Main index | Section 1 | Options |
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 compresses or decompresses each file according to the selected operation mode. If no files are given or file is -, xz reads from standard input and writes the processed data to standard output. xz will refuse (display an error and skip the file) to write compressed data to standard output if it is a terminal. Similarly, xz will refuse to read compressed data from standard input if it is a terminal.
Unless --stdout is specified, files other than - are written to a new file whose name is derived from the source file name:
» | When compressing, the suffix of the target file format (.xz or .lzma) is appended to the source filename to get the target filename. |
» | When decompressing, the .xz or .lzma suffix is removed from the filename to get the target filename. xz also recognizes the suffixes .txz and .tlz, and replaces them with the .tar suffix. |
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 format). |
» | The operation mode is set to decompress and the file doesn't have a suffix of any of the supported file formats (.xz, .txz, .lzma, or .tlz). |
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.
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.amp; XZ_DEFAULTS=--memlimit=150MiB. It is possible to set the limits separately for compression and decompression by using --memlimit-compress=limit and --memlimit-decompress=limit. Using these two options outside XZ_DEFAULTS is rarely useful because a single run of xz cannot do both compression and decompression and --memlimit=limit (or -M limit) is shorter to type on the command line.
If the specified memory usage limit is exceeded when decompressing, xz will display an error and decompressing the file will fail. If the limit is exceeded when compressing, xz will try to scale the settings down so that the limit is no longer exceeded (except when using --format=raw or --no-adjust). This way the operation won't fail unless the limit is very small. The scaling of the settings is done in steps that don't match the compression level presets, e.g. if the limit is only slightly less than the amount required for xz -9, the settings will be scaled down only a little, not all the way down to xz -8.
It is possible to insert padding between the concatenated parts or after the last part. The padding must consist of null bytes and the size of the padding must be a multiple of four bytes. This can be useful e.g. if the .xz file is stored on a medium that measures file sizes in 512-byte blocks.
Concatenation and padding are not allowed with .lzma files or raw streams.
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. |
-z, --compress | |
Compress. This is the default operation mode when no operation mode option is specified and no other operation mode is implied from the command name (for example, unxz implies --decompress). | |
-d, --decompress, --uncompress | |
Decompress. | |
-t, --test | |
Test the integrity of compressed files. This option is equivalent to --decompress --stdout except that the decompressed data is discarded instead of being written to standard output. No files are created or removed. | |
-l, --list | |
Print information about compressed files. No uncompressed output is produced, and no files are created or removed. In list mode, the program cannot read the compressed data from standard input or from other unseekable sources. | |
The default listing shows basic information about files, one file per line. To get more detailed information, use also the --verbose option. For even more information, use --verbose twice, but note that this may be slow, because getting all the extra information requires many seeks. The width of verbose output exceeds 80 characters, so piping the output to e.g.amp; 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. | |
-k, --keep | |||||||
Don't delete the input files. | |||||||
-f, --force | |||||||
This option has several effects:
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-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, 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 each filename must be terminated with the null character. | |||||||
-F format, --format=format | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Specify the file
format to compress or decompress:
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-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:
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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:
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-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).
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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:
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Column descriptions:
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-e, --extreme | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Use a slower variant of the selected compression preset level (-0 ... -9) to hopefully get a little bit better compression ratio, but with bad luck this can also make it worse. Decompressor memory usage is not affected, but compressor memory usage increases a little at preset levels -0 ... -3. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Since there are two presets with dictionary sizes
4 MiB and 8 MiB, the presets
-3e and
-5e use slightly faster settings (lower CompCPU) than
-4e and
-6e, respectively.
That way no two presets are identical.
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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 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:
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For 32-bit xz there is a special case: if the limit would be over 4020 MiB, the limit is set to 4020 MiB. (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 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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
-M limit, --memlimit=limit, --memory=limit | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
This is equivalent to specifying --memlimit-compress=limit --memlimit-decompress=limit. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
--no-adjust | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Display an error and exit if the compression settings exceed the memory usage limit. The default is to adjust the settings downwards so that the memory usage limit is not exceeded. Automatic adjusting is always disabled when creating raw streams (--format=raw). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
-T threads, --threads=threads | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Specify the number of worker threads to use. Setting threads to a special value 0 makes xz use as many threads as there are CPU cores on the system. The actual number of threads can be less 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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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 hasn't been implemented yet. It will only work on files that contain multiple blocks with size information in block headers. All files compressed in multi-threaded mode meet this condition, but files compressed in single-threaded mode don't even if --block-size=size is used. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A filter chain is comparable to piping on the command line. When compressing, the uncompressed input goes to the first filter, whose output goes to the next filter (if any). The output of the last filter gets written to the compressed file. The maximum number of filters in the chain is four, but typically a filter chain has only one or two filters.
