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2013-06-21
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.
xz compresses or decompresses each file according to the selected
operation mode. If no files are given or file is -, xz reads from
standard input and writes the processed data to standard output. xz
will refuse (display an error and skip the file) to write compressed
data to standard output if it is a terminal. Similarly, xz will
refuse to read compressed data from standard input if it is a
terminal.
Unless --stdout is specified, files other than - are written to a new
file whose name is derived from the source file name:
+ When compressing, the suffix of the target file format (.xz or
.lzma) is appended to the source filename to get the target
filename.
+ When decompressing, the .xz or .lzma suffix is removed from the
filename to get the target filename. xz also recognizes the
suffixes .txz and .tlz, and replaces them with the .tar suffix.
If the target file already exists, an error is displayed and the file
is skipped.
Unless writing to standard output, xz will display a warning and skip
the file if any of the following applies:
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+ File is not a regular file. Symbolic links are not followed, and
thus they are not considered to be regular files.
+ File has more than one hard link.
+ File has setuid, setgid, or sticky bit set.
+ The operation mode is set to compress and the file already has a
suffix of the target file format (.xz or .txz when compressing to
the .xz format, and .lzma or .tlz when compressing to the .lzma
format).
+ The operation mode is set to decompress and the file doesn't have a
suffix of any of the supported file formats (.xz, .txz, .lzma, or
.tlz).
After successfully compressing or decompressing the file, xz copies
the owner, group, permissions, access time, and modification time from
the source file to the target file. If copying the group fails, the
permissions are modified so that the target file doesn't become
accessible to users who didn't have permission to access the source
file. xz doesn't support copying other metadata like access control
lists or extended attributes yet.
Once the target file has been successfully closed, the source file is
removed unless --keep was specified. The source file is never removed
if the output is written to standard output.
Sending SIGINFO or SIGUSR1 to the xz process makes it print progress
information to standard error. This has only limited use since when
standard error is a terminal, using --verbose will display an
automatically updating progress indicator.
Memory usage
The memory usage of xz varies from a few hundred kilobytes to several
gigabytes depending on the compression settings. The settings used
when compressing a file determine the memory requirements of the
decompressor. Typically the decompressor needs 5 % to 20 % of the
amount of memory that the compressor needed when creating the file.
For example, decompressing a file created with xz -9 currently
requires 65 MiB of memory. Still, it is possible to have .xz files
that require several gigabytes of memory to decompress.
Especially users of older systems may find the possibility of very
large memory usage annoying. To prevent uncomfortable surprises, xz
has a built-in memory usage limiter, which is disabled by default.
While some operating systems provide ways to limit the memory usage of
processes, relying on it wasn't deemed to be flexible enough (e.g.
using ulimit(1) to limit virtual memory tends to cripple mmap(2)).
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The memory usage limiter can be enabled with the command line option
--memlimit=limit. Often it is more convenient to enable the limiter
by default by setting the environment variable XZ_DEFAULTS, e.g.
XZ_DEFAULTS=--memlimit=150MiB. It is possible to set the limits
separately for compression and decompression by using
--memlimit-compress=limit and --memlimit-decompress=limit. Using
these two options outside XZ_DEFAULTS is rarely useful because a
single run of xz cannot do both compression and decompression and
--memlimit=limit (or -M limit) is shorter to type on the command line.
If the specified memory usage limit is exceeded when decompressing, xz
will display an error and decompressing the file will fail. If the
limit is exceeded when compressing, xz will try to scale the settings
down so that the limit is no longer exceeded (except when using
--format=raw or --no-adjust). This way the operation won't fail
unless the limit is very small. The scaling of the settings is done
in steps that don't match the compression level presets, e.g. if the
limit is only slightly less than the amount required for xz -9, the
settings will be scaled down only a little, not all the way down to xz
-8.
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.
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.
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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 e.g.
less -S may be convenient if the terminal isn't wide enough.
The exact output may vary between xz versions and different
locales. For machine-readable output, --robot --list should be
used.
Operation modifiers
-k, --keep
Don't delete the input files.
-f, --force
This option has several effects:
+ If the target file already exists, delete it before
compressing or decompressing.
