unxz

NAME

SYNOPSIS

COMMAND ALIASES

DESCRIPTION

•

When compressing, the suffix of the target file format (.xz or .lzma) is appended to the source filename to get the target filename.

•

When decompressing, the .xz, .lzma, or .lz 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.

•

File is not a regular file. Symbolic links are not followed, and thus they are not considered to be regular files.

•

File has more than one hard link.

•

File has setuid, setgid, or sticky bit set.

•

The operation mode is set to compress and the file already has a suffix of the target file format (.xz or .txz when compressing to the .xz format, and .lzma or .tlz when compressing to the .lzma format).

•

The operation mode is set to decompress and the file doesn't have a suffix of any of the supported file formats (.xz, .txz, .lzma, .tlz, or .lz).

Memory usage

Concatenation and padding with .xz files

OPTIONS

Integer suffixes and special values

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.

Operation mode

-z, --compress

Compress. This is the default operation mode when no operation mode option is specified and no other operation mode is implied from the command name (for example, unxz implies --decompress).

-d, --decompress, --uncompress

Decompress.

-t, --test

Test the integrity of compressed files. This option is equivalent to --decompress --stdout except that the decompressed data is discarded instead of being written to standard output. No files are created or removed.

-l, --list

Print information about compressed files. No uncompressed output is produced, and no files are created or removed. In list mode, the program cannot read the compressed data from standard input or from other unseekable sources.

The default listing shows basic information about files, one file per line. To get more detailed information, use also the --verbose option. For even more information, use --verbose twice, but note that this may be slow, because getting all the extra information requires many seeks. The width of verbose output exceeds 80 characters, so piping the output to, for example, less -S may be convenient if the terminal isn't wide enough.

The exact output may vary between xz versions and different locales. For machine-readable output, --robot --list should be used.

Operation modifiers

-k, --keep

Don't delete the input files.

Since xz 5.2.6, this option also makes xz 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. In earlier versions this was only done with --force.

-f, --force

This option has several effects:

-c, --stdout, --to-stdout

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

--single-stream

Decompress only the first .xz stream, and silently ignore possible remaining input data following the stream. Normally such trailing garbage makes xz display an error.

xz never decompresses more than one stream from .lzma files or raw streams, but this option still makes xz ignore the possible trailing data after the .lzma file or raw stream.

This option has no effect if the operation mode is not --decompress or --test.

--no-sparse

Disable creation of sparse files. By default, if decompressing into a regular file, xz tries to make the file sparse if the decompressed data contains long sequences of binary zeros. It also works when writing to standard output as long as standard output is connected to a regular file and certain additional conditions are met to make it safe. Creating sparse files may save disk space and speed up the decompression by reducing the amount of disk I/O.

-S .suf, --suffix=.suf

When compressing, use .suf as the suffix for the target file instead of .xz or .lzma. If not writing to standard output and the source file already has the suffix .suf, a warning is displayed and the file is skipped.

When decompressing, recognize files with the suffix .suf in addition to files with the .xz, .txz, .lzma, .tlz, or .lz 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 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:

-C check, --check=check

Specify the type of the integrity check. The check is calculated from the uncompressed data and stored in the .xz file. This option has an effect only when compressing into the .xz format; the .lzma format doesn't support integrity checks. The integrity check (if any) is verified when the .xz file is decompressed.

Supported check types:

Integrity of the .xz headers is always verified with CRC32. It is not possible to change or disable it.

--ignore-check

Don't verify the integrity check of the compressed data when decompressing. The CRC32 values in the .xz headers will still be verified normally.

Do not use this option unless you know what you are doing. Possible reasons to use this option:

-0 ... -9

Select a compression preset level. The default is -6. If multiple preset levels are specified, the last one takes effect. If a custom filter chain was already specified, setting a compression preset level clears the custom filter chain.

The differences between the presets are more significant than with gzip(1) and bzip2(1). The selected compression settings determine the memory requirements of the decompressor, thus using a too high preset level might make it painful to decompress the file on an old system with little RAM. Specifically, it's not a good idea to blindly use -9 for everything like it often is with gzip(1) and bzip2(1).