Many filters have limitations on where they can be in the filter chain: some filters can work only as the last filter in the chain, some only as a non-last filter, and some work in any position in the chain. Depending on the filter, this limitation is either inherent to the filter design or exists to prevent security issues.
A custom filter chain is specified by using one or more filter options in the order they are wanted in the filter chain. That is, the order of filter options is significant! When decoding raw streams (--format=raw), the filter chain is specified in the same order as it was specified when compressing.
Filters take filter-specific options as a comma-separated list. Extra commas in options are ignored. Every option has a default value, so you need to specify only those you want to change.
To see the whole filter chain and options, use xz -vv (that is, use --verbose twice). This works also for viewing the filter chain options used by presets.
--lzma1[=options]
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--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:
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When decoding raw streams (--format=raw), LZMA2 needs only the dictionary size. LZMA1 needs also lc, lp, and pb. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
--x86[=options]
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--powerpc[=options]
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--ia64[=options]
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--arm[=options]
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--armthumb[=options]
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--sparc[=options] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Add a branch/call/jump (BCJ) filter to the filter chain. These filters can be used only as a non-last filter in the filter chain. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A BCJ filter converts relative addresses in the machine code to their absolute counterparts. This doesn't change the size of the data, but it increases redundancy, which can help LZMA2 to produce 0-15 % smaller .xz file. The BCJ filters are always reversible, so using a BCJ filter for wrong type of data doesn't cause any data loss, although it may make the compression ratio slightly worse. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
It is fine to apply a BCJ filter on a whole executable; there's no need to apply it only on the executable section. Applying a BCJ filter on an archive that contains both executable and non-executable files may or may not give good results, so it generally isn't good to blindly apply a BCJ filter when compressing binary packages for distribution. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
These BCJ filters are very fast and use insignificant amount of memory. If a BCJ filter improves compression ratio of a file, it can improve decompression speed at the same time. This is because, on the same hardware, the decompression speed of LZMA2 is roughly a fixed number of bytes of compressed data per second. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
These BCJ filters have known problems related to
the compression ratio:
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Both of the above problems will be fixed in the future in a new filter. The old BCJ filters will still be useful in embedded systems, because the decoder of the new filter will be bigger and use more memory. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Different instruction sets have different alignment:
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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:
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--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:
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-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:
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When standard error is not a terminal, --verbose will make xz print the filename, compressed size, uncompressed size, compression ratio, and possibly also the speed and elapsed time on a single line to standard error after compressing or decompressing the file. The speed and elapsed time are included only when the operation took at least a few seconds. If the operation didn't finish, e.g. due to user interruption, also the completion percentage is printed if the size of the input file is known. | |||||||||||||||
-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. | |||||||||||||||
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 development releases. |
S | Stability. 0 is alpha, 1 is beta, and 2 is stable. S should be always 2 when YYY is even. |
Examples: 4.999.9beta is 49990091 and 5.0.0 is 50000002.
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. |
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: | |||||||||||||||||||
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The columns of the stream lines: | |||||||||||||||||||
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The columns of the block lines: | |||||||||||||||||||
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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: | |||||||||||||||||||
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The columns of the summary lines: | |||||||||||||||||||
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The columns of the totals line: | |||||||||||||||||||
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If --verbose was specified twice, additional columns are included on the totals line: | |||||||||||||||||||
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0 | All is good. |
1 | An error occurred. |
2 | Something worth a warning occurred, but no actual errors occurred. |
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):
| ||
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:
| |||
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.
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.
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 files.
These limitations shouldn't be a problem in practice, since practically all .lzma files have been compressed with settings that liblzma will accept.
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.