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+ 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.
--no-sparse
Disable creation of sparse files. By default, if decompressing
into a regular file, xz tries to make the file sparse if the
decompressed data contains long sequences of binary zeros. It
also works when writing to standard output as long as standard
output is connected to a regular file and certain additional
conditions are met to make it safe. Creating sparse files may
save disk space and speed up the decompression by reducing the
amount of disk I/O.
-S .suf, --suffix=.suf
When compressing, use .suf as the suffix for the target file
instead of .xz or .lzma. If not writing to standard output and
the source file already has the suffix .suf, a warning is
displayed and the file is skipped.
When decompressing, recognize files with the suffix .suf in
addition to files with the .xz, .txz, .lzma, or .tlz suffix. If
the source file has the suffix .suf, the suffix is removed to get
the target filename.
When compressing or decompressing raw streams (--format=raw), the
suffix must always be specified unless writing to standard
output, because there is no default suffix for raw streams.
--files[=file]
Read the filenames to process from file; if file is omitted,
filenames are read from standard input. Filenames must be
terminated with the newline character. A dash (-) is taken as a
regular filename; it doesn't mean standard input. If filenames
are given also as command line arguments, they are processed
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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.
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
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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.
-0 ... -9
Select a compression preset level. The default is -6. If
multiple preset levels are specified, the last one takes effect.
If a custom filter chain was already specified, setting a
compression preset level clears the custom filter chain.
The differences between the presets are more significant than
with gzip(1) and bzip2(1). The selected compression settings
determine the memory requirements of the decompressor, thus using
a too high preset level might make it painful to decompress the
file on an old system with little RAM. Specifically, it's not a
good idea to blindly use -9 for everything like it often is with
gzip(1) and bzip2(1).
-0 ... -3
These are somewhat fast presets. -0 is sometimes faster
than gzip -9 while compressing much better. The higher ones
often have speed comparable to bzip2(1) with comparable or
better compression ratio, although the results depend a lot
on the type of data being compressed.
-4 ... -6
Good to very good compression while keeping decompressor
memory usage reasonable even for old systems. -6 is the
default, which is usually a good choice e.g. for
distributing files that need to be decompressible even on
systems with only 16 MiB RAM. (-5e or -6e may be worth
considering too. See --extreme.)
-7 ... -9
These are like -6 but with higher compressor and
decompressor memory requirements. These are useful only
when compressing files bigger than 8 MiB, 16 MiB, and
32 MiB, respectively.
On the same hardware, the decompression speed is approximately a
constant number of bytes of compressed data per second. In other
words, the better the compression, the faster the decompression
will usually be. This also means that the amount of uncompressed
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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. Memory requirements of some of the future
multithreaded modes may be dramatically higher than that of
the single-threaded mode.
+ DecMem contains the decompressor memory requirements. That
is, the compression settings determine the memory requirements
of the decompressor. The exact decompressor memory usage is
slightly more than the LZMA2 dictionary size, but the values
in the table have been rounded up to the next full MiB.
-e, --extreme
Use a slower variant of the selected compression preset level (-0
... -9) to hopefully get a little bit better compression ratio,
but with bad luck this can also make it worse. Decompressor
memory usage is not affected, but compressor memory usage
increases a little at preset levels -0 ... -3.
Since there are two presets with dictionary sizes 4 MiB and
8 MiB, the presets -3e and -5e use slightly faster settings
(lower CompCPU) than -4e and -6e, respectively. That way no two
presets are identical.
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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.
--memlimit-compress=limit
Set a memory usage limit for compression. If this option is
specified multiple times, the last one takes effect.
If the compression settings exceed the limit, xz will adjust the
settings downwards so that the limit is no longer exceeded and
display a notice that automatic adjustment was done. Such
adjustments are not made when compressing with --format=raw or if
--no-adjust has been specified. In those cases, an error is
displayed and xz will exit with exit status 1.
The limit can be specified in multiple ways:
+ The limit can be an absolute value in bytes. Using an integer
suffix like MiB can be useful. Example:
--memlimit-compress=80MiB
+ The limit can be specified as a percentage of total physical
memory (RAM). This can be useful especially when setting the
XZ_DEFAULTS environment variable in a shell initialization
script that is shared between different computers. That way
the limit is automatically bigger on systems with more memory.
