unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Page:
Section:
Apropos / Subsearch:
optional field

COMPRESS(1)                 General Commands Manual                COMPRESS(1)



NAME
       compress, uncompress.real - compress and expand data (version 4.2)

SYNOPSIS
       compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ...  ]
       uncompress.real [ -f ] [ -v ] [ -c ] [ -V ] [ name ...  ]

DESCRIPTION
       Note that the program that would normally be installed as uncompress is
       installed for Debian as uncompress.real.  This has been done  to  avoid
       conflicting with the more-commonly-used program with the same name that
       is part of the gzip package.

       Compress reduces the size of the named files using adaptive  Lempel-Ziv
       coding.   Whenever  possible,  each  file  is  replaced by one with the
       extension .Z, while keeping the same ownership modes, access and  modi-
       fication  times.  If no files are specified, the standard input is com-
       pressed to the standard output.  Compress will only attempt to compress
       regular files.  In particular, it will ignore symbolic links. If a file
       has multiple hard links, compress will refuse to compress it unless the
       -f flag is given.

       If  -f  is not given and compress is run in the foreground, the user is
       prompted as to whether an existing file should be overwritten.

       Compressed files can be restored to their original  form  using  uncom-
       press.real.

       uncompress.real  takes a list of files on its command line and replaces
       each file whose name ends with .Z and which  begins  with  the  correct
       magic  number  with  an  uncompressed  file without the .Z.  The uncom-
       pressed file will have the mode, ownership and timestamps of  the  com-
       pressed file.

       The -c option makes compress/uncompress.real write to the standard out-
       put; no files are changed.

       If the -r flag is specified, compress will operate recursively. If  any
       of  the  file names specified on the command line are directories, com-
       press will descend into the directory and compress  all  the  files  it
       finds there.

       The  -V  flag  tells  each  of  these programs to print its version and
       patchlevel, along with any preprocessor flags specified during compila-
       tion, on stderr before doing any compression or uncompression.

       Compress uses the modified Lempel-Ziv algorithm popularized in "A Tech-
       nique for High Performance Data Compression", Terry A. Welch, IEEE Com-
       puter,  vol. 17, no. 6 (June 1984), pp. 8-19.  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  (default
       16).  Bits must be between 9 and 16.  The default can be changed in the
       source to allow compress to be run on a smaller machine.

       After the bits limit is attained, compress periodically checks the com-
       pression  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.

       Note that the -b flag is omitted for uncompress.real,  since  the  bits
       parameter  specified  during  compression is encoded within the output,
       along with a magic number to ensure that neither decompression of  ran-
       dom 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%.
       Compression is generally much better than that achieved by Huffman cod-
       ing  (as used in pack), or adaptive Huffman coding (compact), and takes
       less time to compute.

       Under the -v option, a message is printed yielding  the  percentage  of
       reduction for each file compressed.

SEE ALSO
       pack(1), compact(1)

DIAGNOSTICS
       Exit status is normally 0; if the last file is larger after (attempted)
       compression, the status is 2; if an error occurs, exit status is 1.

       Usage: compress [-dfvcVr] [-b maxbits] [file ...]
               Invalid options were specified on the command line.
       Missing maxbits
               Maxbits must follow -b.
       file: not in compressed format
               The file specified to uncompress.real has not been compressed.
       file: compressed with xx bits, can only handle yy bits
               File was compressed by a program that could deal with more bits
               than  the  compress  code on this machine.  Recompress the file
               with smaller bits.
       file: already has .Z suffix -- no change
               The file is assumed to be already compressed.  Rename the  file
               and try again.
       file: filename too long to tack on .Z
               The  file  cannot be compressed because its name is longer than
               12 characters.  Rename and try again.  This  message  does  not
               occur on BSD systems.
       file already exists; do you wish to overwrite (y or n)?
               Respond  "y" if you want the output file to be replaced; "n" if
               not.
       uncompress: corrupt input
               A SIGSEGV violation was detected which usually means  that  the
               input file has been corrupted.
       Compression: xx.xx%
               Percentage  of  the input saved by compression.  (Relevant only
               for -v.)
       -- not a regular file or directory: ignored
               When the input file is not a regular file or directory, (e.g. a
               symbolic  link,  socket,  FIFO,  device file), it is left unal-
               tered.
       -- has xx other links: unchanged
               The input file has links; it is left unchanged.  See ln(1)  for
               more  information. Use the -f flag to force compression of mul-
               tiply-linked files.
       -- file unchanged
               No savings is achieved by compression.  The input remains  vir-
               gin.

BUGS
       Although  compressed  files  are compatible between machines with large
       memory, -b12 should be used for file transfer to architectures  with  a
       small  process  data  space  (64KB or less, as exhibited by the DEC PDP
       series, the Intel 80286, etc.)

       Invoking compress with a -r flag will occasionally cause it to  produce
       spurious error warnings of the form

        "<filename>.Z already has .Z suffix - ignored"

       These  warnings  can  be ignored. See the comments in compress42.c:com-
       pdir() in the source distribution for an explanation.




                                     local                         COMPRESS(1)