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authorLasse Collin <lasse.collin@tukaani.org>2007-12-09 00:42:33 +0200
committerLasse Collin <lasse.collin@tukaani.org>2007-12-09 00:42:33 +0200
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+
+Introduction to the lzma command line tool
+------------------------------------------
+
+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.
+