compress.1 (5169B)
1 .TH COMPRESS 1 2 .SH NAME 3 compress, uncompress, zcat \- compress and expand data 4 .SH SYNOPSIS 5 .B compress 6 [ 7 .B \-f 8 ] [ 9 .B \-v 10 ] [ 11 .B \-c 12 ] [ 13 .B \-V 14 ] [ 15 .B \-b 16 .I bits 17 ] [ 18 .I "name \&..." 19 ] 20 .PP 21 .B uncompress 22 [ 23 .B \-f 24 ] [ 25 .B \-v 26 ] [ 27 .B \-c 28 ] [ 29 .B \-V 30 ] [ 31 .I "name \&..." 32 ] 33 .PP 34 .B zcat 35 [ 36 .B \-V 37 ] [ 38 .I "name \&..." 39 ] 40 .SH DESCRIPTION 41 .I Compress 42 reduces the size of the named files using adaptive Lempel-Ziv coding. 43 Whenever possible, 44 each file is replaced by one with the extension 45 .B "\&.Z," 46 while keeping the same ownership modes, access and modification times. 47 If no files are specified, the standard input is compressed to the 48 standard output. 49 Compressed files can be restored to their original form using 50 .I uncompress 51 or 52 .I zcat. 53 .PP 54 The 55 .B \-f 56 option will force compression of 57 .I name. 58 This is useful for compressing an entire directory, 59 even if some of the files do not actually shrink. 60 If 61 .B \-f 62 is not given and 63 .I compress 64 is run in the foreground, 65 the user is prompted as to whether an existing file should be overwritten. 66 .PP 67 The 68 .B \-c 69 option makes 70 .I compress/uncompress 71 write to the standard output; no files are changed. 72 The nondestructive behavior of 73 .I zcat 74 is identical to that of 75 .I uncompress 76 .B \-c. 77 .PP 78 .I Compress 79 uses the modified Lempel-Ziv algorithm popularized in 80 "A Technique for High Performance Data Compression", 81 Terry A. Welch, 82 .I "IEEE Computer," 83 vol. 17, no. 6 (June 1984), pp. 8-19. 84 Common substrings in the file are first replaced by 9-bit codes 257 and up. 85 When code 512 is reached, the algorithm switches to 10-bit codes and 86 continues to use more bits until the 87 limit specified by the 88 .B \-b 89 flag is reached (default 16). 90 .I Bits 91 must be between 9 and 16. The default can be changed in the source to allow 92 .I compress 93 to be run on a smaller machine. 94 .PP 95 After the 96 .I bits 97 limit is attained, 98 .I compress 99 periodically checks the compression ratio. If it is increasing, 100 .I compress 101 continues to use the existing code dictionary. However, 102 if the compression ratio decreases, 103 .I compress 104 discards the table of substrings and rebuilds it from scratch. This allows 105 the algorithm to adapt to the next "block" of the file. 106 .PP 107 Note that the 108 .B \-b 109 flag is omitted for 110 .I uncompress, 111 since the 112 .I bits 113 parameter specified during compression 114 is encoded within the output, along with 115 a magic number to ensure that neither decompression of random data nor 116 recompression of compressed data is attempted. 117 .PP 118 .ne 8 119 The amount of compression obtained depends on the size of the 120 input, the number of 121 .I bits 122 per code, and the distribution of common substrings. 123 Typically, text such as source code or English 124 is reduced by 50\-60%. 125 Compression is generally much better than that achieved by 126 Huffman coding (as used in 127 .IR pack ), 128 or adaptive Huffman coding 129 .RI ( compact ), 130 and takes less time to compute. 131 .PP 132 Under the 133 .B \-v 134 option, 135 a message is printed yielding the percentage of 136 reduction for each file compressed. 137 .PP 138 If the 139 .B \-V 140 option is specified, the current version and compile options are printed on 141 stderr. 142 .PP 143 Exit status is normally 0; 144 if the last file is larger after (attempted) compression, the status is 2; 145 if an error occurs, exit status is 1. 146 .SH "SEE ALSO" 147 pack(1), compact(1) 148 .SH "DIAGNOSTICS" 149 Usage: compress [\-dfvcV] [\-b maxbits] [file ...] 150 .in +8 151 Invalid options were specified on the command line. 152 .in -8 153 Missing maxbits 154 .in +8 155 Maxbits must follow 156 .BR \-b \. 157 .in -8 158 .IR file : 159 not in compressed format 160 .in +8 161 The file specified to 162 .I uncompress 163 has not been compressed. 164 .in -8 165 .IR file : 166 compressed with 167 .I xx 168 bits, can only handle 169 .I yy 170 bits 171 .in +8 172 .I File 173 was compressed by a program that could deal with 174 more 175 .I bits 176 than the compress code on this machine. 177 Recompress the file with smaller 178 .IR bits \. 179 .in -8 180 .IR file : 181 already has .Z suffix -- no change 182 .in +8 183 The file is assumed to be already compressed. 184 Rename the file and try again. 185 .in -8 186 .IR file : 187 filename too long to tack on .Z 188 .in +8 189 The file cannot be compressed because its name is longer than 190 12 characters. 191 Rename and try again. 192 This message does not occur on BSD systems. 193 .in -8 194 .I file 195 already exists; do you wish to overwrite (y or n)? 196 .in +8 197 Respond "y" if you want the output file to be replaced; "n" if not. 198 .in -8 199 uncompress: corrupt input 200 .in +8 201 A SIGSEGV violation was detected which usually means that the input file has 202 been corrupted. 203 .in -8 204 Compression: 205 .I "xx.xx%" 206 .in +8 207 Percentage of the input saved by compression. 208 (Relevant only for 209 .BR \-v \.) 210 .in -8 211 -- not a regular file: unchanged 212 .in +8 213 When the input file is not a regular file, 214 (e.g. a directory), it is 215 left unaltered. 216 .in -8 217 -- has 218 .I xx 219 other links: unchanged 220 .in +8 221 The input file has links; it is left unchanged. See 222 .IR ln "(1)" 223 for more information. 224 .in -8 225 -- file unchanged 226 .in +8 227 No savings is achieved by 228 compression. The input remains virgin. 229 .in -8 230 .SH SOURCE 231 .B \*9/src/cmd/compress/compress.c 232 .SH "BUGS" 233 Although compressed files are compatible between machines with large memory, 234 .BR \-b \12 235 should be used for file transfer to architectures with 236 a small process data space (64KB or less, as exhibited by the DEC PDP 237 series, the Intel 80286, etc.)