*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->Tru64 Unix man pages -> compress (1)              



NAME    [Toc]    [Back]

       compress, uncompress, zcat - Compresses and expands data

SYNOPSIS    [Toc]    [Back]

       compress [-cCdfFnqvV] [-b bits] [file...]

       uncompress [-cCfFnqvV] [file...]

       zcat [-n] [file...]

STANDARDS    [Toc]    [Back]

       Interfaces  documented  on  this reference page conform to
       industry standards as follows:

       compress:  XCU5.0

       uncompress:  XCU5.0

       zcat:  XCU5.0

       Refer to the standards(5) reference page for more information
 about industry standards and associated tags.

OPTIONS    [Toc]    [Back]

       Specifies  the  maximum  number  of bits to use to replace
       common substrings in the file.  The default  for  bits  is
       16,  with  values  of 9 through 16 acceptable.  First, the
       algorithm uses 9-bit codes 257 through 512.  Then it  uses
       10-bit  codes, continuing until the bits limit is reached.
       (This option applies to the compress command only.)

              After the bits limit is attained, the compress command
 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 the beginning.
              This lets the algorithm adapt to the next block  of
              the file.

              [Tru64 UNIX]  The -b option must be the last option
              on the command line.  Makes compress and uncompress
              write to the standard output; no files are changed.
              The nondestructive behavior of zcat is identical to
              that of uncompress -c.  [Tru64 UNIX]  Produces output
   compatible   with   compress   2.0.    [Tru64
              UNIX]  Uncompresses  a  file.   When  used with the
              compress command, forces the  compression  of  file
              even  if  no reduction in the size of the file will

              With both the compress and uncompress commands,  if
              you run the command in the background and -f is not
              specified, you are prompted as to whether an existing
    file    should   be   overwritten.    [Tru64
              UNIX]  Specifies that no  header  has  been  added.
              [Tru64  UNIX]  Specifies a quiet operation. This is
              the default.  Diagnostics messages,  which  display
              if  you specify the -v option, do not print. If the
              -q and -v options are both specified, the last  one
              on   the  command  line  is  honored.   Prints  the
              percentage reduction of each file when  compressing
              the  file.   Prints messages to standard error concerning
 the expansion of each file when uncompressing
  the  file.  If  the -q and -v options are both
              specified, the last one on the command line is honored.
  [Tru64 UNIX]  Specifies a version.

OPERANDS    [Toc]    [Back]

       The path name of a file to be processed.  If - (hyphen) is
       used, standard input will be read.

              The compress command will  create  an  output  file
              name  by  appending  a  suffix of to the input file

              With the uncompress and zcat commands, if the  file
              name has a suffix, the command will look for a file
              of that name.  If the suffix is not specified,  the
              command will append the suffix before searching for
              the file.

              The uncompress command will store output in a  file
              named  with the suffix removed if it was specified.
              If no suffix is specified, the  uncompress  command
              will accept file as the name of the output file and
              append a suffix before searching for the file.

DESCRIPTION    [Toc]    [Back]

       The compress command reduces the size of the  named  files
       using adaptive Lempel-Ziv coding.

       Whenever  possible,  each file is replaced by one with the
       extension while keeping the same ownership modes,  access,
       and  modification times.  If no files are specified, standard
 input is compressed to the standard output.

       Compressed files can be restored to their original form by
       using the uncompress or zcat command.

       The  uncompress  command replaces the compressed file with
       an uncompressed version of the file, identical to the file
       that  was  originally compressed with compress; the suffix
       is removed.  When issuing an uncompress command,  you  can
       refer  to  the  compressed target file with or without the
       suffix. If you  do  not  specify  the  suffix,  uncompress
       assumes it.

       The zcat command writes the uncompressed version of a compressed
 file to  standard  output.   The  compressed  (.Z)
       files  remain  intact.  The  zcat  command is identical to
       uncompress -c.  When issuing a zcat command, you can refer
       to  the compressed target file with or without the suffix;
       if you do not specify the suffix, zcat assumes it.

