Several changes: Added a BUGS section with several bugs. And
-- Made the synopses more precise. -- Added argument to flag in option description. -- Moved -b default and limits to option description (to un-hide). -- Noted several behaviors that were not mentioned. -- A few more trivial changes. PR: docs/46787 Approved by: keramida MFC after: 3 days
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@ -45,33 +45,50 @@
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.Nd compress and expand data
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.Sh SYNOPSIS
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.Nm
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.Op Fl cfv
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.Op Fl fv
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.Op Fl b Ar bits
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.Op Ar
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.Nm
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.Fl c
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.Op Fl b Ar bits
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.Op Ar file
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.Nm uncompress
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.Op Fl cfv
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.Op Fl f
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.Op Ar
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.Nm uncompress
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.Fl c
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.Op Ar file
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.Sh DESCRIPTION
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The
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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 reduces the size of files using adaptive Lempel-Ziv coding.
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Each
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.Ar file
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is renamed to the same name plus the extension
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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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user ID, and group ID as allowed by permissions are retained in the
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new file.
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A
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.Ar file
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argument with a
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.Dq .Z
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extension will be ignored except it will cause an
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error exit after other arguments are processed.
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If compression would not reduce the size of a
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.Ar file ,
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the file is ignored.
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.Pp
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The
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.Nm uncompress
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utility restores the compressed files to their original form, renaming the
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utility restores compressed files to their original form, renaming the
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files by deleting the
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.Dq .Z
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extensions.
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A file specification need not include the file's
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.Dq .Z
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extension.
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If a file's name in its file system does not have a
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.Dq .Z
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extension, it will not be uncompressed and it will cause
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an error exit after other arguments are processed.
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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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input device is a terminal, the user is prompted (on the standard error
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@ -79,6 +96,10 @@ output) for confirmation.
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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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.Pp
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As many of the modification time, access time, file flags, file mode,
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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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.Pp
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If no files are specified or a
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.Ar file
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argument is a single dash
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@ -86,24 +107,36 @@ argument is a single dash
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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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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 are not retained
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in the output file.
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl b
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Specify the
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.Ar bits
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code limit (see below).
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.Bl -tag -width ".Fl b Ar bits"
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.It Fl b Ar bits
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The code size (see below) is limited to
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.Ar bits ,
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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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Compressed or uncompressed output is written to the standard output.
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No files are modified.
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The
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.Fl v
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option is ignored.
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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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.It Fl f
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Force compression of
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.Ar file ,
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even if it is not actually reduced in size.
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Additionally, files are overwritten without prompting for confirmation.
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Files are overwritten without prompting for confirmation.
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Also, for
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.Nm compress ,
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files are compressed even if they are not actually reduced in size.
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.It Fl v
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Print the percentage reduction of each file.
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Ignored by
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.Nm uncompress
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or if the
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.Fl c
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option is also used.
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.El
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.Pp
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The
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@ -114,13 +147,9 @@ 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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limit specified by the
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.Fl b
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flag is reached (the default is 16).
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.Ar Bits
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must be between 9 and 16.
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option or its default is reached.
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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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After the 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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@ -134,7 +163,7 @@ the algorithm to adapt to the next "block" of the file.
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.Pp
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The
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.Fl b
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flag is omitted for
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option is unavailable for
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.Nm uncompress
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since the
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.Ar bits
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@ -157,10 +186,10 @@ time to compute.
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.Pp
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The
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.Nm compress
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utility exits 2 if attempting to compress the file would not reduce its size
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utility exits 2 if attempting to compress a file would not reduce its size
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and the
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.Fl f
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option was not specified.
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option was not specified and if no other error occurs.
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.Sh SEE ALSO
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.Xr gunzip 1 ,
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.Xr gzexe 1 ,
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@ -188,3 +217,42 @@ The
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.Nm
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command appeared in
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.Bx 4.3 .
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.Sh BUGS
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.Pp
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Some of these might be considered otherwise-undocumented features.
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.Pp
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.Nm compress :
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If the utility does not compress a file because doing so would not
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reduce it size, and a file of the same name except with an
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.Dq .Z
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extension exists, the named file is not really ignored as stated above;
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it causes a prompt to confirm the overwriting of the file with the extension.
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If the operation is confirmed, that file is deleted.
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.Pp
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.Nm uncompress :
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If an empty file is compressed (using
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.Fl f ) ,
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the resulting
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.Dq .Z
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file is also empty.
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That seems right, but if
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.Nm uncompress
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is then used on that file, an error will occur.
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.Pp
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Both utilities: If a
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.Sq Fl
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argument is used and the utility prompts the user, the standard input
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is taken as the user's reply to the prompt.
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.Pp
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Both utilities:
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If the specified file does not exist, but a similarly-named one with (for
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.Nm compress )
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or without (for
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.Nm uncompress )
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a
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.Dq .Z
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extension does exist, the utility will waste the user's time by not
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immediately emitting an error message about the missing file and
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continuing.
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Instead, it first asks for confirmation to overwrite
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the existing file and then doesn't overwrite it.
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