XZ(1) XZ Utils XZ(1)NAME
xz, unxz, xzcat, lzma, unlzma, lzcat - Compress or decompress .xz and
xz [option]... [file]...
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.
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 also the legacy .lzma format and raw compressed streams
with no container format headers are supported.
xz compresses or decompresses each file according to the selected oper‐
ation 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 stan‐
dard output if it is a terminal. Similarly, xz will refuse to read com‐
pressed 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 file‐
· When decompressing, the .xz or .lzma suffix is removed from the
filename to get the target filename. xz also recognizes the suf‐
fixes .txz and .tlz, and replaces them with the .tar suffix.
If the target file already exists, an error is displayed and the file
Unless writing to standard output, xz will display a warning and skip
the file if any of the following applies:
· File is not a regular file. Symbolic links are not followed, thus
they are never considered to be regular files.
· File has more than one hardlink.
· 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 for‐
· 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
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 permis‐
sions 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
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 automati‐
cally updating progress indicator.
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 affect also the memory usage of the decompressor.
Typically the decompressor needs only 5 % to 20 % of the amount of RAM
that the compressor needed when creating the file. Still, the worst-
case memory usage of the decompressor is several gigabytes.
To prevent uncomfortable surprises caused by huge memory usage, xz has
a built-in memory usage limiter. The default limit is 40 % of total
physical RAM. While operating systems provide ways to limit the memory
usage of processes, relying on it wasn't deemed to be flexible enough.
When compressing, if the selected compression settings exceed the mem‐
ory usage limit, the settings are automatically adjusted downwards and
a notice about this is displayed. As an exception, if the memory usage
limit is exceeded when compressing with --format=raw, an error is dis‐
played and xz will exit with exit status 1.
If source file cannot be decompressed without exceeding the memory
usage limit, an error message is displayed and the file is skipped.
Note that compressed files may contain many blocks, which may have been
compressed with different settings. Typically all blocks will have
roughly the same memory requirements, but it is possible that a block
later in the file will exceed the memory usage limit, and an error
about too low memory usage limit gets displayed after some data has
already been decompressed.
The absolute value of the active memory usage limit can be seen near
the bottom of the output of --long-help. The default limit can be
overriden with --memory=limit.
Integer suffixes and special values
In most places where an integer argument is expected, an optional suf‐
fix is supported to easily indicate large integers. There must be no
space between the integer and the suffix.
k or kB
The integer is multiplied by 1,000 (10^3). For example, 5k or
5kB equals 5000.
Ki or KiB
The integer is multiplied by 1,024 (2^10).
M or MB
The integer is multiplied by 1,000,000 (10^6).
Mi or MiB
The integer is multiplied by 1,048,576 (2^20).
G or GB
The integer is multiplied by 1,000,000,000 (10^9).
Gi or GiB
The integer is multiplied by 1,073,741,824 (2^30).
A special value max can be used to indicate the maximum integer value
supported by the option.
If multiple operation mode options are given, the last one takes
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
Test the integrity of compressed files. No files are created or
removed. This option is equivalent to --decompress --stdout
except that the decompressed data is discarded instead of being
written to standard output.
View information about the 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.
This feature has not been implemented yet.
Keep (don't delete) the input files.
This option has several effects:
· If the target file already exists, delete it before compress‐
ing or decompressing.
· Compress or decompress even if the input is not a regular
file, has more than one hardlink, or has setuid, setgid, or
sticky bit set. The setuid, setgid, and sticky bits are not
copied to the target file.
· If combined with --decompress --stdout and xz doesn't recog‐
nize the type of the source file, xz will copy the source
file as is to standard output. This allows using xzcat
--force like cat(1) for files that have not been compressed
with xz. Note that in future, xz might support new com‐
pressed 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.
· Allow writing compressed data to a terminal, and reading com‐
pressed data from a terminal.
-c, --stdout, --to-stdout
Write the compressed or decompressed data to standard output
instead of a file. This implies --keep.
