xzcat Man page

XZ(1) XZ Utils XZ(1)


xz, unxz, xzcat, lzma, unlzma, lzcat – Compress or decompress .xz and
.lzma files


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 and bzip2. The native file format is the .xz
format, but the legacy .lzma format used by LZMA Utils and raw com‐
pressed streams with no container format headers are also 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
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 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
is skipped.

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, 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 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 per‐
missions 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 automati‐
cally 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 decom‐
pressor. 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)).

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 sepa‐
rately for compression and decompression by using –memlimit-com‐
press=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 –for‐
mat=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 use‐
ful 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


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.

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.

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

-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

-t, –test
Test the integrity of compressed files. This option is equiva‐
lent 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 out‐
put 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

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 compress‐
ing or decompressing.

· Compress or decompress even if the input is a symbolic link
to a regular file, has more than one hard link, or has the
setuid, setgid, or sticky bit set. The setuid, setgid, and
sticky bits are not copied to the target file.

· When used with –decompress –stdout and xz cannot recognize
the type of the source file, copy the source file as is to
standard output. This allows xzcat –force to be used like
cat 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 for‐

-c, –stdout, –to-stdout
Write the compressed or decompressed data to standard output
instead of a file. This implies –keep.

Decompress only the first .xz stream, and silently ignore possi‐
ble 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 –decom‐
press or –test.

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 dis‐
played 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 ter‐
minated with the newline character. A dash (-) is taken as a
regular filename; it doesn’t mean standard input. If filenames
are given also as command line arguments, they are processed
before the filenames read from file.

–files0[=file] This is identical to –files[=file] except that each filename
must be terminated with the null character.

Basic file format and compression options
-F format, –format=format
Specify the file format to compress or decompress:

auto This is the default. When compressing, auto is equiva‐
lent 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

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 fil‐
ter 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 calcu‐
lated 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 decom‐

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 neg‐

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 mul‐
tiple preset levels are specified, the last one takes effect.
If a custom filter chain was already specified, setting a com‐
pression preset level clears the custom filter chain.

The differences between the presets are more significant than
with gzip and bzip2. The selected compression settings
determine the memory requirements of the decompressor, thus
using a too high preset level might make it painful to decom‐
press 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 and bzip2.

-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 with compa‐
rable 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 dis‐
tributing 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 decom‐
pressor memory requirements. These are useful only when
compressing files bigger than 8 MiB, 16 MiB, and 32 MiB,

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 decom‐
pression 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:

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 mat‐

· 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 ver‐
sions. Memory requirements of some of the future multi‐
threaded 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 require‐
ments of the decompressor. The exact decompressor memory
usage is slighly more than the LZMA2 dictionary size, but the
values in the table have been rounded up to the next full

-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. Decom‐
pressor 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.

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.

–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.

When compressing to the .xz format, split the input data into
blocks of size bytes. The blocks are compressed independently
from each other.

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 inte‐
ger suffix like MiB can be useful. Example: –memlimit-com‐

· 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 mem‐
ory. 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.

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

Display an error and exit if the compression settings exceed the
memory usage limit. The default is to adjust the settings down‐
wards so that the memory usage limit is not exceeded. Automatic
adjusting is always disabled when creating raw streams (–for‐

-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 com‐
plicating 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 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 command line. When com‐
pressing, 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 fil‐

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 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.

–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:

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
–extreme. 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. The algorithm tries to find repeating byte
sequences (matches) in the uncompressed data, and replace
them with references to the data currently in the dictio‐
nary. The bigger the dictionary, the higher is the
chance to find a match. Thus, increasing dictionary size
usually improves compression ratio, but a dictionary big‐
ger 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 cur‐
rently 1.5 GiB (1536 MiB). The decompressor already sup‐
ports 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 addi‐
tion, 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 uncom‐
pressed data.

The default value (3) is usually good. If you want maxi‐
mum 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 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 hav‐
ing 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

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 com‐
pressed 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:
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)

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 nor‐
mal 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.

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 the expense of speed. The
default depends on the preset.

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 set‐
ting the depth over 1000 unless you are prepared to
interrupt the compression in case it is taking far too

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

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 gener‐
ally 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 sec‐

These BCJ filters have known problems related to the compression

· 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 simi‐
lar 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.

