Utils release.
Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.
Memory limit information
xz --robot --info-memory prints a single line with three
tab-separated columns:
1. Total amount of physical memory (RAM) in bytes
2. Memory usage limit for compression in bytes. A special
value of zero indicates the default setting, which for
single-threaded mode is the same as no limit.
3. Memory usage limit for decompression in bytes. A spe-
cial value of zero indicates the default setting, which
for single-threaded mode is the same as no limit.
In the future, the output of xz --robot --info-memory may
have more columns, but never more than a single line.
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 speci-
fied. There are as many stream lines as there are
streams in the .xz file.
block
This line type is used only when --verbose was speci-
fied. 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
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not interleaved.
summary
This line type is used only when --verbose was speci-
fied 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 out-
put. It shows the total counts and sizes.
The columns of the file lines:
2. Number of streams in the file
3. Total number of blocks in the stream(s)
4. Compressed size of the file
5. Uncompressed size of the file
6. Compression ratio, for example 0.123. If ratio is
over 9.999, three dashes (---) are displayed
instead of the ratio.
7. Comma-separated list of integrity check names. The
following strings are used for the known check
types: None, CRC32, CRC64, and SHA-256. For
unknown check types, Unknown-N is used, where N is
the Check ID as a decimal number (one or two
digits).
8. Total size of stream padding in the file
The columns of the stream lines:
2. Stream number (the first stream is 1)
3. Number of blocks in the stream
4. Compressed start offset
5. Uncompressed start offset
6. Compressed size (does not include stream padding)
7. Uncompressed size
8. Compression ratio
9. Name of the integrity check
10. Size of stream padding
The columns of the block lines:
2. Number of the stream containing this block
3. Block number relative to the beginning of the
stream (the first block is 1)
4. Block number relative to the beginning of the file
5. Compressed start offset relative to the beginning
of the file
6. Uncompressed start offset relative to the beginning
of the file
7. Total compressed size of the block (includes
headers)
8. Uncompressed size
9. Compression ratio
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10. Name of the integrity check
If --verbose was specified twice, additional columns are
included on the block lines. These are not displayed with a
single --verbose, because getting this information requires
many seeks and can thus be slow:
11. Value of the integrity check in hexadecimal
12. Block header size
13. Block flags: c indicates that compressed size is
present, and u indicates that uncompressed size is
present. If the flag is not set, a dash (-) is
shown instead to keep the string length fixed. New
flags may be added to the end of the string in the
future.
14. Size of the actual compressed data in the block
(this excludes the block header, block padding, and
check fields)
15. Amount of memory (in bytes) required to decompress
this block with this xz version
16. Filter chain. Note that most of the options used
at compression time cannot be known, because only
the options that are needed for decompression are
stored in the .xz headers.
The columns of the summary lines:
2. Amount of memory (in bytes) required to decompress
this file with this xz version
3. yes or no indicating if all block headers have both
compressed size and uncompressed size stored in
them
Since xz 5.1.2alpha:
4. Minimum xz version required to decompress the file
The columns of the totals line:
2. Number of streams
3. Number of blocks
4. Compressed size
5. Uncompressed size
6. Average compression ratio
7. Comma-separated list of integrity check names that
were present in the files
8. Stream padding size
9. Number of files. This is here to keep the order of
the earlier columns the same as on file lines.
If --verbose was specified twice, additional columns are
included on the totals line:
10. Maximum amount of memory (in bytes) required to
decompress the files with this xz version
11. yes or no indicating if all block headers have both
compressed size and uncompressed size stored in
them
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Since xz 5.1.2alpha:
12. Minimum xz version required to decompress the file
Future versions may add new line types and new columns can
be added to the existing line types, but the existing
columns won't be changed.
EXIT STATUS
0 All is good.
1 An error occurred.
2 Something worth a warning occurred, but no actual
errors occurred.
Notices (not warnings or errors) printed on standard error
don't affect the exit status.
