1 COMPRESS(1)                      User Commands                     COMPRESS(1)
   2 
   3 
   4 
   5 NAME
   6        compress, uncompress, zcat - compress, uncompress files or display
   7        expanded files
   8 
   9 SYNOPSIS
  10        compress [-fv/] [-b bits] [file]...
  11 
  12 
  13        compress -c [-fv] [-b bits] [file]
  14 
  15 
  16        uncompress [-fv] [-c | -/] [file]...
  17 
  18 
  19        zcat [file]...
  20 
  21 
  22 DESCRIPTION
  23    compress
  24        The compress utility attempts to reduce the size of the named files by
  25        using adaptive Lempel-Ziv coding. Except when the output is to the
  26        standard output, each file is replaced by one with the extension .Z,
  27        while keeping the same ownership modes, change times and modification
  28        times, ACLs, and extended attributes. The compress utility also
  29        attempts to set the owner and group of file.Z to the owner and group of
  30        file, but does not fail if this cannot be done. If appending the .Z to
  31        the file pathname would make the pathname exceed 1023 bytes, the
  32        command fails. If no files are specified, the standard input is
  33        compressed to the standard output.
  34 
  35 
  36        The amount of compression obtained depends on the size of the input,
  37        the number of bits per code, and the distribution of common substrings.
  38        Typically, text such as source code or English is reduced by 50-60%.
  39        Compression is generally much better than that achieved by Huffman
  40        coding (as used in pack(1)) and it takes less time to compute. The bits
  41        parameter specified during compression is encoded within the compressed
  42        file, along with a magic number to ensure that neither decompression of
  43        random data nor recompression of compressed data is subsequently
  44        allowed.
  45 
  46    uncompress
  47        The uncompress utility restores files to their original state after
  48        they have been compressed using the compress utility. If no files are
  49        specified, the standard input is uncompressed to the standard output.
  50 
  51 
  52        This utility supports the uncompressing of any files produced by
  53        compress. For files produced by compress on other systems, uncompress
  54        supports 9- to 16-bit compression (see -b).
  55 
  56    zcat
  57        The zcat utility writes to standard output the uncompressed form of
  58        files that have been compressed using compress. It is the equivalent of
  59        uncompress -c. Input files are not affected.
  60 
  61 OPTIONS
  62        The following options are supported:
  63 
  64        -b bits
  65                   Sets the upper limit (in bits) for common substring codes.
  66                   bits must be between 9 and 16 (16 is the default). Lowering
  67                   the number of bits result in larger, less compressed files.
  68 
  69 
  70        -c
  71                   Writes to the standard output; no files are changed and no
  72                   .Z files are created. The behavior of zcat is identical to
  73                   that of `uncompress -c'.
  74 
  75 
  76        -f
  77                   When compressing, forces compression of file, even if it
  78                   does not actually reduce the size of the file, or if the
  79                   corresponding file.Z file already exists.
  80 
  81                   If the -f option is not specified, and the process is not
  82                   running in the background, prompts to verify whether an
  83                   existing file should be overwritten.  If the response is
  84                   affirmative, the existing file is overwritten. When
  85                   uncompressing, does not prompt for overwriting files. If the
  86                   -f option is not specified, and the process is not running
  87                   in the background, prompts to verify whether an existing
  88                   file should be overwritten. If the standard input is not a
  89                   terminal and -f is not specified, writes a diagnostic
  90                   message to standard error and exits with a status greater
  91                   than 0.
  92 
  93 
  94        -v
  95                   Verbose. Writes to standard error messages concerning the
  96                   percentage reduction or expansion of each file.
  97 
  98 
  99        -/
 100                   When compressing or decompressing, copies any extended
 101                   system attributes associated with the source file to the
 102                   target file and copies any extended system attributes
 103                   associated with extended attributes of the source file to
 104                   the corresponding extended attributes associated with the
 105                   target file. If any extended system attributes cannot be
 106                   copied, the original file is retained, a diagnostic is
 107                   written to stderr, and the final exit status is non-zero.
 108 
 109 
 110 OPERANDS
 111        The following operand is supported:
 112 
 113        file
 114                A path name of a file to be compressed by compress,
 115                uncompressed by uncompress, or whose uncompressed form is
 116                written to standard out by zcat. If file is -, or if no file is
 117                specified, the standard input is used.
