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Annotation of 43BSDReno/usr.bin/compress/compress.1, revision 1.1

1.1     ! root        1: .\" Copyright (c) 1986, 1990 The Regents of the University of California.
        !             2: .\" All rights reserved.
        !             3: .\"
        !             4: .\" This code is derived from software contributed to Berkeley by
        !             5: .\" James A. Woods, derived from original work by Spencer Thomas
        !             6: .\" and Joseph Orost.
        !             7: .\"
        !             8: .\" Redistribution and use in source and binary forms are permitted provided
        !             9: .\" that: (1) source distributions retain this entire copyright notice and
        !            10: .\" comment, and (2) distributions including binaries display the following
        !            11: .\" acknowledgement:  ``This product includes software developed by the
        !            12: .\" University of California, Berkeley and its contributors'' in the
        !            13: .\" documentation or other materials provided with the distribution and in
        !            14: .\" all advertising materials mentioning features or use of this software.
        !            15: .\" Neither the name of the University nor the names of its contributors may
        !            16: .\" be used to endorse or promote products derived from this software without
        !            17: .\" specific prior written permission.
        !            18: .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
        !            19: .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
        !            20: .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
        !            21: .\"
        !            22: .\"     @(#)compress.1 6.8 (Berkeley) 7/24/90
        !            23: .\"
        !            24: .Dd July 24, 1990
        !            25: .Dt COMPRESS 1
        !            26: .Os BSD 4.3
        !            27: .Sh NAME
        !            28: .Nm compress ,
        !            29: .Nm uncompress ,
        !            30: .Nm zcat
        !            31: .Nd compress and expand data
        !            32: .Sh SYNOPSIS
        !            33: .Nm compress
        !            34: .Op Fl f
        !            35: .Op Fl v
        !            36: .Op Fl c
        !            37: .Op Fl b Ar bits
        !            38: .Ar
        !            39: .br
        !            40: .Nm uncompress
        !            41: .Op Fl f
        !            42: .Op Fl v
        !            43: .Op Fl c
        !            44: .Ar
        !            45: .br
        !            46: .Nm zcat
        !            47: .Ar
        !            48: .Sh DESCRIPTION
        !            49: .Nm Compress
        !            50: reduces the size of the named files using adaptive Lempel-Ziv coding.
        !            51: Whenever possible,
        !            52: each
        !            53: .Ar file
        !            54: is replaced by one with the extension
        !            55: .Ar \&.Z ,
        !            56: while keeping the same ownership modes, access and modification times.
        !            57: If no files are specified, the standard input is compressed to the
        !            58: standard output.
        !            59: Compressed files can be restored to their original form using
        !            60: .Nm uncompress
        !            61: or
        !            62: .Nm zcat
        !            63: .Tw Ds
        !            64: .Tp Fl f
        !            65: Force compression of
        !            66: .Ar file ,
        !            67: even if it does not actually shrink
        !            68: or the corresponding
        !            69: .Ar file.Z
        !            70: file already exists.
        !            71: Except when run in the background under
        !            72: .Pa /bin/sh ,
        !            73: if
        !            74: .Fl f
        !            75: is not given the user is prompted as to whether an existing
        !            76: .Ar file.Z
        !            77: file should be overwritten.
        !            78: .Pp
        !            79: .Tp Fl c
        !            80: (``cat'') makes
        !            81: .Nm compress/uncompress
        !            82: write to the standard output; no files are changed.
        !            83: The nondestructive behavior of
        !            84: .Nm zcat
        !            85: is identical to that of
        !            86: .Nm uncompress
        !            87: .Fl c.
        !            88: .Tp Fl b
        !            89: Specify
        !            90: .Ar bits
        !            91: code limit (see below).
        !            92: .Tp Fl v
        !            93: Print the percentage reduction of each file.
        !            94: .Tp
        !            95: .Pp
        !            96: .Nm Compress
        !            97: uses the modified Lempel-Ziv algorithm popularized in
        !            98: "A Technique for High Performance Data Compression",
        !            99: Terry A. Welch,
        !           100: .Em IEEE Computer ,
        !           101: vol. 17, no. 6 (June 1984), pp. 8-19.
        !           102: Common substrings in the file are first replaced by 9-bit codes 257 and up.
        !           103: When code 512 is reached, the algorithm switches to 10-bit codes and
        !           104: continues to use more bits until the
        !           105: limit specified by the
        !           106: .Fl b
        !           107: flag is reached (default 16).
        !           108: .Ar Bits
        !           109: must be between 9 and 16.  The default can be changed in the source to allow
        !           110: .Nm compress
        !           111: to be run on a smaller machine.
        !           112: .Pp
        !           113: After the
        !           114: .Ar bits
        !           115: limit is attained,
        !           116: .Nm compress
        !           117: periodically checks the compression ratio.  If it is increasing,
        !           118: .Nm compress
        !           119: continues to use the existing code dictionary.  However,
        !           120: if the compression ratio decreases,
        !           121: .Nm compress
        !           122: discards the table of substrings and rebuilds it from scratch.  This allows
        !           123: the algorithm to adapt to the next "block" of the file.
