108 lines
3.9 KiB
Plaintext
108 lines
3.9 KiB
Plaintext
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Introduction to the lzma command line tool
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Overview
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The lzma command line tool is similar to gzip and bzip2, but for
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compressing and uncompressing .lzma files.
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Supported file formats
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By default, the tool creates files in the new .lzma format. This can
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be overriden with --format=FMT command line option. Use --format=alone
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to create files in the old LZMA_Alone format.
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By default, the tool uncompresses both the new .lzma format and
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LZMA_Alone format. This is to make it transparent to switch from
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the old LZMA_Alone format to the new .lzma format. Since both
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formats use the same filename suffix, average user should never
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notice which format was used.
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Differences to gzip and bzip2
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Standard input and output
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Both gzip and bzip2 refuse to write compressed data to a terminal and
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read compressed data from a terminal. With gzip (but not with bzip2),
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this can be overriden with the `--force' option. lzma follows the
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behavior of gzip here.
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Usage of LZMA_OPT environment variable
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gzip and bzip2 read GZIP and BZIP2 environment variables at startup.
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These variables may contain extra command line options.
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gzip and bzip2 allow passing not only options, but also end-of-options
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indicator (`--') and filenames via the environment variable. No quoting
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is supported with the filenames.
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Here are examples with gzip. bzip2 behaves identically.
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bash$ echo asdf > 'foo bar'
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bash$ GZIP='"foo bar"' gzip
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gzip: "foo: No such file or directory
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gzip: bar": No such file or directory
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bash$ GZIP=-- gzip --help
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gzip: --help: No such file or directory
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lzma silently ignores all non-option arguments given via the
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environment variable LZMA_OPT. Like on the command line, everything
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after `--' is taken as non-options, and thus ignored in LZMA_OPT.
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bash$ LZMA_OPT='--help' lzma --version # Displays help
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bash$ LZMA_OPT='-- --help' lzma --version # Displays version
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Filter chain presets
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Like in gzip and bzip2, lzma supports numbered presets from 1 to 9
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where 1 is the fastest and 9 the best compression. 1 and 2 are for
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fast compressing with small memory usage, 3 to 6 for good compression
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ratio with medium memory usage, and 7 to 9 for excellent compression
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ratio with higher memory requirements. The default is 7 if memory
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usage limit allows.
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In future, there will probably be an option like --preset=NAME, which
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will contain more special presets for specific file types.
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It's also possible that there will be some heuristics to select good
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filters. For example, the tool could detect when a .tar archive is
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being compressed, and enable x86 filter only for those files in the
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.tar archive that are ELF or PE executables for x86.
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Specifying custom filter chains
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Custom filter chains are specified by using long options with the name
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of the filters in correct order. For example, to pass the input data to
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the x86 filter and the output of that to the LZMA filter, the following
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command will do:
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lzma --x86 --lzma filename
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Some filters accept options, which are specified as a comma-separated
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list of key=value pairs:
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lzma --delta=distance=4 --lzma=dict=4Mi,lc=8,lp=2 filename
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Memory usage control
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By default, the command line tool limits memory usage to 1/3 of the
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available physical RAM. If no preset or custom filter chain has been
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given, the default preset will be used. If the memory limit is too
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low for the default preset, the tool will silently switch to lower
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preset.
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When a preset or a custom filter chain has been specified and the
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memory limit is too low, an error message is displayed and no files
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are processed.
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If the decoder hits the memory usage limit, an error is displayed and
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no more files are processed.
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