 118 
 119 
 120 USAGE
 121        See largefile(5) for the description of the behavior of compress,
 122        uncompress, and zcat when encountering files greater than or equal to 2
 123        Gbyte (2^31 bytes).
 124 
 125 ENVIRONMENT VARIABLES
 126        See environ(5) for descriptions of the following environment variables
 127        that affect the execution of compress, uncompress, and zcat: LANG,
 128        LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES, and NLSPATH.
 129 
 130 
 131        Affirmative responses are processed using the extended regular
 132        expression defined for the yesexpr keyword in the LC_MESSAGES category
 133        of the user's locale. The locale specified in the LC_COLLATE category
 134        defines the behavior of ranges, equivalence classes, and multi-
 135        character collating elements used in the expression defined for
 136        yesexpr. The locale specified in LC_CTYPE determines the locale for
 137        interpretation of sequences of bytes of text data a characters, the
 138        behavior of character classes used in the expression defined for the
 139        yesexpr. See locale(5).
 140 
 141 EXIT STATUS
 142        The following error values are returned:
 143 
 144        0
 145              Successful completion.
 146 
 147 
 148        1
 149              An error occurred.
 150 
 151 
 152        2
 153              One or more files were not compressed because they would have
 154              increased in size (and the -f option was not specified).
 155 
 156 
 157        >2
 158              An error occurred.
 159 
 160 
 161 ATTRIBUTES
 162        See attributes(5) for descriptions of the following attributes:
 163 
 164 
 165 
 166 
 167        +--------------------+-------------------+
 168        |  ATTRIBUTE TYPE    |  ATTRIBUTE VALUE  |
 169        +--------------------+-------------------+
 170        |CSI                 | Enabled           |
 171        +--------------------+-------------------+
 172        |Interface Stability | Committed         |
 173        +--------------------+-------------------+
 174        |Standard            | See standards(5). |
 175        +--------------------+-------------------+
 176 
 177 SEE ALSO
 178        ln(1), pack(1), fgetattr(3C), fsetattr(3C), attributes(5), environ(5),
 179        largefile(5), locale(5), standards(5)
 180 
 181 DIAGNOSTICS
 182        Usage: compress [-fv/] [-b maxbits] [file... ]
 183        compress c [-fv] [-b maxbits] [file]
 184 
 185            Invalid options were specified on the command line.
 186 
 187 
 188        Usage: uncompress [-fv] [-c | -/] [file]...
 189 
 190            Invalid options were specified on the command line.
 191 
 192 
 193        Missing maxbits
 194 
 195            Maxbits must follow -b, or invalid maxbits, not a numeric value.
 196 
 197 
 198        file: not in compressed format
 199 
 200            The file specified to uncompress has not been compressed.
 201 
 202 
 203        file: compressed with xxbits, can only handle yybits
 204 
 205            file was compressed by a program that could deal with more bits
 206            than the compress code on this machine. Recompress the file with
 207            smaller bits.
 208 
 209 
 210        file: already has .Z suffix -- no change
 211 
 212            The file is assumed to be already compressed. Rename the file and
 213            try again.
 214 
 215 
 216        file already exists; do you wish to overwrite (yes or no)?
 217 
 218            Respond y if you want the output file to be replaced; n if not.
 219 
 220 
 221        uncompress: corrupt input
 222 
 223            A SIGSEGV violation was detected, which usually means that the
 224            input file is corrupted.
 225 
 226 
 227        Compression:xx.xx%
 228 
 229            Percentage of the input saved by compression. (Relevant only for
 230            -v.)
 231 
 232 
 233        - - not a regular file: unchanged
 234 
 235            When the input file is not a regular file, (such as a directory),
 236            it is left unaltered.
 237 
 238 
 239        - - has xx other links: unchanged
 240 
 241            The input file has links; it is left unchanged. See ln(1) for more
 242            information.
 243 
 244 
 245        - - file unchanged
 246 
 247            No savings are achieved by compression. The input remains
 248            uncompressed.
 249 
 250 
 251        - -filename too long to tack on .Z
 252 
 253            The path name is too long to append the .Z suffix.
 254 
 255 
 256        - -cannot preserve extended attributes. file unchanged
 257 
 258            Extended system attributes could not be copied.
 259 
 260 
 261 NOTES
 262        Although compressed files are compatible between machines with large
 263        memory, -b 12 should be used for file transfer to architectures with a
 264        small process data space (64KB or less).
 265 
 266 
 267        compress should be more flexible about the existence of the .Z suffix.
 268 
 269 
 270 
 271                                February 5, 2020                    COMPRESS(1)