        !           124: .Pp
        !           125: Note that the
        !           126: .Fl b
        !           127: flag is omitted for
        !           128: .Ar uncompress
        !           129: since the
        !           130: .Ar bits
        !           131: parameter specified during compression
        !           132: is encoded within the output, along with
        !           133: a magic number to ensure that neither decompression of random data nor
        !           134: recompression of compressed data is attempted.
        !           135: .Pp
        !           136: .ne 8
        !           137: The amount of compression obtained depends on the size of the
        !           138: input, the number of
        !           139: .Ar bits
        !           140: per code, and the distribution of common substrings.
        !           141: Typically, text such as source code or English
        !           142: is reduced by 50\-60%.
        !           143: Compression is generally much better than that achieved by
        !           144: Huffman coding (as used in
        !           145: .Xr pack ) ,
        !           146: or adaptive Huffman coding (as
        !           147: used in
        !           148: .Xr compact ) ,
        !           149: and takes less time to compute.
        !           150: .Pp
        !           151: If an error occurs, exit status is 1, else
        !           152: if the last file was not compressed because it became larger, the status
        !           153: is 2; else the status is 0.
        !           154: .Sh DIAGNOSTICS
        !           155: Usage: compress
        !           156: .Op Fl fvc
        !           157: .Op Fl b Ar maxbits
        !           158: .Ar
        !           159: .Dl Invalid options were specified on the command line.
        !           160: .Pp
        !           161: Missing maxbits
        !           162: .Df I
        !           163: Maxbits must follow
        !           164: .Fl b .
        !           165: .De
        !           166: .Pp
        !           167: .Ar file :
        !           168: not in compressed format
        !           169: .Df I
        !           170: The file specified to
        !           171: .Ar uncompress
        !           172: has not been compressed.
        !           173: .De
        !           174: .Pp
        !           175: .Ar file :
        !           176: compressed with
        !           177: .Ar xx
        !           178: bits, can only handle
        !           179: .Ar yy
        !           180: bits
        !           181: .Df I
        !           182: .Ar File
        !           183: was compressed by a program that could deal with
        !           184: more
        !           185: .Ar bits
        !           186: than the compress code on this machine.
        !           187: Recompress the file with smaller
        !           188: .Ar bits .
        !           189: .De
        !           190: .Pp
        !           191: .Ar file :
        !           192: already has .Z suffix -- no change
        !           193: .Df I
        !           194: The file is assumed to be already compressed.
        !           195: Rename the file and try again.
        !           196: .De
        !           197: .Pp
        !           198: .Ar file :
        !           199: filename too long to tack on .Z
        !           200: .Df I
        !           201: The file cannot be compressed because its name is longer than
        !           202: 12 characters.
        !           203: Rename and try again.
        !           204: This message does not occur on BSD systems.
        !           205: .De
        !           206: .Pp
        !           207: .Ar file
        !           208: already exists; do you wish to overwrite (y or n)?
        !           209: .Df I
        !           210: Respond "y" if you want the output file to be replaced; "n" if not.
        !           211: .De
        !           212: .Pp
        !           213: uncompress: corrupt input
        !           214: .Df I
        !           215: A SIGSEGV violation was detected which usually means that the input file is
        !           216: corrupted.
        !           217: .De
        !           218: .Pp
        !           219: Compression:
        !           220: .Em xx.xx%
        !           221: .Df I
        !           222: Percentage of the input saved by compression.
        !           223: (Relevant only for
        !           224: .Fl v . )
        !           225: .De
        !           226: .Pp
        !           227: -- not a regular file: unchanged
        !           228: .Df I
        !           229: When the input file is not a regular file,
        !           230: (e.g. a directory), it is
        !           231: left unaltered.
        !           232: .De
        !           233: .Pp
        !           234: -- has
        !           235: .Ar xx
        !           236: other links: unchanged
        !           237: .Df I
        !           238: The input file has links; it is left unchanged.  See
        !           239: .Xr ln 1
        !           240: for more information.
        !           241: .De
        !           242: .Pp
        !           243: -- file unchanged
        !           244: .Df I
        !           245: No savings is achieved by
        !           246: compression.  The input remains virgin.
        !           247: .De
        !           248: .Pp
        !           249: .Sh FILES
        !           250: .Tw file.Z
        !           251: .Tp Pa file.Z
        !           252: compressed file is file.Z
        !           253: .Tp
        !           254: .Sh HISTORY
        !           255: Appeared in 4.3 BSD.
        !           256: .Sh "BUGS"
        !           257: Although compressed files are compatible between machines with large memory,
        !           258: .Cx Fl b
        !           259: .Ar 12
        !           260: .Cx
        !           261: should be used for file transfer to architectures with
        !           262: a small process data space (64KB or less, as exhibited by the DEC PDP
        !           263: series, the Intel 80286, etc.)
        !           264: .Pp
        !           265: .Nm Compress
        !           266: should be more flexible about the existence of the `.Z' suffix.

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