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(-)/tmp/compress.1 (-32 / +46 lines)
Lines 54-70 Link Here
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.Sh DESCRIPTION
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.Sh DESCRIPTION
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The
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The
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.Nm
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.Nm
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utility reduces the size of the named files using adaptive Lempel-Ziv coding.
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utility attempts to reduce the size of the
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named files using adaptive Lempel-Ziv coding.
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Each
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Each
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.Ar file
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.Ar file
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is renamed to the same name plus the extension
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is renamed to the same name plus the extension
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.Dq .Z .
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.Dq .Z .
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As many of the modification time, access time, file flags, file mode,
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If attempted compression would not reduce the size of a
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user ID, and group ID as allowed by permissions are retained in the
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new file.
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If compression would not reduce the size of a
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.Ar file ,
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.Ar file ,
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the file is ignored.
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or if the filename ends with
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.Dq .Z ,
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the file is not compressed.  Exceptions are discussed below.
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.Pp
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.Pp
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The
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The
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.Nm uncompress
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.Nm uncompress
Lines 72-109 Link Here
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files by deleting the
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files by deleting the
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.Dq .Z
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.Dq .Z
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extension.
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extension.
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If a specified filename doesn't end with
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.Dq .Z ,
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one will be assumed unless a file with the extended filename does not
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exist, in which case the named file will be removed (because of a bug).
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.Pp
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.Pp
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If renaming the files would cause files to be overwritten and the standard
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If renaming the files would cause files to be overwritten and the standard
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input device is a terminal, the user is prompted (on the standard error
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input device is a terminal, the user is prompted (on the standard error
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output) for confirmation.
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output) for confirmation.
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If prompting is not possible or confirmation is not received, the files
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If prompting is not possible or confirmation is not received, the files
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are not overwritten.
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are not overwritten.
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As many of the file attributes (modification time, access time,
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file flags, file mode, user ID, and group ID) as allowed by
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file permissions are retained in the new file.
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.Pp
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.Pp
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If no files are specified or a
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If no files are specified or a
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.Ar file
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.Ar file
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argument is a single dash
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argument is a single dash
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.Pq Sq Fl ,
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.Pq Sq Fl ,
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the standard input is compressed or uncompressed to the standard output.
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the standard input is compressed or uncompressed to the standard output.
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If either the input and output files are not regular files, the checks for
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If either the input and output are not regular files, the checks for
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reduction in size and file overwriting are not performed, the input file is
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reduction in size and file overwriting are not performed, the input file is
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not removed, and the attributes of the input file are not retained.
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not removed, and the attributes of the input file
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are not retained in the output.
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.Pp
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.Pp
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The options are as follows:
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The options are as follows:
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.Bl -tag -width indent
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.Bl -tag -width ".Fl b Ar bits"
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.It Fl b
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.It Fl b Ar bits
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Specify the
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The code size (see below) is limited to
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.Ar bits
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.Ar bits ,
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code limit (see below).
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which must be in the range 9..16.
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The default is 16.
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.It Fl c
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.It Fl c
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Compressed or uncompressed output is written to the standard output.
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Compressed or uncompressed output is written to the standard output.
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No files are modified.
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Compression is attempted even if the results will be larger than the
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original.
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No files are modified.  The
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.Fl v
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flag is ignored.
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.It Fl f
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.It Fl f
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Force compression of
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Files are overwritten without prompting for confirmation.
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.Ar file ,
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For compression, proceed even if the
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even if it is not actually reduced in size.
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results will be larger than the original.
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Additionally, files are overwritten without prompting for confirmation.
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.It Fl v
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.It Fl v
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Print the percentage reduction of each file.
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The size reduction of each file is printed as a percentage to
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the standard output.  Ignored for uncompressiong or if the
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.Fl c
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flag is also used.
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.El
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.El
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.Pp
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.Pp
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The
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The
Lines 112-132 Link Here
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Common substrings in the file are first replaced by 9-bit codes 257 and up.
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Common substrings in the file are first replaced by 9-bit codes 257 and up.
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When code 512 is reached, the algorithm switches to 10-bit codes and
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When code 512 is reached, the algorithm switches to 10-bit codes and
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continues to use more bits until the
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continues to use more bits until the
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limit specified by the
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code size limit (see
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.Fl b
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.Fl b )
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flag is reached (the default is 16).
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is reached.
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.Ar Bits
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Then the compression ratio is periodically checked.
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must be between 9 and 16.
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.Pp
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After the
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.Ar bits
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limit is reached,
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.Nm
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periodically checks the compression ratio.
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If it is increasing,
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If it is increasing,
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.Nm
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.Nm
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continues to use the existing code dictionary.
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continues to use the existing code dictionary;
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However, if the compression ratio decreases,
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but if the compression ratio is decreasing,
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.Nm
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.Nm
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discards the table of substrings and rebuilds it from scratch.  This allows
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discards the table of substrings and rebuilds it from scratch.  This allows
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the algorithm to adapt to the next "block" of the file.
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the algorithm to adapt to the next "block" of the file.
Lines 187-189 Link Here
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.Nm
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.Nm
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command appeared in
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command appeared in
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.Bx 4.3 .
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.Bx 4.3 .
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.Sh BUGS
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For uncompression, if a specified filename doesn't end with
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.Dq .Z
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and a file with the filename extended with
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.Dq .Z
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does not exist, the named file will be removed.

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