The above means that once --rsyncable has been implemented, the resulting files won't necessarily be rsyncable unless both old and new files have been compressed with the same xz version. This problem can be fixed if a part of the encoder implementation is frozen to keep rsyncable output stable across xz versions.
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.
xz foo
Decompress bar.xz into bar and don't remove bar.xz even if decompression is successful:
xz -dk bar.xz
Create baz.tar.xz with the preset -4e (-4 --extreme), which is slower than e.g. the default -6, but needs less memory for compression and decompression (48 MiB and 5 MiB, respectively):
tar cf - baz | xz -4e > baz.tar.xz
A mix of compressed and uncompressed files can be decompressed to standard output with a single command:
xz -dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt
find . -type f \! -name '*.xz' -print0 \ | xargs -0r -P4 -n16 xz -T1
The -P option to xargs(1) sets the number of parallel xz processes. The best value for the -n option depends on how many files there are to be compressed. If there are only a couple of files, the value should probably be 1; with tens of thousands of files, 100 or even more may be appropriate to reduce the number of xz processes that xargs(1) will eventually create.
The option -T1 for xz is there to force it to single-threaded mode, because xargs(1) is used to control the amount of parallelization.
xz --robot --list *.xz | awk '/^totals/{print $5-$4}'
A script may want to know that it is using new enough xz. The following sh(1) script checks that the version number of the xz tool is at least 5.0.0. This method is compatible with old beta versions, which didn't support the --robot option:
if ! eval "$(xz --robot --version 2> /dev/null)" || [ "$XZ_VERSION" -lt 50000002 ]; then echo "Your xz is too old." fi unset XZ_VERSION LIBLZMA_VERSION
Set a memory usage limit for decompression using XZ_OPT, but if a limit has already been set, don't increase it:
NEWLIM=$((123 << 20)) # 123 MiB OLDLIM=$(xz --robot --info-memory | cut -f3) if [ $OLDLIM -eq 0 -o $OLDLIM -gt $NEWLIM ]; then XZ_OPT="$XZ_OPT --memlimit-decompress=$NEWLIM" export XZ_OPT fi
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 uncompressed file is waste of memory, so the above command isn't useful for small files.
Sometimes the compression time doesn't matter, but the decompressor memory usage has to be kept low e.g. to make it possible to decompress the file on an embedded system. The following command uses -6e (-6 --extreme) as a base and sets the dictionary to only 64 KiB. The resulting file can be decompressed with XZ Embedded (that's why there is --check=crc32) using about 100 KiB of memory.
xz --check=crc32 --lzma2=preset=6e,dict=64KiB foo
If you want to squeeze out as many bytes as possible, adjusting the number of literal context bits (lc) and number of position bits (pb) can sometimes help. Adjusting the number of literal position bits (lp) might help too, but usually lc and pb are more important. E.g. a source code archive contains mostly US-ASCII text, so something like the following might give slightly (like 0.1 %) smaller file than xz -6e (try also without lc=4):
xz --lzma2=preset=6e,pb=0,lc=4 source_code.tar
Using another filter together with LZMA2 can improve compression with certain file types. E.g. to compress a x86-32 or x86-64 shared library using the x86 BCJ filter:
xz --x86 --lzma2 libfoo.so
Note that the order of the filter options is significant. If --x86 is specified after --lzma2, xz will give an error, because there cannot be any filter after LZMA2, and also because the x86 BCJ filter cannot be used as the last filter in the chain.
The Delta filter together with LZMA2 can give good results with bitmap images. It should usually beat PNG, which has a few more advanced filters than simple delta but uses Deflate for the actual compression.
The image has to be saved in uncompressed format, e.g. as uncompressed TIFF. The distance parameter of the Delta filter is set to match the number of bytes per pixel in the image. E.g. 24-bit RGB bitmap needs dist=3, and it is also good to pass pb=0 to LZMA2 to accommodate the three-byte alignment:
xz --delta=dist=3 --lzma2=pb=0 foo.tiff
If multiple images have been put into a single archive (e.g.amp; .tar), the Delta filter will work on that too as long as all images have the same number of bytes per pixel.
XZ Utils: <https://tukaani.org/xz/>
XZ Embedded: <https://tukaani.org/xz/embedded.html>
LZMA SDK: <http://7-zip.org/sdk.html>
2020-02-01 | XZ (1) | Tukaani |
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