xz-archive/doc/lzma-intro.txt

Introduction to the lzma command line tool
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Overview

    The lzma command line tool is similar to gzip and bzip2, but for
    compressing and uncompressing .lzma files.


Supported file formats

    By default, the tool creates files in the new .lzma format. This can
    be overriden with --format=FMT command line option. Use --format=alone
    to create files in the old LZMA_Alone format.

    By default, the tool uncompresses both the new .lzma format and
    LZMA_Alone format. This is to make it transparent to switch from
    the old LZMA_Alone format to the new .lzma format. Since both
    formats use the same filename suffix, average user should never
    notice which format was used.


Differences to gzip and bzip2

  Standard input and output

    Both gzip and bzip2 refuse to write compressed data to a terminal and
    read compressed data from a terminal. With gzip (but not with bzip2),
    this can be overriden with the `--force' option. lzma follows the
    behavior of gzip here.

  Usage of LZMA_OPT environment variable

    gzip and bzip2 read GZIP and BZIP2 environment variables at startup.
    These variables may contain extra command line options.

    gzip and bzip2 allow passing not only options, but also end-of-options
    indicator (`--') and filenames via the environment variable. No quoting
    is supported with the filenames.

    Here are examples with gzip. bzip2 behaves identically.

        bash$ echo asdf > 'foo bar'
        bash$ GZIP='"foo bar"' gzip
        gzip: "foo: No such file or directory
        gzip: bar": No such file or directory

        bash$ GZIP=-- gzip --help
        gzip: --help: No such file or directory

    lzma silently ignores all non-option arguments given via the
    environment variable LZMA_OPT. Like on the command line, everything
    after `--' is taken as non-options, and thus ignored in LZMA_OPT.

        bash$ LZMA_OPT='--help' lzma --version     # Displays help
        bash$ LZMA_OPT='-- --help' lzma --version  # Displays version


Filter chain presets

    Like in gzip and bzip2, lzma supports numbered presets from 1 to 9
    where 1 is the fastest and 9 the best compression. 1 and 2 are for
    fast compressing with small memory usage, 3 to 6 for good compression
    ratio with medium memory usage, and 7 to 9 for excellent compression
    ratio with higher memory requirements. The default is 7 if memory
    usage limit allows.

    In future, there will probably be an option like --preset=NAME, which
    will contain more special presets for specific file types.

    It's also possible that there will be some heuristics to select good
    filters. For example, the tool could detect when a .tar archive is
    being compressed, and enable x86 filter only for those files in the
    .tar archive that are ELF or PE executables for x86.


Specifying custom filter chains

    Custom filter chains are specified by using long options with the name
    of the filters in correct order. For example, to pass the input data to
    the x86 filter and the output of that to the LZMA filter, the following
    command will do:

        lzma --x86 --lzma filename

    Some filters accept options, which are specified as a comma-separated
    list of key=value pairs:

        lzma --delta=distance=4 --lzma=dict=4Mi,lc=8,lp=2 filename


Memory usage control

    By default, the command line tool limits memory usage to 1/3 of the
    available physical RAM. If no preset or custom filter chain has been
    given, the default preset will be used. If the memory limit is too
    low for the default preset, the tool will silently switch to lower
    preset.

    When a preset or a custom filter chain has been specified and the
    memory limit is too low, an error message is displayed and no files
    are processed.

    If the decoder hits the memory usage limit, an error is displayed and
    no more files are processed.