Example: --memlimit-compress=70%
+ The limit can be reset back to its default value by setting it
to 0. This is currently equivalent to setting the limit to
max (no memory usage limit). Once multithreading support has
been implemented, there may be a difference between 0 and max
for the multithreaded case, so it is recommended to use 0
instead of max until the details have been decided.
See also the section Memory usage.
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--memlimit-decompress=limit
Set a memory usage limit for decompression. This also affects
the --list mode. If the operation is not possible without
exceeding the limit, xz will display an error and decompressing
the file will fail. See --memlimit-compress=limit for possible
ways to specify the limit.
-M limit, --memlimit=limit, --memory=limit
This is equivalent to specifying --memlimit-compress=limit
--memlimit-decompress=limit.
--no-adjust
Display an error and exit if the compression settings exceed the
memory usage limit. The default is to adjust the settings
downwards so that the memory usage limit is not exceeded.
Automatic adjusting is always disabled when creating raw streams
(--format=raw).
-T threads, --threads=threads
Specify the number of worker threads to use. The actual number
of threads can be less than threads if using more threads would
exceed the memory usage limit.
Multithreaded compression and decompression are not implemented
yet, so this option has no effect for now.
As of writing (2010-09-27), it hasn't been decided if threads
will be used by default on multicore systems once support for
threading has been implemented. Comments are welcome. The
complicating factor is that using many threads will increase the
memory usage dramatically. Note that if multithreading will be
the default, it will probably be done so that single-threaded and
multithreaded modes produce the same output, so compression ratio
won't be significantly affected if threading will be enabled by
default.
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
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filters in the chain is four, but typically a filter chain has only
one or two filters.
Many filters have limitations on where they can be in the filter
chain: some filters can work only as the last filter in the chain,
some only as a non-last filter, and some work in any position in the
chain. Depending on the filter, this limitation is either inherent to
the filter design or exists to prevent security issues.
A custom filter chain is specified by using one or more filter options
in the order they are wanted in the filter chain. That is, the order
of filter options is significant! When decoding raw streams
(--format=raw), the filter chain is specified in the same order as it
was specified when compressing.
Filters take filter-specific options as a comma-separated list. Extra
commas in options are ignored. Every option has a default value, so
you need to specify only those you want to change.
To see the whole filter chain and options, use xz -vv (that is, use
--verbose twice). This works also for viewing the filter chain
options used by presets.
--lzma1[=options]
--lzma2[=options]
Add LZMA1 or LZMA2 filter to the filter chain. These filters can
be used only as the last filter in the chain.
LZMA1 is a legacy filter, which is supported almost solely due to
the legacy .lzma file format, which supports only LZMA1. LZMA2
is an updated version of LZMA1 to fix some practical issues of
LZMA1. The .xz format uses LZMA2 and doesn't support LZMA1 at
all. Compression speed and ratios of LZMA1 and LZMA2 are
practically the same.
LZMA1 and LZMA2 share the same set of options:
preset=preset
Reset all LZMA1 or LZMA2 options to preset. Preset consist
of an integer, which may be followed by single-letter preset
modifiers. The integer can be from 0 to 9, matching the
command line options -0 ... -9. The only supported modifier
is currently e, which matches --extreme. If no preset is
specified, the default values of LZMA1 or LZMA2 options are
taken from the preset 6.
dict=size
Dictionary (history buffer) size indicates how many bytes of
the recently processed uncompressed data is kept in memory.
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The algorithm tries to find repeating byte sequences
(matches) in the uncompressed data, and replace them with
references to the data currently in the dictionary. The
bigger the dictionary, the higher is the chance to find a
match. Thus, increasing dictionary size usually improves
compression ratio, but a dictionary bigger than the
uncompressed file is waste of memory.
Typical dictionary size is from 64 KiB to 64 MiB. The
minimum is 4 KiB. The maximum for compression is currently
1.5 GiB (1536 MiB). The decompressor already supports
dictionaries up to one byte less than 4 GiB, which is the
maximum for the LZMA1 and LZMA2 stream formats.
