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Annotation of researchv9/cmd/compress/compress.1, revision 1.1

1.1     ! root        1: .PU
        !             2: .TH COMPRESS 1 local
        !             3: .SH NAME
        !             4: compress, uncompress, zcat \- compress and expand data
        !             5: .SH SYNOPSIS
        !             6: .ll +8
        !             7: .B compress
        !             8: [
        !             9: .B \-f
        !            10: ] [
        !            11: .B \-v
        !            12: ] [
        !            13: .B \-c
        !            14: ] [
        !            15: .B \-V
        !            16: ] [
        !            17: .B \-b
        !            18: .I bits
        !            19: ] [
        !            20: .I "name \&..."
        !            21: ]
        !            22: .ll -8
        !            23: .br
        !            24: .B uncompress
        !            25: [
        !            26: .B \-f
        !            27: ] [
        !            28: .B \-v
        !            29: ] [
        !            30: .B \-c
        !            31: ] [
        !            32: .B \-V
        !            33: ] [
        !            34: .I "name \&..."
        !            35: ]
        !            36: .br
        !            37: .B zcat
        !            38: [
        !            39: .B \-V
        !            40: ] [
        !            41: .I "name \&..."
        !            42: ]
        !            43: .SH DESCRIPTION
        !            44: .I Compress
        !            45: reduces the size of the named files using adaptive Lempel-Ziv coding.
        !            46: Whenever possible,
        !            47: each file is replaced by one with the extension
        !            48: .B "\&.Z,"
        !            49: while keeping the same ownership modes, access and modification times.
        !            50: If no files are specified, the standard input is compressed to the
        !            51: standard output.
        !            52: Compressed files can be restored to their original form using
        !            53: .I uncompress
        !            54: or
        !            55: .I zcat.
        !            56: .PP
        !            57: The
        !            58: .B \-f
        !            59: option will force compression of
        !            60: .I name.
        !            61: This is useful for compressing an entire directory,
        !            62: even if some of the files do not actually shrink.
        !            63: If
        !            64: .B \-f
        !            65: is not given and
        !            66: .I compress
        !            67: is run in the foreground,
        !            68: the user is prompted as to whether an existing file should be overwritten.
        !            69: .PP
        !            70: The
        !            71: .B \-c
        !            72: option makes
        !            73: .I compress/uncompress
        !            74: write to the standard output; no files are changed.
        !            75: The nondestructive behavior of
        !            76: .I zcat
        !            77: is identical to that of
        !            78: .I uncompress
        !            79: .B \-c.
        !            80: .PP
        !            81: .I Compress
        !            82: uses the modified Lempel-Ziv algorithm popularized in
        !            83: "A Technique for High Performance Data Compression",
        !            84: Terry A. Welch,
        !            85: .I "IEEE Computer,"
        !            86: vol. 17, no. 6 (June 1984), pp. 8-19.
        !            87: Common substrings in the file are first replaced by 9-bit codes 257 and up.
        !            88: When code 512 is reached, the algorithm switches to 10-bit codes and
        !            89: continues to use more bits until the
        !            90: limit specified by the
        !            91: .B \-b
        !            92: flag is reached (default 16).
        !            93: .I Bits
        !            94: must be between 9 and 16.  The default can be changed in the source to allow
        !            95: .I compress
        !            96: to be run on a smaller machine.
        !            97: .PP
        !            98: After the
        !            99: .I bits
        !           100: limit is attained,
        !           101: .I compress
        !           102: periodically checks the compression ratio.  If it is increasing,
        !           103: .I compress
        !           104: continues to use the existing code dictionary.  However,
        !           105: if the compression ratio decreases,
        !           106: .I compress
        !           107: discards the table of substrings and rebuilds it from scratch.  This allows
        !           108: the algorithm to adapt to the next "block" of the file.
        !           109: .PP
        !           110: Note that the
        !           111: .B \-b
        !           112: flag is omitted for
        !           113: .I uncompress,
        !           114: since the 
        !           115: .I bits
        !           116: parameter specified during compression
        !           117: is encoded within the output, along with
        !           118: a magic number to ensure that neither decompression of random data nor
        !           119: recompression of compressed data is attempted. 