ENVIRONMENT
xz parses space-separated lists of options from the environ-
ment 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 argu-
ments.
XZ_DEFAULTS
User-specific or system-wide default options. Typi-
cally 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 possi-
ble to set the options directly on the xz command line.
This is the case e.g. when xz is run by a script or
tool, e.g. GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
Scripts may use XZ_OPT e.g. to set script-specific
default compression options. It is still recommended
to allow users to override XZ_OPT if that is reason-
able, e.g. 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 prob-
lems.
Compression preset levels
The numbering of the compression level presets is not ident-
ical in xz and LZMA Utils. The most important difference is
how dictionary sizes are mapped to different presets. Dic-
tionary 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.
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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 dic-
tionary 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 set-
tings 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 con-
siders 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
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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 sup-
port all the check types, or at least are able to decompress
the file without verifying the integrity check if the par-
ticular 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 compres-
sion level (-6), and remove foo if compression is success-
ful:
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 e.g. the default -6, but needs less memory
for compression and decompression (48 MiB and 5 MiB, respec-
tively):
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 paral-
lelize 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.
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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 custom-
izing 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
(e.g. 32 MiB) to compress well, but you want to compress it
quicker than xz -8 would do, a preset with a low CompCPU
value (e.g. 1) can be modified to use a bigger dictionary:
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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 dic-
tionary while keeping the CompCPU value low. The most obvi-
ous 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 hun-
dred megabytes, it may be useful to use an even bigger dic-
tionary 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 compres-
sor and decompressor. Remember that using a dictionary
bigger than the size of the uncompressed file is waste of
memory, so the above command isn't useful for small files.
Sometimes the compression time doesn't matter, but the
decompressor memory usage has to be kept low e.g. to make it
possible to decompress the file on an embedded system. The
following command uses -6e (-6 --extreme) as a base and sets
the dictionary to only 64 KiB. The resulting file can be
decompressed with XZ Embedded (that's why there is
--check=crc32) using about 100 KiB of memory.
xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo
If you want to squeeze out as many bytes as possible,
adjusting the number of literal context bits (lc) and number
of position bits (pb) can sometimes help. Adjusting the
number of literal position bits (lp) might help too, but
usually lc and pb are more important. E.g. a source code
archive contains mostly US-ASCII text, so something like the
following might give slightly (like 0.1 %) smaller file than
xz -6e (try also without lc=4):
xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar
Using another filter together with LZMA2 can improve
compression with certain file types. E.g. to compress a
x86-32 or x86-64 shared library using the x86 BCJ filter:
xz --x86 --lzma2 libfoo.so
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Note that the order of the filter options is significant.
If --x86 is specified after --lzma2, xz will give an error,
because there cannot be any filter after LZMA2, and also
because the x86 BCJ filter cannot be used as the last filter
in the chain.
The Delta filter together with LZMA2 can give good results
with bitmap images. It should usually beat PNG, which has a
few more advanced filters than simple delta but uses Deflate
for the actual compression.
The image has to be saved in uncompressed format, e.g. as
uncompressed TIFF. The distance parameter of the Delta
filter is set to match the number of bytes per pixel in the
image. E.g. 24-bit RGB bitmap needs dist=3, and it is also
good to pass pb=0 to LZMA2 to accommodate the three-byte
alignment:
xz --delta=dist=3 --lzma2=pb=0 foo.tiff
If multiple images have been put into a single archive (e.g.
.tar), the Delta filter will work on that too as long as all
images have the same number of bytes per pixel.
SEE ALSO
xzdec(1), xzdiff(1), xzgrep(1), xzless(1), xzmore(1),
gzip(1), bzip2(1), 7z(1)
XZ Utils: <http://tukaani.org/xz/>
XZ Embedded: <http://tukaani.org/xz/embedded.html>
LZMA SDK: <http://7-zip.org/sdk.html>
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See also lzma(1)
See also unlzma(1)
See also unxz(1)
See also xz(1)
See also xzcat(1)
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