       [Tru64 UNIX]  The compress command uses the modified  Lempel-Ziv
  algorithm  popularized  in  "A Technique for High
       Performance Data Compression," Terry A. Welch,  IEEE  Computer,
 vol. 17, no. 6 (June 1984), pp. 8-19.

       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, files containing
       source code or plain text are reduced by 50 to 60 percent.
       Compression is generally much better than that achieved by
       Huffman coding (as used in the pack command)  or  adaptive
       Huffman coding, and takes less time to compute.

NOTES    [Toc]    [Back]

       [Tru64  UNIX]  The  compress  command may not preserve the
       extended  file  attributes  (property  list)  of  a  file,
       including  the  access  control  list (ACL) if any. Verify
       that any ACLs have not  been  removed  or  modified  after
       using compress or uncompress.

EXIT STATUS    [Toc]    [Back]

       The following exit values are returned:

   compress command
       Successful  completion.   An  error occurred.  One or more
       files were not compressed because they became larger.   An
       error occurred.

   uncompress and zcat commands
       Successful completion.  An error occurred.

DIAGNOSTICS    [Toc]    [Back]

       Invalid  arguments  were  specified  on  the command line.
       maxbits must follow -b.  The file cannot  be  uncompressed
       because  it was never compressed.  The 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 is assumed to be  already  compressed.
       Rename the file and try again.  Respond y, or the locale's
       equivalent of a y, if you  want  the  output  file  to  be
       replaced;  n,  or  the locale's equivalent of a n, if not.
       (The LC_MESSAGES variable determines the locale's  equivalent
  of y or n.)  A SIGSEGV violation was detected, which
       usually means that the input file is corrupted.   Percentage
  of  the input saved by compression (relevant only for
       -v).  When the input file is  not  a  regular  file,  (for
       example,  a  directory),  it is left unaltered.  The input
       file has links; it is left unchanged.  (See the ln command
       for  more information.)  No saving is achieved by compression.
  The input remains unchanged.

EXAMPLES    [Toc]    [Back]

       To compress folder and print the savings, enter:  compress
       -v folder

              The  system  responds  with a message like: folder:
              Compression: 43.94% -- replaced with  folder.Z  The
              following command displays the uncompressed version
              of the testlog.Z file: zcat testlog.Z


       The following environment variables affect  the  execution
       of  compress,  uncompress,  and  zcat:  Provides a default
       value for  the  internationalization  variables  that  are
       unset or null. If LANG is unset or null, the corresponding
       value from the default locale is  used.   If  any  of  the
       internationalization variables contain an invalid setting,
       the utility behaves as if none of the variables  had  been
       defined.   If  set  to a non-empty string value, overrides
       the values of all  the  other  internationalization  variables.
   Determines  the  locale for the interpretation of
       sequences of bytes of text data as characters  (for  example,
  single-byte  as  opposed  to multibyte characters in
       arguments).  Determines the locale for the format and contents
  of  diagnostic  messages written to standard error.
       Determines the location of message catalogues for the processing

SEE ALSO    [Toc]    [Back]

       Commands:  pack(1), pcat(1), unpack(1)

       Files:  acl(4)

       Standards:  standards(5)

[ Back ]
 Similar pages
Name OS Title
pcat Tru64 Compresses and expands files
pack Tru64 Compresses and expands files
gunzip Tru64 Compresses or expands files.
gzip Tru64 Compresses or expands files.
gzcat Tru64 Compresses or expands files.
unpack Tru64 Compresses and expands files
dmMPEG1AudioEncode IRIX Compresses a single block of audio data using MPEG1 audio compression algorithm.
expand_alias HP-UX recursively expands the sendmail aliases
logrotate Linux rotates, compresses, and mails system logs
dcecp_name HP-UX A dcecp object that compares and expands DCE names
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service