On the same hardware, the decompression speed is approximately a constant number of bytes of compressed data per second. In other words, the better the compression, the faster the decompression will usually be. This also means that the amount of uncompressed output produced per second can vary a lot.

The following table summarises the features of the presets:

Column descriptions:

-e, --extreme

Use a slower variant of the selected compression preset level (-0 ... -9) to hopefully get a little bit better compression ratio, but with bad luck this can also make it worse. Decompressor memory usage is not affected, but compressor memory usage increases a little at preset levels -0 ... -3.

Since there are two presets with dictionary sizes 4 MiB and 8 MiB, the presets -3e and -5e use slightly faster settings (lower CompCPU) than -4e and -6e, respectively. That way no two presets are identical.

For example, there are a total of four presets that use 8 MiB dictionary, whose order from the fastest to the slowest is -5, -6, -5e, and -6e.

--fast

--best

These are somewhat misleading aliases for -0 and -9, respectively. These are provided only for backwards compatibility with LZMA Utils. Avoid using these options.

--block-size=size

When compressing to the .xz format, split the input data into blocks of size bytes. The blocks are compressed independently from each other, which helps with multi-threading and makes limited random-access decompression possible. This option is typically used to override the default block size in multi-threaded mode, but this option can be used in single-threaded mode too.

In multi-threaded mode about three times size bytes will be allocated in each thread for buffering input and output. The default size is three times the LZMA2 dictionary size or 1 MiB, whichever is more. Typically a good value is 2–4 times the size of the LZMA2 dictionary or at least 1 MiB. Using size less than the LZMA2 dictionary size is waste of RAM because then the LZMA2 dictionary buffer will never get fully used. The sizes of the blocks are stored in the block headers, which a future version of xz will use for multi-threaded decompression.

In single-threaded mode no block splitting is done by default. Setting this option doesn't affect memory usage. No size information is stored in block headers, thus files created in single-threaded mode won't be identical to files created in multi-threaded mode. The lack of size information also means that a future version of xz won't be able decompress the files in multi-threaded mode.

--block-list=sizes

When compressing to the .xz format, start a new block after the given intervals of uncompressed data.

The uncompressed sizes of the blocks are specified as a comma-separated list. Omitting a size (two or more consecutive commas) is a shorthand to use the size of the previous block.

If the input file is bigger than the sum of sizes, the last value in sizes is repeated until the end of the file. A special value of 0 may be used as the last value to indicate that the rest of the file should be encoded as a single block.

If one specifies sizes that exceed the encoder's block size (either the default value in threaded mode or the value specified with --block-size=size), the encoder will create additional blocks while keeping the boundaries specified in sizes. For example, if one specifies --block-size=10MiB --block-list=5MiB,10MiB,8MiB,12MiB,24MiB and the input file is 80 MiB, one will get 11 blocks: 5, 10, 8, 10, 2, 10, 10, 4, 10, 10, and 1 MiB.

In multi-threaded mode the sizes of the blocks are stored in the block headers. This isn't done in single-threaded mode, so the encoded output won't be identical to that of the multi-threaded mode.

--flush-timeout=timeout

When compressing, if more than timeout milliseconds (a positive integer) has passed since the previous flush and reading more input would block, all the pending input data is flushed from the encoder and made available in the output stream. This can be useful if xz is used to compress data that is streamed over a network. Small timeout values make the data available at the receiving end with a small delay, but large timeout values give better compression ratio.

This feature is disabled by default. If this option is specified more than once, the last one takes effect. The special timeout value of 0 can be used to explicitly disable this feature.

This feature is not available on non-POSIX systems.

This feature is still experimental. Currently xz is unsuitable for decompressing the stream in real time due to how xz does buffering.

--memlimit-compress=limit

Set a memory usage limit for compression. If this option is specified multiple times, the last one takes effect.

If the compression settings exceed the limit, xz will attempt to adjust the settings downwards so that the limit is no longer exceeded and display a notice that automatic adjustment was done. The adjustments are done in this order: reducing the number of threads, switching to single-threaded mode if even one thread in multi-threaded mode exceeds the limit, and finally reducing the LZMA2 dictionary size.