        !           120: .PP
        !           121: .ne 8
        !           122: The amount of compression obtained depends on the size of the
        !           123: input, the number of
        !           124: .I bits
        !           125: per code, and the distribution of common substrings.
        !           126: Typically, text such as source code or English
        !           127: is reduced by 50\-60%.
        !           128: Compression is generally much better than that achieved by
        !           129: Huffman coding (as used in
        !           130: .IR pack ),
        !           131: or adaptive Huffman coding
        !           132: .RI ( compact ),
        !           133: and takes less time to compute.
        !           134: .PP
        !           135: Under the
        !           136: .B \-v
        !           137: option,
        !           138: a message is printed yielding the percentage of
        !           139: reduction for each file compressed.
        !           140: .PP
        !           141: If the
        !           142: .B \-V
        !           143: option is specified, the current version and compile options are printed on
        !           144: stderr.
        !           145: .PP
        !           146: Exit status is normally 0;
        !           147: if the last file is larger after (attempted) compression, the status is 2;
        !           148: if an error occurs, exit status is 1.
        !           149: .SH "SEE ALSO"
        !           150: pack(1), compact(1)
        !           151: .SH "DIAGNOSTICS"
        !           152: Usage: compress [\-dfvcV] [\-b maxbits] [file ...]
        !           153: .in +8
        !           154: Invalid options were specified on the command line.
        !           155: .in -8
        !           156: Missing maxbits
        !           157: .in +8
        !           158: Maxbits must follow
        !           159: .BR \-b \.
        !           160: .in -8
        !           161: .IR file :
        !           162: not in compressed format
        !           163: .in +8
        !           164: The file specified to
        !           165: .I uncompress
        !           166: has not been compressed.
        !           167: .in -8
        !           168: .IR file :
        !           169: compressed with 
        !           170: .I xx
        !           171: bits, can only handle 
        !           172: .I yy
        !           173: bits
        !           174: .in +8
        !           175: .I File
        !           176: was compressed by a program that could deal with
        !           177: more 
        !           178: .I bits
        !           179: than the compress code on this machine.
        !           180: Recompress the file with smaller
        !           181: .IR bits \.
        !           182: .in -8
        !           183: .IR file :
        !           184: already has .Z suffix -- no change
        !           185: .in +8
        !           186: The file is assumed to be already compressed.
        !           187: Rename the file and try again.
        !           188: .in -8
        !           189: .IR file :
        !           190: filename too long to tack on .Z
        !           191: .in +8
        !           192: The file cannot be compressed because its name is longer than
        !           193: 12 characters.
        !           194: Rename and try again.
        !           195: This message does not occur on BSD systems.
        !           196: .in -8
        !           197: .I file
        !           198: already exists; do you wish to overwrite (y or n)?
        !           199: .in +8
        !           200: Respond "y" if you want the output file to be replaced; "n" if not.
        !           201: .in -8
        !           202: uncompress: corrupt input
        !           203: .in +8
        !           204: A SIGSEGV violation was detected which usually means that the input file has
        !           205: been corrupted.
        !           206: .in -8
        !           207: Compression: 
        !           208: .I "xx.xx%"
        !           209: .in +8
        !           210: Percentage of the input saved by compression.
        !           211: (Relevant only for
        !           212: .BR \-v \.)
        !           213: .in -8
        !           214: -- not a regular file: unchanged
        !           215: .in +8
        !           216: When the input file is not a regular file,
        !           217: (e.g. a directory), it is
        !           218: left unaltered.
        !           219: .in -8
        !           220: -- has 
        !           221: .I xx 
        !           222: other links: unchanged
        !           223: .in +8
        !           224: The input file has links; it is left unchanged.  See
        !           225: .IR ln "(1)"
        !           226: for more information.
        !           227: .in -8
        !           228: -- file unchanged
        !           229: .in +8
        !           230: No savings is achieved by
        !           231: compression.  The input remains virgin.
        !           232: .in -8
        !           233: .SH "BUGS"
        !           234: Although compressed files are compatible between machines with large memory,
        !           235: .BR \-b \12
        !           236: should be used for file transfer to architectures with 
        !           237: a small process data space (64KB or less, as exhibited by the DEC PDP
        !           238: series, the Intel 80286, etc.)

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