Different instruction sets have have different alignment:

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 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
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:

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.

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 indi‐
cate 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

· Amount of uncompressed data consumed (compressing) or pro‐
duced (decompressing).

· Compression ratio, which is calculated by dividing the amount
of compressed data processed so far by the amount of uncom‐
pressed data processed so far.

· Compression or decompression speed. This is measured as the
amount of uncompressed data consumed (compression) or pro‐
duced (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, compres‐
sion 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 per‐
centage is printed if the size of the input file is known.

-Q, –no-warn
Don’t set the exit status to 2 even if a condition worth a 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.

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.

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

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

The robot mode is activated with the –robot option. It makes the out‐
put of xz easier to parse by other programs. Currently –robot is sup‐
ported only together with –version, –info-memory, and –list. It
will be supported for normal compression and decompression in the

xz –robot –version will print the version number of xz and liblzma in
the following format:


X Major version.

YYY Minor version. Even numbers are stable. Odd numbers are alpha
or beta versions.

ZZZ Patch level for stable releases or just a counter for develop‐
ment 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

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.

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

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
7. Comma-separated list of integrity check names. The follow‐
ing 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
6. Uncompressed start offset relative to the beginning of the
7. Total compressed size of the block (includes headers)
8. Uncompressed size
9. Compression ratio
10. Name of the integrity check

If –verbose was specified twice, additional columns are included on
the block lines. These are not displayed with a single –verbose,
because getting this information requires many seeks and can thus be
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 com‐
pression 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 com‐
pressed 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 ear‐
lier 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 com‐
pressed size and uncompressed size stored in them
Since xz 5.1.2alpha:
12. Minimum xz version required to decompress the file

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

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 parses space-separated lists of options from the environment vari‐
ables 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

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_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:

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

Scripts may use XZ_OPT e.g. to set script-specific default com‐
pression options. It is still recommended to allow users to
override XZ_OPT if that is reasonable, e.g. in sh scripts one
may use something like this:

export XZ_OPT

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 exist‐
ing scripts. There are some incompatibilities though, which may some‐
times cause problems.

Compression preset levels
The numbering of the compression level presets is not identical in xz
and LZMA Utils. The most important difference is how dictionary sizes
are mapped to different presets. Dictionary size is roughly equal to
the decompressor memory usage.

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:

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 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
uncompressed size.

Unsupported .lzma files
The .lzma format allows lc values up to 8, and lp values up to 4. LZMA
Utils can decompress files with any lc and lp, but always creates files
with lc=3 and lp=0. Creating files with other lc and lp is possible
with xz and with LZMA SDK.

The implementation of the LZMA1 filter in liblzma requires that the sum
of lc and lp must not exceed 4. Thus, .lzma files, which exceed this
limitation, cannot be decompressed with xz.

LZMA Utils creates only .lzma files which have a dictionary size of 2^n
(a power of 2) but accepts files with any dictionary size. liblzma
accepts only .lzma files which have a dictionary size of 2^n or 2^n +
2^(n-1). This is to decrease false positives when detecting .lzma

These limitations shouldn’t be a problem in practice, since practically
all .lzma files have been compressed with settings that liblzma will

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

If there is data left after the first .lzma stream, xz considers the
file to be corrupt unless –single-stream was used. This may break
obscure scripts which have assumed that trailing garbage is ignored.

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
different build options are used.

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 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 veri‐
fying the integrity check if the particular check is not supported.

XZ Embedded supports BCJ filters, but only with the default start off‐

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 decompres‐
sion 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 stan‐
dard 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 and xargs can be used to parallelize com‐
pression of many files:

find . -type f \! -name ‘*.xz’ -print0 \
| xargs -0r -P4 -n16 xz -T1

The -P option to xargs 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 will
eventually create.

The option -T1 for xz is there to force it to single-threaded mode,
because xargs 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 follow‐
ing sh 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.”

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 pre‐ set. 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: 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: 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 uncom‐ pressed 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 fil‐ ters 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.


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

XZ Utils:
XZ Embedded:

Tukaani 2012-07-01 XZ(1)

Ils en parlent aussi

Blog Note – Linux File compression and Archive command