Dictionary size and match finder (mf) together determine the
memory usage of the LZMA1 or LZMA2 encoder. The same (or
bigger) dictionary size is required for decompressing that
was used when compressing, thus the memory usage of the
decoder is determined by the dictionary size used when
compressing. The .xz headers store the dictionary size
either as 2^n or 2^n + 2^(n-1), so these sizes are somewhat
preferred for compression. Other sizes will get rounded up
when stored in the .xz headers.
lc=lc
Specify the number of literal context bits. The minimum is
0 and the maximum is 4; the default is 3. In addition, the
sum of lc and lp must not exceed 4.
All bytes that cannot be encoded as matches are encoded as
literals. That is, literals are simply 8-bit bytes that are
encoded one at a time.
The literal coding makes an assumption that the highest lc
bits of the previous uncompressed byte correlate with the
next byte. E.g. in typical English text, an upper-case
letter is often followed by a lower-case letter, and a
lower-case letter is usually followed by another lower-case
letter. In the US-ASCII character set, the highest three
bits are 010 for upper-case letters and 011 for lower-case
letters. When lc is at least 3, the literal coding can take
advantage of this property in the uncompressed data.
The default value (3) is usually good. If you want maximum
compression, test lc=4. Sometimes it helps a little, and
sometimes it makes compression worse. If it makes it worse,
test e.g. lc=2 too.
lp=lp
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Specify the number of literal position bits. The minimum is
0 and the maximum is 4; the default is 0.
Lp affects what kind of alignment in the uncompressed data
is assumed when encoding literals. See pb below for more
information about alignment.
pb=pb
Specify the number of position bits. The minimum is 0 and
the maximum is 4; the default is 2.
Pb affects what kind of alignment in the uncompressed data
is assumed in general. The default means four-byte
alignment (2^pb=2^2=4), which is often a good choice when
there's no better guess.
When the aligment is known, setting pb accordingly may
reduce the file size a little. E.g. with text files having
one-byte alignment (US-ASCII, ISO-8859-*, UTF-8), setting
pb=0 can improve compression slightly. For UTF-16 text,
pb=1 is a good choice. If the alignment is an odd number
like 3 bytes, pb=0 might be the best choice.
Even though the assumed alignment can be adjusted with pb
and lp, LZMA1 and LZMA2 still slightly favor 16-byte
alignment. It might be worth taking into account when
designing file formats that are likely to be often
compressed with LZMA1 or LZMA2.
mf=mf
Match finder has a major effect on encoder speed, memory
usage, and compression ratio. Usually Hash Chain match
finders are faster than Binary Tree match finders. The
default depends on the preset: 0 uses hc3, 1-3 use hc4, and
the rest use bt4.
The following match finders are supported. The memory usage
formulas below are rough approximations, which are closest
to the reality when dict is a power of two.
hc3 Hash Chain with 2- and 3-byte hashing
Minimum value for nice: 3
Memory usage:
dict * 7.5 (if dict <= 16 MiB);
dict * 5.5 + 64 MiB (if dict > 16 MiB)
hc4 Hash Chain with 2-, 3-, and 4-byte hashing
Minimum value for nice: 4
Memory usage:
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dict * 7.5 (if dict <= 32 MiB);
dict * 6.5 (if dict > 32 MiB)
bt2 Binary Tree with 2-byte hashing
Minimum value for nice: 2
Memory usage: dict * 9.5
bt3 Binary Tree with 2- and 3-byte hashing
Minimum value for nice: 3
Memory usage:
dict * 11.5 (if dict <= 16 MiB);
dict * 9.5 + 64 MiB (if dict > 16 MiB)
bt4 Binary Tree with 2-, 3-, and 4-byte hashing
Minimum value for nice: 4
Memory usage:
dict * 11.5 (if dict <= 32 MiB);
dict * 10.5 (if dict > 32 MiB)
mode=mode
Compression mode specifies the method to analyze the data
produced by the match finder. Supported modes are fast and
normal. The default is fast for presets 0-3 and normal for
presets 4-9.