When compressing with --format=raw or if --no-adjust has been specified, only the number of threads may be reduced since it can be done without affecting the compressed output.

If the limit cannot be met even with the adjustments described above, an error is displayed and xz will exit with exit status 1.

The limit can be specified in multiple ways:

For 32-bit xz there is a special case: if the limit would be over 4020 MiB, the limit is set to 4020 MiB. On MIPS32 2000 MiB is used instead. (The values 0 and max aren't affected by this. A similar feature doesn't exist for decompression.) This can be helpful when a 32-bit executable has access to 4 GiB address space (2 GiB on MIPS32) while hopefully doing no harm in other situations.

See also the section Memory usage.

--memlimit-decompress=limit

Set a memory usage limit for decompression. This also affects the --list mode. If the operation is not 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.

--memlimit-mt-decompress=limit

Set a memory usage limit for multi-threaded decompression. This can only affect the number of threads; this will never make xz refuse to decompress a file. If limit is too low to allow any multi-threading, the limit is ignored and xz will continue in single-threaded mode. Note that if also --memlimit-decompress is used, it will always apply to both single-threaded and multi-threaded modes, and so the effective limit for multi-threading will never be higher than the limit set with --memlimit-decompress.

In contrast to the other memory usage limit options, --memlimit-mt-decompress=limit has a system-specific default limit. xz --info-memory can be used to see the current value.

This option and its default value exist because without any limit the threaded decompressor could end up allocating an insane amount of memory with some input files. If the default limit is too low on your system, feel free to increase the limit but never set it to a value larger than the amount of usable RAM as with appropriate input files xz will attempt to use that amount of memory even with a low number of threads. Running out of memory or swapping will not improve decompression performance.

See --memlimit-compress=limit for possible ways to specify the limit. Setting limit to 0 resets the limit to the default system-specific value.

-M limit, --memlimit=limit, --memory= limit

This is equivalent to specifying --memlimit-compress=limit --memlimit-decompress=limit --memlimit-mt-decompress=limit.

--no-adjust

Display an error and exit if the memory usage limit cannot be met without adjusting settings that affect the compressed output. That is, this prevents xz from switching the encoder from multi-threaded mode to single-threaded mode and from reducing the LZMA2 dictionary size. Even when this option is used the number of threads may be reduced to meet the memory usage limit as that won't affect the compressed output.

Automatic adjusting is always disabled when creating raw streams (--format=raw).

-T threads, --threads=threads

Specify the number of worker threads to use. Setting threads to a special value 0 makes xz use up to as many threads as the processor(s) on the system support. The actual number of threads can be fewer than threads if the input file is not big enough for threading with the given settings or if using more threads would exceed the memory usage limit.

The single-threaded and multi-threaded compressors produce different output. Single-threaded compressor will give the smallest file size but only the output from the multi-threaded compressor can be decompressed using multiple threads. Setting threads to 1 will use the single-threaded mode. Setting threads to any other value, including 0, will use the multi-threaded compressor even if the system supports only one hardware thread. (xz 5.2.x used single-threaded mode in this situation.)

To use multi-threaded mode with only one thread, set threads to +1. The + prefix has no effect with values other than 1. A memory usage limit can still make xz switch to single-threaded mode unless --no-adjust is used. Support for the + prefix was added in xz 5.4.0.

If an automatic number of threads has been requested and no memory usage limit has been specified, then a system-specific default soft limit will be used to possibly limit the number of threads. It is a soft limit in sense that it is ignored if the number of threads becomes one, thus a soft limit will never stop xz from compressing or decompressing. This default soft limit will not make xz switch from multi-threaded mode to single-threaded mode. The active limits can be seen with xz --info-memory.

Currently the only threading method is to split the input into blocks and compress them independently from each other. The default block size depends on the compression level and can be overridden with the --block-size=size option.

Threaded decompression only works on files that contain multiple blocks with size information in block headers. All large enough files compressed in multi-threaded mode meet this condition, but files compressed in single-threaded mode don't even if --block-size=size has been used.