-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 dis‐
played and the file is skipped.
When decompressing, recognize also 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.
Read the filenames to process from file; if file is omitted,
filenames are read from standard input. Filenames must be termi‐
nated with the newline character. If filenames are given also as
command line arguments, they are processed before the filenames
read from file.
This is identical to --files[=file] except that the filenames
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 equiva‐
lent to xz. When decompressing, the format of the input file
is autodetected. Note that raw streams (created with --for‐
mat=raw) cannot be autodetected.
· xz: Compress to the .xz file format, or accept only .xz files
· lzma or 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
· raw: Compress or uncompress a raw stream (no headers). This
is meant for advanced users only. To decode raw streams, you
need to set not only --format=raw but also specify the filter
chain, which would normally be stored in the container format
-C check, --check=check
Specify the type of the integrity check, which is calculated
from the uncompressed data. This option has an effect only when
compressing into the .xz format; the .lzma format doesn't sup‐
port 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 usu‐
ally 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
· 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 negli‐
· sha256: Calculate SHA-256. This is somewhat slower than CRC32
Integrity of the .xz headers is always verified with CRC32. It
is not possible to change or disable it.
-0 ... -9
Select compression preset. If a preset level is specified multi‐
ple times, the last one takes effect.
The compression preset levels can be categorised roughly into
-0 ... -2
Fast presets with relatively low memory usage. -1 and -2
should give compression speed and ratios comparable to
bzip2 -1 and bzip2 -9, respectively. Currently -0 is not
very good (not much faster than -1 but much worse com‐
pression). In future, -0 may be indicate some fast algo‐
rithm instead of LZMA2.
-3 ... -5
Good compression ratio with low to medium memory usage.
These are significantly slower than levels 0-2.
-6 ... -9
Excellent compression with medium to high memory usage.
These are also slower than the lower preset levels. The
default is -6. Unless you want to maximize the compres‐
sion ratio, you probably don't want a higher preset level
than -7 due to speed and memory usage.
The exact compression settings (filter chain) used by each pre‐
set may vary between xz versions. The settings may also vary
between files being compressed, if xz determines that modified
settings will probably give better compression ratio without
significantly affecting compression time or memory usage.
Because the settings may vary, the memory usage may vary too.
The following table lists the maximum memory usage of each pre‐
set level, which won't be exceeded even in future versions of
FIXME: The table below is just a rough idea.
Preset Compression Decompression
-0 6 MiB 1 MiB
-1 6 MiB 1 MiB
-2 10 MiB 1 MiB
-3 20 MiB 2 MiB
-4 30 MiB 3 MiB
-5 60 MiB 6 MiB
-6 100 MiB 10 MiB
-7 200 MiB 20 MiB
-8 400 MiB 40 MiB
-9 800 MiB 80 MiB
When compressing, xz automatically adjusts the compression set‐
tings downwards if the memory usage limit would be exceeded, so
it is safe to specify a high preset level even on systems that
don't have lots of RAM.
--fast and --best
These are somewhat misleading aliases for -0 and -9, respec‐
tively. These are provided only for backwards compatibility
with LZMA Utils. Avoid using these options.
Especially the name of --best is misleading, because the defini‐
tion of best depends on the input data, and that usually people
don't want the very best compression ratio anyway, because it
would be very slow.
Modify the compression preset (-0 ... -9) so that a little bit
better compression ratio can be achieved without increasing mem‐
ory usage of the compressor or decompressor (exception: compres‐
sor memory usage may increase a little with presets -0 ... -2).
The downside is that the compression time will increase dramati‐
cally (it can easily double).
-M limit, --memory=limit
Set the memory usage limit. If this option is specied multiple
times, the last one takes effect. The limit can be specified in
· The limit can be an absolute value in bytes. Using an integer
suffix like MiB can be useful. Example: --memory=80MiB
· The limit can be specified as a percentage of physical RAM.