Usually fast is used with Hash Chain match finders and
normal with Binary Tree match finders. This is also what
the presets do.
nice=nice
Specify what is considered to be a nice length for a match.
Once a match of at least nice bytes is found, the algorithm
stops looking for possibly better matches.
Nice can be 2-273 bytes. Higher values tend to give better
compression ratio at the expense of speed. The default
depends on the preset.
depth=depth
Specify the maximum search depth in the match finder. The
default is the special value of 0, which makes the
compressor determine a reasonable depth from mf and nice.
Reasonable depth for Hash Chains is 4-100 and 16-1000 for
Binary Trees. Using very high values for depth can make the
encoder extremely slow with some files. Avoid setting the
depth over 1000 unless you are prepared to interrupt the
compression in case it is taking far too long.
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When decoding raw streams (--format=raw), LZMA2 needs only the
dictionary size. LZMA1 needs also lc, lp, and pb.
--x86[=options]
--powerpc[=options]
--ia64[=options]
--arm[=options]
--armthumb[=options]
--sparc[=options]
Add a branch/call/jump (BCJ) filter to the filter chain. These
filters can be used only as a non-last filter in the filter
chain.
A BCJ filter converts relative addresses in the machine code to
their absolute counterparts. This doesn't change the size of the
data, but it increases redundancy, which can help LZMA2 to
produce 0-15 % smaller .xz file. The BCJ filters are always
reversible, so using a BCJ filter for wrong type of data doesn't
cause any data loss, although it may make the compression ratio
slightly worse.
It is fine to apply a BCJ filter on a whole executable; there's
no need to apply it only on the executable section. Applying a
BCJ filter on an archive that contains both executable and non-
executable files may or may not give good results, so it
generally isn't good to blindly apply a BCJ filter when
compressing binary packages for distribution.
These BCJ filters are very fast and use insignificant amount of
memory. If a BCJ filter improves compression ratio of a file, it
can improve decompression speed at the same time. This is
because, on the same hardware, the decompression speed of LZMA2
is roughly a fixed number of bytes of compressed data per second.
These BCJ filters have known problems related to the compression
ratio:
+ Some types of files containing executable code (e.g. object
files, static libraries, and Linux kernel modules) have the
addresses in the instructions filled with filler values.
These BCJ filters will still do the address conversion, which
will make the compression worse with these files.
+ Applying a BCJ filter on an archive containing multiple
similar executables can make the compression ratio worse than
not using a BCJ filter. This is because the BCJ filter
doesn't detect the boundaries of the executable files, and
doesn't reset the address conversion counter for each
executable.
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Both of the above problems will be fixed in the future in a new
filter. The old BCJ filters will still be useful in embedded
systems, because the decoder of the new filter will be bigger and
use more memory.
Different instruction sets have have different alignment:
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 converting
between relative and absolute addresses. The offset must be
a multiple of the alignment of the filter (see the table
above). The default is zero. In practice, the default is
good; specifying a custom offset is almost never useful.
--delta[=options]
Add the Delta filter to the filter chain. The Delta filter can
be only used as a non-last filter in the filter chain.
Currently only simple byte-wise delta calculation is supported.
It can be useful when compressing e.g. uncompressed bitmap images
or uncompressed PCM audio. However, special purpose algorithms
may give significantly better results than Delta + LZMA2. This
is true especially with audio, which compresses faster and better
e.g. with flac(1).
Supported options:
dist=distance
Specify the distance of the delta calculation in bytes.
distance must be 1-256. The default is 1.
For example, with dist=2 and eight-byte input A1 B1 A2 B3 A3
B5 A4 B7, the output will be A1 B1 01 02 01 02 01 02.
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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 terminal, xz
will display a progress indicator. Specifying --verbose twice
will give even more verbose output.
The progress indicator shows the following information:
+ Completion percentage is shown if the size of the input file
is known. That is, the percentage cannot be shown in pipes.
+ Amount of compressed data produced (compressing) or consumed
(decompressing).
+ Amount of uncompressed data consumed (compressing) or produced
(decompressing).
+ Compression ratio, which is calculated by dividing the amount
of compressed data processed so far by the amount of
uncompressed data processed so far.
