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compress(1)               BSD General Commands Manual              compress(1)


     compress, uncompress -- compress and expand data


     compress [-fv] [-b bits] [file ...]
     compress -c [-b bits] [file]
     uncompress [-fv] [file ...]
     uncompress -c [file]


     The compress utility reduces the size of files using adaptive Lempel-Ziv
     coding.  Each file is renamed to the same name plus the extension .Z.  A
     file argument with a .Z extension will be ignored except it will cause an
     error exit after other arguments are processed.  If compression would not
     reduce the size of a file, the file is ignored.

     The uncompress utility restores compressed files to their original form,
     renaming the files by deleting the .Z extensions.  A file specification
     need not include the file's .Z extension.  If a file's name in its file
     system does not have a .Z extension, it will not be uncompressed and it
     will cause an error exit after other arguments are processed.

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

     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

     If no files are specified or a file argument is a single dash (`-'), the
     standard input is compressed or uncompressed to the standard output.  If
     either the input and 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 in
     the output file.

     The options are as follows:

     -b bits  The code size (see below) is limited to bits, which must be in
              the range 9..16.  The default is 16.

     -c       Compressed or uncompressed output is written to the standard
              output.  No files are modified.  The -v option is ignored.  Com-
              pression is attempted even if the results will be larger than
              the original.

     -f       Files are overwritten without prompting for confirmation.  Also,
              for compress, files are compressed even if they are not actually
              reduced in size.

     -v       Print the percentage reduction of each file.  Ignored by
              uncompress or if the -c option is also used.

     The compress utility uses a modified Lempel-Ziv algorithm.  Common sub-
     strings 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 option or its
     default is reached.

     After the 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 dis-
     cards the table of substrings and rebuilds it from scratch.  This allows
     the algorithm to adapt to the next "block" of the file.

     The -b option is unavailable 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.  Typ-
     ically, text such as source code or English is reduced by 50-60%.  Com-
     pression 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.


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

     The compress utility exits 2 if attempting to compress a file would not
     reduce its size and the -f option was not specified and if no other error


     gunzip(1), gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)

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


     The compress and uncompress utilities conform to IEEE Std 1003.1-2001


     The compress command appeared in 4.3BSD.


     Some of these might be considered otherwise-undocumented features.

     compress: If the utility does not compress a file because doing so would
     not reduce its size, and a file of the same name except with an .Z exten-
     sion exists, the named file is not really ignored as stated above; it
     causes a prompt to confirm the overwriting of the file with the exten-
     sion.  If the operation is confirmed, that file is deleted.

     uncompress: If an empty file is compressed (using -f), the resulting .Z
     file is also empty.  That seems right, but if uncompress is then used on
     that file, an error will occur.

     Both utilities: If a `-' argument is used and the utility prompts the
     user, the standard input is taken as the user's reply to the prompt.

     Both utilities: If the specified file does not exist, but a similarly-
     named one with (for compress) or without (for uncompress) a .Z extension
     does exist, the utility will waste the user's time by not immediately
     emitting an error message about the missing file and continuing.
     Instead, it first asks for confirmation to overwrite the existing file
     and then does not overwrite it.

BSD                              May 17, 2002                              BSD

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