· The limit can be reset back to its default value (currently
40 % of physical RAM) by setting it to 0.
· The memory usage limiting can be effectively disabled by set‐
ting limit to max. This isn't recommended. It's usually bet‐
ter to use, for example, --memory=90%.
The current limit can be seen near the bottom of the output of
the --long-help option.
-T threads, --threads=threads
Specify the maximum number of worker threads to use. The default
is the number of available CPU cores. You can see the current
value of threads near the end of the output of the --long-help
The actual number of worker threads can be less than threads if
using more threads would exceed the memory usage limit. In
addition to CPU-intensive worker threads, xz may use a few aux‐
iliary threads, which don't use a lot of CPU time.
Multithreaded compression and decompression are not implemented
yet, so this option has no effect for now.
Custom compressor filter chains
A custom filter chain allows specifying the compression settings in
detail instead of relying on the settings associated to the preset lev‐
els. When a custom filter chain is specified, the compression preset
level options (-0 ... -9 and --extreme) are silently ignored.
A filter chain is comparable to piping on the UN*X 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
Many filters have limitations 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 fil‐
ter 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 (--for‐
mat=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.
Add LZMA1 or LZMA2 filter to the filter chain. These filter 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:
Reset all LZMA1 or LZMA2 options to preset. Preset con‐
sist of an integer, which may be followed by single-let‐
ter 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
The default preset is 6, from which the default values
for the rest of the LZMA1 or LZMA2 options are taken.
Dictionary (history buffer) size indicates how many bytes
of the recently processed uncompressed data is kept in
memory. One method to reduce size of the uncompressed
data is to store distance-length pairs, which indicate
what data to repeat from the dictionary buffer. The big‐
ger the dictionary, the better the compression ratio usu‐
ally is, but dictionaries bigger than the uncompressed
data are waste of RAM.
Typical dictionary size is from 64 KiB to 64 MiB. The
minimum is 4 KiB. The maximum for compression is cur‐
rently 1.5 GiB. The decompressor already supports dictio‐
naries up to one byte less than 4 GiB, which is the maxi‐
mum for LZMA1 and LZMA2 stream formats.
Dictionary size has the biggest effect on compression
ratio. Dictionary size and match finder together deter‐
mine the memory usage of the LZMA1 or LZMA2 encoder. The
same 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
lc=lc Specify the number of literal context bits. The minimum
is 0 and the maximum is 4; the default is 3. In addi‐
tion, the sum of lc and lp must not exceed 4.
lp=lp Specify the number of literal position bits. The minimum
is 0 and the maximum is 4; the default is 0.
pb=pb Specify the number of position bits. The minimum is 0 and
the maximum is 4; the default is 2.
Compression mode specifies the function used to analyze
the data produced by the match finder. Supported modes
are fast and normal. The default is fast for presets 0-2
and normal for presets 3-9.
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. Hash
Chains are usually used together with mode=fast and
Binary Trees with mode=normal. The memory usage formulas
are only rough estimates, which are closest to 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
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
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 bet‐
ter compression ratio at expense of speed. The default
depends on the preset level.
Specify the maximum search depth in the match finder. The
default is the special value 0, which makes the compres‐
sor determine a reasonable depth from mf and nice.
Using very high values for depth can make the encoder
extremely slow with carefully crafted files. Avoid set‐
ting the depth over 1000 unless you are prepared to
interrupt the compression in case it is taking too long.
When decoding raw streams (--format=raw), LZMA2 needs only the
value of dict. LZMA1 needs also lc, lp, and pb.
Add a branch/call/jump (BCJ) filter to the filter chain. These
filters can be used only as 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 allows e.g. LZMA2 to
get better compression ratio.
The BCJ filters are always reversible, so using a BCJ filter for
wrong type of data doesn't cause any data loss. However, apply‐
ing a BCJ filter for wrong type of data is a bad idea, because
it tends to make the compression ratio worse.