+ Compression or decompression speed. This is measured as the
amount of uncompressed data consumed (compression) or produced
(decompression) per second. It is shown after a few seconds
have passed since xz started processing the file.
+ Elapsed time in the format M:SS or H:MM:SS.
+ Estimated remaining time is shown only when the size of the
input file is known and a couple of seconds have already
passed since xz started processing the file. The time is
shown in a less precise format which never has any colons,
e.g. 2 min 30 s.
When standard error is not a terminal, --verbose will make xz
print the filename, compressed size, uncompressed size,
compression ratio, and possibly also the speed and elapsed time
on a single line to standard error after compressing or
decompressing the file. The speed and elapsed time are included
only when the operation took at least a few seconds. If the
operation didn't finish, e.g. due to user interruption, also the
completion percentage is printed if the size of the input file is
known.
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-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.
YYY Minor version. Even numbers are stable. Odd numbers are alpha
or beta versions.
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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 special value of
zero indicates the default setting, which for single-threaded mode
is the same as no limit.
In the future, the output of xz --robot --info-memory may have more
columns, but never more than a single line.
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.
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summary
This line type is used only when --verbose was specified twice.
This line is printed after all block lines. Like the file line,
the summary line contains overall information about the .xz file.
totals
This line is always the very last line of the list output. It
shows the total counts and sizes.
The columns of the file lines:
2. Number of streams in the file
3. Total number of blocks in the stream(s)
4. Compressed size of the file
5. Uncompressed size of the file
6. Compression ratio, for example 0.123. If ratio is over
9.999, three dashes (---) are displayed instead of the ratio.
7. Comma-separated list of integrity check names. The following
strings are used for the known check types: None, CRC32,
CRC64, and SHA-256. For unknown check types, Unknown-N is
used, where N is the Check ID as a decimal number (one or two
digits).
8. Total size of stream padding in the file
The columns of the stream lines:
2. Stream number (the first stream is 1)
3. Number of blocks in the stream
4. Compressed start offset
5. Uncompressed start offset
6. Compressed size (does not include stream padding)
7. Uncompressed size
8. Compression ratio
9. Name of the integrity check
10. Size of stream padding
The columns of the block lines:
2. Number of the stream containing this block
3. Block number relative to the beginning of the stream (the
first block is 1)
4. Block number relative to the beginning of the file
5. Compressed start offset relative to the beginning of the file
6. Uncompressed start offset relative to the beginning of the
file
7. Total compressed size of the block (includes headers)
8. Uncompressed size
9. Compression ratio
10. Name of the integrity check
If --verbose was specified twice, additional columns are included on
the block lines. These are not displayed with a single --verbose,
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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
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
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.
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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
e.g. when xz is run by a script or tool, e.g. GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
Scripts may use XZ_OPT e.g. to set script-specific default
compression options. It is still recommended to allow users to
override XZ_OPT if that is reasonable, e.g. in sh(1) scripts one
may use something like this:
XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT
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
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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.
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
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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. This may break obscure scripts which have assumed
that trailing garbage is ignored.
NOTES
Compressed output may vary
The exact compressed output produced from the same uncompressed input
file may vary between XZ Utils versions even if compression options
are identical. This is because the encoder can be improved (faster or
better compression) without affecting the file format. The output can
vary even between different builds of the same XZ Utils version, if
different build options are used.
The above means that once --rsyncable has been implemented, the
resulting files won't necessarily be rsyncable unless both old and new
files have been compressed with the same xz version. This problem can
be fixed if a part of the encoder implementation is frozen to keep
rsyncable output stable across xz versions.
Embedded .xz decompressors
Embedded .xz decompressor implementations like XZ Embedded don't
necessarily support files created with integrity check types other
than none and crc32. Since the default is --check=crc64, you must use
--check=none or --check=crc32 when creating files for embedded
systems.
Outside embedded systems, all .xz format decompressors support all the
check types, or at least are able to decompress the file without
verifying the integrity check if the particular check is not
supported.
XZ Embedded supports BCJ filters, but only with the default start
offset.