Custom compressor filter chains

--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:

When decoding raw streams (--format=raw), LZMA2 needs only the dictionary size. LZMA1 needs also lc, lp, and pb.

--x86[=options]

--arm[=options]

--armthumb[=options]

--arm64[=options]

--powerpc[=options]

--ia64[=options]

--sparc[=options]

Add a branch/call/jump (BCJ) filter to the filter chain. These filters can be used only as a non-last filter in the filter chain.

A BCJ filter converts relative addresses in the machine code to their absolute counterparts. This doesn't change the size of the data but it increases redundancy, which can help LZMA2 to produce 0–15 % smaller .xz file. The BCJ filters are always reversible, so using a BCJ filter for wrong type of data doesn't cause any data loss, although it may make the compression ratio slightly worse. The BCJ filters are very fast and use an insignificant amount of memory.

These BCJ filters have known problems related to the compression ratio:

Different instruction sets have different alignment: the executable file must be aligned to a multiple of this value in the input data to make the filter work.

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 or even pb=4,lp=4,lc=0 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:

--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, for example, 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, for example, with flac(1).

Supported options:

Other options

-q, --quiet

Suppress warnings and notices. Specify this twice to suppress errors too. This option has no effect on the exit status. That is, even if a warning was suppressed, the exit status to indicate a warning is still used.

-v, --verbose

Be verbose. If standard error is connected to a terminal, xz will display a progress indicator. Specifying --verbose twice will give even more verbose output.

The progress indicator shows the following information:

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) and how many processor threads 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

Version

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 development releases.

S

Stability. 0 is alpha, 1 is beta, and 2 is stable. S should be always 2 when YYY is even.

Memory limit information

1.

Total amount of physical memory (RAM) in bytes.

2.

Memory usage limit for compression in bytes (--memlimit-compress). A special value of 0 indicates the default setting which for single-threaded mode is the same as no limit.

3.

Memory usage limit for decompression in bytes (--memlimit-decompress). A special value of 0 indicates the default setting which for single-threaded mode is the same as no limit.

4.

Since xz 5.3.4alpha: Memory usage for multi-threaded decompression in bytes (--memlimit-mt-decompress). This is never zero because a system-specific default value shown in the column 5 is used if no limit has been specified explicitly. This is also never greater than the value in the column 3 even if a larger value has been specified with --memlimit-mt-decompress.

5.

Since xz 5.3.4alpha: A system-specific default memory usage limit that is used to limit the number of threads when compressing with an automatic number of threads (--threads=0) and no memory usage limit has been specified (--memlimit-compress). This is also used as the default value for --memlimit-mt-decompress.

6.

Since xz 5.3.4alpha: Number of available processor threads.

List mode

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.

EXIT STATUS

0

All is good.

1

An error occurred.

2

Something worth a warning occurred, but no actual errors occurred.

ENVIRONMENT

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 when xz is run by a script or tool, for example, GNU tar(1):

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

Scripts may use XZ_OPT, for example, to set script-specific default compression options. It is still recommended to allow users to override XZ_OPT if that is reasonable. For example, in sh(1) scripts one may use something like this:

XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT

LZMA UTILS COMPATIBILITY

Compression preset levels

Streamed vs. non-streamed .lzma files

Unsupported .lzma files

Trailing garbage

NOTES

Compressed output may vary

Embedded .xz decompressors

EXAMPLES

Basics

xz foo

xz -dk bar.xz

tar cf - baz | xz -4e > baz.tar.xz

xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt

Parallel compression of many files

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

Robot mode

xz --robot --list *.xz | awk '/^totals/{print $5-$4}'

if ! eval "$(xz --robot --version 2> /dev/null)" ||
        [ "$XZ_VERSION" -lt 50000002 ]; then
    echo "Your xz is too old."
fi
unset XZ_VERSION LIBLZMA_VERSION

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

xz --lzma2=preset=1,dict=32MiB foo.tar

xz -vv --lzma2=dict=192MiB big_foo.tar

xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo

xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar

xz --x86 --lzma2 libfoo.so

xz --delta=dist=3 --lzma2=pb=0 foo.tiff

SEE ALSO