Different instruction sets have have different alignment:
Filter Alignment Notes
x86 1 32-bit and 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 fil‐
ter. 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 usu‐
ally good to stick to LZMA2's default four-byte alignment when
compressing x86 executables.
All BCJ filters support the same options:
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 ta‐
ble above). The default is zero. In practice, the
default is good; specifying a custom offset is almost
Specifying a non-zero start offset is probably useful
only if the executable has multiple sections, and there
are many cross-section jumps or calls. Applying a BCJ
filter separately for each section with proper start off‐
set and then compressing the result as a single chunk may
give some improvement in compression ratio compared to
applying the BCJ filter with the default offset for the
Add Delta filter to the filter chain. The Delta filter can be
used only as 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 algo‐
rithms may give significantly better results than Delta + LZMA2.
This is true especially with audio, which compresses faster and
better e.g. with FLAC.
Specify the distance of the delta calculation as 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.
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 indi‐
cate a warning is still used.
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 (useful mostly for debug‐
Don't set the exit status to 2 even if a condition worth a warn‐
ing 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.
Display a help message describing the most commonly used
options, and exit successfully.
Display a help message describing all features of xz, and exit
Display the version number of xz and liblzma.
0 All is good.
1 An error occurred.
2 Something worth a warning occurred, but no actual errors
Notices (not warnings or errors) printed on standard error don't affect
the exit status.
XZ_OPT A space-separated list of options is parsed from XZ_OPT before
parsing the options given on the command line. Note that only
options are parsed from XZ_OPT; all non-options are silently
ignored. Parsing is done with getopt_long(3) which is used also
for the command line arguments.
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
Compression preset levels
The numbering of the compression level presets is not identical in xz
and LZMA Utils. The most important difference is how dictionary sizes
are mapped to different presets. Dictionary size is roughly equal to
the decompressor memory usage.
Level xz LZMA Utils
-1 64 KiB 64 KiB
-2 512 KiB 1 MiB
-3 1 MiB 512 KiB
-4 2 MiB 1 MiB
-5 4 MiB 2 MiB
-6 8 MiB 4 MiB
-7 16 MiB 8 MiB
-8 32 MiB 16 MiB
-9 64 MiB 32 MiB
The dictionary size differences affect the compressor memory usage too,
but there are some other differences between LZMA Utils and XZ Utils,
which make the difference even bigger:
Level xz LZMA Utils 4.32.x
-1 2 MiB 2 MiB
-2 5 MiB 12 MiB
-3 13 MiB 12 MiB
-4 25 MiB 16 MiB
-5 48 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 8 MiB dictionary by default.
Streamed vs. non-streamed .lzma files
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 exam‐
ple 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 exam‐
ple, 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
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 dictionary size of 2^n
(a power of 2), but accepts files with any dictionary size. liblzma
accepts only .lzma files which have dictionary size of 2^n or 2^n +
2^(n-1). This is to decrease false positives when autodetecting .lzma
These limitations shouldn't be a problem in practice, since practically
all .lzma files have been compressed with settings that liblzma will
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.
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 bet‐
ter compression) without affecting the file format. The output can vary
even between different builds of the same XZ Utils version, if differ‐
ent build options are used or if the endianness of the hardware is dif‐
ferent for different builds.
The above means that implementing --rsyncable to create rsyncable .xz
files is not going to happen without freezing a part of the encoder
implementation, which can then be used with --rsyncable.
Embedded .xz decompressors
Embedded .xz decompressor implementations like XZ Embedded don't neces‐
sarily support files created with 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 veri‐
fying the integrity check if the particular check is not supported.
XZ Embedded supports BCJ filters, but only with the default start off‐
SEE ALSOxzdec(1), gzip(1), bzip2(1)
XZ Utils: <http://tukaani.org/xz/>
XZ Embedded: <http://tukaani.org/xz/embedded.html>
LZMA SDK: <http://7-zip.org/sdk.html>
Tukaani 2009-08-27 XZ(1)