EXAMPLES
Basics
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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 e.g. the default -6, but needs less memory for compression and
decompression (48 MiB and 5 MiB, respectively):
tar cf - baz | xz -4e > baz.tar.xz
A mix of compressed and uncompressed files can be decompressed to
standard output with a single command:
xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt
Parallel compression of many files
On GNU and *BSD, find(1) and xargs(1) can be used to parallelize
compression of many files:
find . -type f \! -name '*.xz' -print0 \
| xargs -0r -P4 -n16 xz -T1
The -P option to xargs(1) sets the number of parallel xz processes.
The best value for the -n option depends on how many files there are
to be compressed. If there are only a couple of files, the value
should probably be 1; with tens of thousands of files, 100 or even
more may be appropriate to reduce the number of xz processes that
xargs(1) will eventually create.
The option -T1 for xz is there to force it to single-threaded mode,
because xargs(1) is used to control the amount of parallelization.
Robot mode
Calculate how many bytes have been saved in total after compressing
multiple files:
xz --robot --list *.xz | awk '/^totals/{print $5-$4}'
A script may want to know that it is using new enough xz. The
following sh(1) script checks that the version number of the xz tool
is at least 5.0.0. This method is compatible with old beta versions,
which didn't support the --robot option:
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if ! eval "$(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 (e.g. 32 MiB)
to compress well, but you want to compress it quicker than xz -8 would
do, a preset with a low CompCPU value (e.g. 1) can be modified to use
a bigger dictionary:
xz --lzma2=preset=1,dict=32MiB foo.tar
With certain files, the above command may be faster than xz -6 while
compressing significantly better. However, it must be emphasized that
only some files benefit from a big dictionary while keeping the
CompCPU value low. The most obvious situation, where a big dictionary
can help a lot, is an archive containing very similar files of at
least a few megabytes each. The dictionary size has to be
significantly bigger than any individual file to allow LZMA2 to take
full advantage of the similarities between consecutive files.
If very high compressor and decompressor memory usage is fine, and the
file being compressed is at least several hundred megabytes, it may be
useful to use an even bigger dictionary than the 64 MiB that xz -9
would use:
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xz -vv --lzma2=dict=192MiB big_foo.tar
Using -vv (--verbose --verbose) like in the above example can be
useful to see the memory requirements of the compressor and
decompressor. Remember that using a dictionary bigger than the size
of the uncompressed file is waste of memory, so the above command
isn't useful for small files.
Sometimes the compression time doesn't matter, but the decompressor
memory usage has to be kept low e.g. to make it possible to decompress
the file on an embedded system. The following command uses -6e (-6
--extreme) as a base and sets the dictionary to only 64 KiB. The
resulting file can be decompressed with XZ Embedded (that's why there
is --check=crc32) using about 100 KiB of memory.
xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo
If you want to squeeze out as many bytes as possible, adjusting the
number of literal context bits (lc) and number of position bits (pb)
can sometimes help. Adjusting the number of literal position bits
(lp) might help too, but usually lc and pb are more important. E.g. a
source code archive contains mostly US-ASCII text, so something like
the following might give slightly (like 0.1 %) smaller file than xz
-6e (try also without lc=4):
xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar
Using another filter together with LZMA2 can improve compression with
certain file types. E.g. to compress a x86-32 or x86-64 shared
library using the x86 BCJ filter:
xz --x86 --lzma2 libfoo.so
Note that the order of the filter options is significant. If --x86 is
specified after --lzma2, xz will give an error, because there cannot
be any filter after LZMA2, and also because the x86 BCJ filter cannot
be used as the last filter in the chain.
The Delta filter together with LZMA2 can give good results with bitmap
images. It should usually beat PNG, which has a few more advanced
filters than simple delta but uses Deflate for the actual compression.
The image has to be saved in uncompressed format, e.g. as uncompressed
TIFF. The distance parameter of the Delta filter is set to match the
number of bytes per pixel in the image. E.g. 24-bit RGB bitmap needs
dist=3, and it is also good to pass pb=0 to LZMA2 to accommodate the
three-byte alignment:
xz --delta=dist=3 --lzma2=pb=0 foo.tiff
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XZ(1) Tukaani XZ(1)
XZ Utils XZ Utils
2013-06-21
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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