XZ(1)

NAME

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

SYNOPSIS

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.

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 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 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:

o When compressing, the suffix of the target file format (.xz or: .lzma) is appended to the source filename to get the target filename.
o When decompressing, the .xz or .lzma suffix is removed from the: filename to get the target filename. xz also recognizes the suffixes .txz and .tlz, and replaces them with the .tar suffix.
If the target file already exists, an error is displayed and the file is skipped.
Unless writing to standard output, xz will display a warning and skip the file if any of the following applies:
o File is not a regular file. Symbolic links are not followed, thus: they are not considered to be regular files.
o File has more than one hard link.
o File has setuid, setgid, or sticky bit set.
o The operation mode is set to compress, and the file already has a: suffix of the target file format (.xz or .txz when compressing to the .xz format, and .lzma or .tlz when compressing to the .lzma format).
o The operation mode is set to decompress, and the file doesn't have a: suffix of any of the supported file 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 only 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 need 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, but 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-com-press=limit and --memlimit-decompress=limit, respectively. 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 be null bytes and the size of the padding must be a multiple of four bytes. This can be useful if the .xz file is stored on a medium that stores file sizes e.g. as 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 The integer is multiplied by 1,024 (2^10). Also Ki, k, kB, K,
and KB are accepted as synonyms for KiB.; MiB The integer is multiplied by 1,048,576 (2^20). Also Mi, m, M,
and MB are accepted as synonyms for MiB.; GiB The integer is multiplied by 1,073,741,824 (2^30). Also Gi, g,
G, and GB are accepted as synonyms for GiB.; A 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. 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.; -l, --list
List 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 it 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. To get machine-readable output, --robot --list should be used.
Operation modifiers: -k, --keep
Keep (don't delete) the input files.; -f, --force
This option has several effects:

o If the target file already exists, delete it before compress
ing or decompressing.

o Compress or decompress even if the input is a symbolic link

to a regular file, has more than one hard link, or has setuid, setgid, or sticky bit set. The setuid, setgid, and sticky bits are not copied to the target file.

o 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 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 works also 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 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.
--files[=file]: Read the filenames to process from file; if file is omitted, filenames are read from standard input. Filenames must be terminated with the newline character. A dash (-) is taken as a regular filename; it doesn't mean standard input. If filenames are given also as command line arguments, they are processed before the filenames read from file.
--files0[=file]: This is identical to --files[=file] except that 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:

o 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.

o xz: Compress to the .xz file format, or accept only .xz files

when decompressing.

o 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 Utils.

o 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 headers.
-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 support integrity checks. The integrity check (if any) is verified when the .xz file is decompressed.; Supported check types:; o 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.; o crc32: Calculate CRC32 using the polynomial from IEEE-802.3
(Ethernet).; o crc64: Calculate CRC64 using the polynomial from ECMA-182.
This is the default, since it is slightly better than CRC32 at detecting damaged files and the speed difference is negligible.; o 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 compression preset. If a preset level is specified multiple times, the last one takes effect.; The compression preset levels can be categorised roughly into three categories:; -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 compression). In future, -0 may be indicate some fast algorithm 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 compression 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 preset may vary between xz versions. Because the settings may vary, the memory usage may vary slightly too. FIXME The following table lists the maximum memory usage of each preset level, which won't be exceeded even in future versions of xz.; 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
--fast and --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.; Especially the name of --best is misleading, because the definition 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.
-e, --extreme: Modify the compression preset (-0 ... -9) so that a little bit better compression ratio can be achieved without increasing memory usage of the compressor or decompressor (exception: compressor memory usage may increase a little with presets -0 ... -2). The downside is that the compression time will increase dramatically (it can easily double).
--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. Adjustment is never done when compressing with --format=raw or if --no-adjust has been specified. In those cases, an error is displayed and xz will exit with exit status 1.; The limit can be specified in multiple ways:; o The limit can be an absolute value in bytes. Using an integer
suffix like MiB can be useful. Example: --memlimit-com-press=80MiB; o 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%; o The limit can be reset back to its default value by setting
it to 0. This is currently equivalent to setting the limit to max i.e. 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 at least until the details have been decided.; See also the section Memory usage.
--memlimit-decompress=limit: Set a memory usage limit for decompression. This affects also 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 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 (2010-08-07), 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 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 levels. 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 filters.; 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 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 (--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 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:

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.

