compress



COMPRESS(1)                 General Commands Manual                COMPRESS(1)


NAME

     compress, uncompress, zcat - compress and expand data (compress mode)


SYNOPSIS

     compress [-123456789cdfghlNnOqrtv] [-b bits] [-o filename] [-S suffix]
              [file ...]
     uncompress [-cfhlNnqrtv] [-o filename] [file ...]
     zcat [-fghqr] [file ...]


DESCRIPTION

     The compress utility reduces the size of the named files using adaptive
     Lempel-Ziv coding, in compress mode.  If invoked as compress -g, the
     deflate mode of compression is chosen; see gzip(1) for more information.
     Each file is renamed to the same name plus the extension ".Z".  As many
     of the modification time, access time, file flags, file mode, user ID,
     and group ID as allowed by permissions are retained in the new file.  If
     compression would not reduce the size of a file, the file is ignored
     (unless -f is used).

     The uncompress utility restores compressed files to their original form,
     renaming the files by removing the extension (or by using the stored name
     if the -N flag is specified).  It has the ability to restore files
     compressed by both compress and gzip(1), recognising the following
     extensions: ".Z", "-Z", "_Z", ".gz", "-gz", "_gz", ".tgz", "-tgz",
     "_tgz", ".taz", "-taz", and "_taz".  Extensions ending in "tgz" and "taz"
     are not removed when decompressing, instead they are converted to "tar".

     The zcat command is equivalent in functionality to uncompress -c.

     If renaming the files would cause files to be overwritten and the
     standard input device is a terminal, the user is prompted (on the
     standard error output) for confirmation.  If prompting is not possible or
     confirmation is not received, the files are not overwritten.

     If no files are specified, the standard input is compressed or
     uncompressed to the standard output.  If either the input or output files
     are not regular files, the checks for reduction in size and file
     overwriting are not performed, the input file is not removed, and the
     attributes of the input file are not retained.

     By default, when compressing using the deflate scheme (-g), the original
     file name and time stamp are stored in the compressed file.  When
     uncompressing, this information is not used.  Instead, the uncompressed
     file inherits the time stamp of the compressed version and the
     uncompressed file name is generated from the name of the compressed file
     as described above.  These defaults may be overridden by the -N and -n
     flags, described below.

     The options are as follows:

     -1...9  Use the deflate scheme, with compression factor of -1 to -9.
             Compression factor -1 is the fastest, but provides a poorer level
             of compression.  Compression factor -9 provides the best level of
             compression, but is relatively slow.  The default is -6.  This
             option implies -g.

     -b bits
             Specify the bits code limit (see below).

     -c      Compressed or uncompressed output is written to the standard
             output.  No files are modified (force zcat mode).

     -d      Decompress the source files instead of compressing them (force
             uncompress mode).

     -f      Force compression of file, even if it is not actually reduced in
             size.  Additionally, files are overwritten without prompting for
             confirmation.  If the input data is not in a format recognized by
             compress and if the option -c is also given, copy the input data
             without change to the standard output: let zcat behave as cat(1).

     -g      Use the deflate scheme, which reportedly provides better
             compression rates (force gzip(1) mode).

     -h      Print a short help message.

     -l      List information for the specified compressed files.  The
             following information is listed:

             compressed size    Size of the compressed file.

             uncompressed size  Size of the file when uncompressed.

             compression ratio  Ratio of the difference between the compressed
                                and uncompressed sizes to the uncompressed
                                size.

             uncompressed name  Name the file will be saved as when
                                uncompressing.

             If the -v option is specified, the following additional
             information is printed:

             compression method  Name of the method used to compress the file.

             crc                 32-bit CRC (cyclic redundancy code) of the
                                 uncompressed file.

             time stamp          Date and time corresponding to the last data
                                 modification time (mtime) of the compressed
                                 file (if the -n option is specified, the time
                                 stamp stored in the compressed file is
                                 printed instead).

     -N      When uncompressing or listing, use the time stamp and file name
             stored in the compressed file, if any, for the uncompressed
             version.  This information is only available when the deflate
             scheme (-g) is used.

     -n      When compressing, do not store the original file name and time
             stamp in the header of the compressed file.

     -O      Use compress mode (the default).

     -o filename
             Set the output file name.

     -q      Be quiet: suppress all messages.

     -r      Recursive mode: compress will descend into specified directories.

     -S suffix
             Set the suffix for compressed files.

     -t      Test the integrity of each file leaving any files intact.

     -v      Print the percentage reduction of each file and other
             information.

     compress uses a modified Lempel-Ziv algorithm (LZW).  Common substrings
     in the file are first replaced by 9-bit codes 257 and up.  When code 512
     is reached, the algorithm switches to 10-bit codes and continues to use
     more bits until the limit specified by the -b flag is reached.  bits must
     be between 9 and 16 (the default is 16).

     After the bits limit is reached, compress periodically checks the
     compression ratio.  If it is increasing, compress continues to use the
     existing code dictionary.  However, if the compression ratio decreases,
     compress discards the table of substrings and rebuilds it from scratch.
     This allows the algorithm to adapt to the next "block" of the file.

     The -b flag is omitted for uncompress since the bits parameter specified
     during compression is encoded within the output, along with a magic
     number to ensure that neither decompression of random data nor
     recompression of compressed data is attempted.

     The amount of compression obtained depends on the size of the input, the
     number of bits per code, and the distribution of common substrings.
     Typically, text such as source code or English is reduced by 50 - 60%
     using compress.  Compression is generally much better than that achieved
     by Huffman coding (as used in the historical command pack), or adaptive
     Huffman coding (as used in the historical command compact), and takes
     less time to compute.


EXIT STATUS

     The compress utility exits with one of the following values:

           0       Success.
           1       An error occurred.
           2       At least one of the specified files was not compressed
                   since -f was not specified and compression would have
                   resulted in a size increase.
           >2      An error occurred.

     The uncompress and zcat utilities exit 0 on success, and >0 if an error
     occurs.


SEE ALSO

     gzexe(1), gzip(1), zdiff(1), zforce(1), zmore(1), znew(1), compress(3)

     Welch, Terry A., "A Technique for High Performance Data Compression",
     IEEE Computer, 17:6, pp. 8-19, June, 1984.


STANDARDS

     The compress, uncompress, and zcat utilities are compliant with the
     X/Open System Interfaces option of the IEEE Std 1003.1-2008 ("POSIX.1")
     specification.

     The compress flags [-123456789dghlNnOoqrSt], uncompress flags
     [-hlNnoqrt], and the zcat flags [-fghqr] are extensions to that
     specification.

     IEEE Std 1003.1-2008 ("POSIX.1") specifies a maximum bits limit (-b) of
     14 to "achieve portability to all systems".


HISTORY

     The compress command appeared in 4.3BSD.  Deflate compression support was
     added in OpenBSD 2.1.

OpenBSD 6.2                     March 17, 2014                     OpenBSD 6.2

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