The default preset is 6, from which the default values for the rest of the LZMA1 or LZMA2 options are taken.

dict=size

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 bigger the dictionary, the better the compression ratio usually 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 currently 1.5 GiB. The decompressor already supports dictionaries up to one byte less than 4 GiB, which is the maximum for LZMA1 and LZMA2 stream formats.

Dictionary size has the biggest effect on compression ratio. Dictionary size and match finder together determine 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 compressing.

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.

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.

mode=mode

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
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 expense of speed. The default depends on the preset level.
depth=depth: Specify the maximum search depth in the match finder. The default is the special value 0, which makes the compressor 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 setting 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.
--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 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, applying 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 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.

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 offset 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 whole executable.
--delta[=options]: 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 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 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.
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 (useful mostly for debugging).

The progress indicator shows the following information:

o Completion percentage is shown if the size of the input file
is known. That is, percentage cannot be shown in pipes.

o Amount of compressed data produced (compressing) or consumed

(decompressing).

o Amount of uncompressed data consumed (compressing) or pro

duced (decompressing).

o Compression ratio, which is calculated by dividing the amount

of compressed data processed so far by the amount of uncompressed data processed so far.

o Compression or decompression speed. This is 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.

o Elapsed time in the format M:SS or H:MM:SS.

o Estimated remaining time is shown only when the size of the

input file is known and a couple of seconds have already passed since xz started processing the file. The time is shown in a less precise format which never has any colons, e.g. 2 min 30 s.

When standard error is not a terminal, --verbose will make xz print the filename, compressed size, uncompressed size, compression ratio, and possibly also the speed and elapsed time on a single line to standard error after compressing or decompressing the file. The speed and elapsed time are included only when the operation took at least a few seconds. If the operation didn't finish, for example due to user interruption, also the completion percentage is printed if the size of the input file is known.; -Q, --no-warn
Don't set the exit status to 2 even if a condition worth a warning was detected. This option doesn't affect the verbosity level, thus both --quiet and --no-warn have to be used to not display warnings and to not alter the exit status.; --robot
Print messages in a machine-parsable format. This is intended to ease writing frontends that want to use xz instead of liblzma, which may be the case with various scripts. The output with this option enabled is meant to be stable across xz releases. See the section ROBOT MODE for details.; --info-memory
Display, in human-readable format, how much physical memory (RAM) 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 normal 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.; 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 columns:

1. Total amount of physical memory (RAM) as bytes

2. Memory usage limit for compression as bytes. A specialvalue of zero indicates the default setting, which for single-threaded mode is the same as no limit.

3. Memory usage limit for decompression as bytes. A specialvalue 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.
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, 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 (as bytes) required to decompress this block with this xz version; 16. Filter chain. Note that most of the options used at compres sion time cannot be known, because only the options that are needed for decompression are stored in the .xz headers.
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 (as 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
Future versions may add new line types and new columns can be added to the existing line types, but the existing columns won't be changed.

EXIT STATUS

0 All is good.

1 An error occurred.

2 Something worth a warning occurred, but no actual errors: occurred.
Notices (not warnings or errors) printed on standard error don't affect the exit status.

ENVIRONMENT

xz parses space-separated lists of options from the 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 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 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 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 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 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 decompressorsEmbedded .xz decompressor implementations like XZ Embedded don't necessarily 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 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: 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.xz > abcd.txt
Parallel compression of many filesOn 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 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 examples
Calculating how many bytes have been saved in total after compressing multiple files:: xz --robot --list *.xz | awk '/^totals/{print $5-$4}'

docs.sk

comprehensive documentation repository

See also